Words fail me: Symptoms and causes of naming breakdown in aphasia.
Lyndsey Nickels,
Macquarie Centre for Cognitive Science
Macquarie University, Sydney.
Dr L.A. Nickels,
Macquarie Centre for Cognitive Science,
Macquarie University, Sydney, NSW 2109, Australia.
TEL: +61-2-9850-8448
FAX: +61-2-9850-6059
E-mail: lyndsey@maccs.mq.edu.au
To appear in:
R.Berndt (Ed.) Handbook of Neuropsychology, Vol 2: Language and Aphasia
(Second Edition). Amsterdam:Elsevier Science.
Acknowledgements
This chapter was prepared while the author was supported by a Wellcome Trust
Advanced Training Fellowship and an Australian Research Council QEII Fellowship.
Thanks to Wendy Best, Jenny Cole-Virtue and Sarah Ross for helpful comments to
an earlier version, and to Sarah Ross for help formatting the manuscript. Particular
thanks to the people with aphasia I have known, who have taught me so much, and
generously given of their time.
Nickels / 2
People with aphasia have different patterns of language impairment; they may have
problems with understanding spoken or written language, difficulties in reading
aloud, or writing, or difficulties in expressing themselves in speech. Of all of these
different problems, it is very often the difficulty in producing spoken language that is
the most troubling for the person with aphasia. Some may be able to retrieve the
words they want to say with ease but be unable to use these in connected speech.
However, many aphasic people cannot retrieve or pronounce the words that they
need to form sentences. These problems are the topic of this chapter.
Table 1: Excerpt of conversation between the author (LN) and a woman with aphasia
(LMT) triggered by her difficulty in describing a pictured scene.
LMT: Oh dear, see I can’t come and give it to anyone .. I didn’t .. Ican’t /bel/ it can I, I
can’t give it.
LN: Its difficult
LMT: It is, for me you see, its only just simple and I know them and all the rest of it,
but I just can’t, from there to there (points at throat then mouth).
…. I mean I can go, and I can ge.. and I c you know come and say this and know
everything but it won’t come, see that’s the only thing. Funny isn’t it
LN: And when you first had your stroke what was it like then?
LMT: Oh it was terrible then, couldn’t .. I couldn’t hardly get anything really.
LN: Worse than it is now?
LMT: Oh much worse, yes
LN: What happened then?
LMT: Well I just couldn’t, I just couldn’t talk at all and I used to want a cigare.. not a
cigarette.. um er oh you know I mean for my throat, and I couldn’t ask the man if if
Icould please have that or that ‘cos I hadn’t got it there.
Table 1 gives an excerpt of conversation with a woman who has aphasia (LMT) who
is explaining her difficulties in producing words. While conveying her message, her
speech can be described as ‘empty’ with few content words. The one specific lexical
item she attempts to retrieve (as opposed to using alternative strategies such as
gesture) results in an error – cigarette. It is difficult in this case to know what LMT
was aiming for when she retrieved ‘cigarette’, possibly ‘drink’ or ‘water’ or ‘coffee’.
Identifying the relationship between the target and the error assists us in
understanding the nature of the difficulty in word production. Hence, rather than
Nickels / 3
analyse spontaneous speech where, as we have seen, the target can be difficult to
unambiguously determine, picture naming is often used. Picture naming provides a
(relatively) unambiguous target, and a means of varying the target to see whether
different types of words are more or less problematic (e.g. longer words, less
common words). Thus, in table 2 we can see that often LMT’s errors seem unrelated
to their targets. In contrast, the errors made by two other aphasic individuals, CTJ
and CK, do show a clear relationship to the name of the picture: CTJ’s errors seem
to be related in meaning, and CK’s errors have sounds in common.
Table 2: Picture naming errors made by 3 individuals with aphasia: LMT (Nickels,
unpublished data); CTJ & CK (Nickels, 1992b)
Picture
name
triangle
saxophone
skirt
tent
stable
daffodil
camel
LMT’s response
CTJ’s response
CK’s response
a three something, a three,
it’s a three that way, I mean
three, three
/helfnt/ /hnfd/
square
trifle
soul … something to do
with soul.
blouse
/sksfe/ /sk/
/sk/


Horse…
/tent/ …
/skebl/ /skebl/

/dr/ /drv/
Funny horse! Some have
two bumps, its only got
one, people like Iranians
and all that.

umbrella, not an umbrella,
one of these (points to her
skirt).
This is a match, a /mtu:/
a /k:pn/ the garden’s
/k:pn/
a /frna:/ /frn:/ it isn’t but
still it is.
/ks/ /ksl /ksl/
/kstl/
The question, of course, is why is CTJ unable to retrieve the words he wants, and
why is CK unable to correctly pronounce the words she tries to say? Only when we
understand the causes of the surface symptoms will we be able to determine the
most appropriate therapy to remediate those symptoms.
Nickels / 4
Levels of processing in word production
In order to identify the causes of difficulty in word retrieval, we will first outline the
different levels of processing that are thought to occur when producing a single word.
The model we shall use (Fig 1) has been chosen because of its ability to account for
the basic patterns of breakdown in aphasic naming. The levels of processing are
broadly similar to most current models of spoken word production (e.g. Butterworth,
1989; Caramazza, 1997; Dell, Schwartz, Martin et al., 1997; Levelt, Roelofs and
Meyer, 1999), however terminology may differ. Furthermore, where there is one level
in the model used here, other models may dissociate it into two levels (and vice
versa). Similarly, the precise processing assumptions within and between models
may also differ (e.g. decompositional semantics (Dell et al, 1997) versus nondecompositional semantics (Levelt et al., 1999), or strictly feed-forward flow of
activation (Levelt et al, 1999) versus interactive activation (Dell et al., 1997)) 1.
Detailed discussion of these issues is beyond the scope of this chapter.
Figure 1: Levels of processing in spoken word production.
Conceptual
Semantics
Lexical
Semantics
Phonological
Output Lexicon
Phonological
Output Buffer
Phonological
encoding
Speech
Nickels / 5
Whether one is attempting to retrieve a word for use in conversation, or name an
object in your environment or name a picture, the same basic processes occur. First
there must be some activation of the concept corresponding to the
object/picture/idea (for pictures or objects this will be activated via vision and object
recognition processes). We will assume that this occurs at the level of conceptual
semantics. This represents the meanings of things; it is sometimes labelled
‘semantic memory’. The information at this level is non-linguistic and pre-verbal.
Hence, at this level we may have concepts for things that we have no name for.
Similarly, infants clearly have concepts for things before they have any
understanding of language (e.g. for ‘mother’, ‘food’).
We can think of this conceptual representation as activation of a set of semantic
features that describe an item2 e.g. for the concept of DOG our semantic
representation might consist of the features [has fur] [barks] [four legs] [pet]. When
we see a picture of a dog (or think about a dog) we activate all of these features (and
of course many more; Figure 2). This pattern of activation corresponds to our
knowledge of the concept DOG. Of course, some of these features will be part of the
representation of other concepts. It is only the set of features as a whole that can
identify one concept rather than another. Thus, the same feature ‘has fur’ will be
active for many different (animal) concepts.
The conceptual-semantic level is linked to a level of lexical-semantics. At this level
each word that you know is represented individually. Words that sound (or are spelt)
the same, but have different meanings will be separately represented. Thus there will
be a representation (node) for ‘bank’, the financial institution, and for ‘bank’, the part
of a river bed. This level corresponds to the ‘lemma’ level in some models (e.g.
Levelt, 1989), and as such may also include details of a word’s syntactic properties3.
The lexical-semantic level is modality-neutral, in other words the same
representation is accessed whether the concept is to be spoken or written (or, in
comprehension, if the word is heard or read). The lexical-semantic node for an item
is activated by the features that represent the concept to which that node refers. To
use the example above, the lexical semantic node for ‘dog’ will become active when
the conceptual features [has fur], [barks], [four legs] [pet] are activated. However, as
every active semantic feature at the conceptual level will activate every item at the
lexical-semantic level that has that feature, other semantically-related nodes will also
become active, by virtue of the fact that they share features (e.g. the nodes for ‘cat’ ,
‘rabbit’, ’fish’). Nevertheless, the node corresponding to the to-be-expressed-concept
(the target node) will be the most highly active as more features will correspond to
that item than to any other. For example, the feature [pet] will activate the stored
representation for ‘dog’ but also those for ‘cat’ ‘rabbit’ ‘fish’. The node for ‘dog’ will
nevertheless be the most active, as this has received activation from the most
features (i.e. by four features, whereas ‘cat’ and ‘rabbit’ will only be activated by
three, and fish by one).
The lexical-semantic node in turn activates a corresponding representation at the
level of the Phonological Output Lexicon (POL). This is the level at which the sound
structure (phonology) of a word is represented (as there is no regular or rulegoverned correspondence between the meaning of a word and the sounds used to
say it, these correspondences must be learned and represented by 1:1 mappings) 4.
Nickels / 6
Object, picture or
idea
purrs
robin
fur
bark
s
cat
dog
/kt/
/dg/
4legs
pet
rabbit
/rbt/
scales
fish
/f/
Conceptual-Semantic
Level
house
Lexical-Semantic
Level
Phonological
Level
Thus, the lexical-semantic node for ‘dog’ will activate the node for its phonological
representation /dog/. The lexical-semantic node will also activate the stored
orthographic form of the word (its spelling) in the orthographic output lexicon.
Figure 2: An example of how selection of the phonological form associated with a
particular concept may occur. Black lines represent connections currently sending
activation from one level to the next; grey lines represent connections between
nodes that are not currently active; dotted lines represent other connections not
implemented in this example. Active nodes are encircled, the more rings the more
active the node
When the phonological form (the sounds, syllable structure and stress pattern) of the
word has been retrieved from the POL it can be held in the Phonological Output
Buffer until required for output. Prior to output there are a number of further
procedures that are required - this is known as "Phonological Encoding" and involves
'unpacking' the phonology and metrical (stress and syllable) structure, resyllabifying
this information, associating the phonemes with their positions within a syllable,
Nickels / 7
retrieving and/or assembling the motor programmes for articulation (see Levelt et al,
1999 for further details).
When this speech production system (fig. 1) is affected by brain damage, some parts
of the system may be damaged while others may remain intact. Thus, word
production (including picture naming) may break down at any one of the levels of
processing discussed above or combinations of levels, depending on the nature and
extent of the damage. Breakdown at each level will have characteristic symptoms in
terms of the types of naming errors that occur, the factors that affect naming (e.g.
how long a word is, how frequently it occurs in the language, how concrete it is) and
whether it is only naming that is affected or if performance on other tasks is impaired
too (e.g. understanding words, repeating, reading or writing words)5. We will now go
on to examine these characteristics.
Types of error in spoken word production
Errors in the speech of aphasic people for the most part mirror the type of speech
errors made by non-aphasic people (Ellis, 1985). The difference is that for aphasics
these errors occur more frequently. In spontaneous speech, the errors can be
divided into main three groups depending on their relationship to the intended word
(target)6:
1) they may be related in meaning, and called semantic errors, such as saying ‘cat’
or ‘kennel’ when trying to name a picture of a dog.
2) they may be related by virtue of sharing sounds (phonology) with the targets, and
known as phonological errors (or occasionally phonemic, or speech sound
errors; literal paraphasias, Goodglass & Kaplan, 1972). For example, saying ‘log’
or ‘dop’ to a picture of a dog.
3) they may be unrelated to the targets having neither meaning nor sounds in
common. For example, for a picture of a ‘dog’ both ‘mountain’ and ‘mekit’ would
be unrelated errors.
For each of these error types, we will discuss, below, the different possible levels at
which they might arise when the model shown in figure 1 is damaged. Then, we will
briefly describe some other types of responses.
Semantically-related responses
These responses can be subdivided into categorically related errors: co-ordinates or
members of the same category as the target (e.g. saying ‘cat’ for dog), or
superordinates (e.g.’animal’ for ‘dog’) or subordinates of the target (e.g. ‘greyhound’
for ‘dog’, when the depicted dog is not a greyhound); and associatively related errors
(that do not share semantic characteristics): for example, ‘kennel’ to a picture of a
dog, or ‘war’ as a response to a picture of a soldier, or ‘Soul’ for saxophone.
Traditionally, semantically related errors are thought of as a symptom of a semantic
level disorder (either lexical or conceptual) and associated with poor comprehension.
More recently, however, it has been demonstrated that semantic errors may occur
even when the stored meaning is intact (Caramazza and Hillis, 1990; see below).
(See also discussion below, regarding multi-word responses).
Nickels / 8
Phonologically related responses
These errors which are related in sound to the target, may be words of the speaker’s
language or ‘nonwords’ - words that do not exist in that speaker’s language (or at
least are not known to the speaker). As unrelated errors may just happen to share
sounds by chance, it is good practice to use a consistent criterion for relatedness: for
example, (at least) 50% of the phonemes shared between the target and response
(some authors use more lax criteria e.g. Dell et al, 1997). Using this criterion, when
attempting to produce ‘doctor’ /dkt/ - /dkl/ and /dp/ would be classified as
related responses whereas /di:pl:/ and /dp/ would be classified as unrelated.
Most aphasic people produce more phonologically related errors that are nonwords
than words (e.g. Nickels and Howard, 1995a). This is generally argued to reflect the
fact that the incorrect responses are caused by errors affecting single sounds
(phonemes) rather than the whole word. In other words, one sound in the word is
produced incorrectly. These errors might be as a result of a problem after the
phonological form has been successfully retrieved (i.e. during phonological
encoding). However, a problem at this level will result in both phonologically-related
nonwords and words. An error can be a word by chance - if one phoneme is
changed at random short words will often result in real word errors, long words will
rarely do so (e.g. compare the effects of changing a single phoneme in 'cat' to that of
changing a phoneme in ‘caterpillar’). Thus despite being words these errors are not
formed by any lexical process (such as selection of another word from the lexicon)
but are simply chance events.
However, some aphasic people have been reported in the literature who have
produced more real word phonologically related errors than you would predict by
chance alone (e.g. Best 1996; Blanken, 1990, 1998; Martin & Saffran, 1992).
Because they share phonological form with the target these real word errors are
often referred to as FORMAL PARAPHASIAS, (e.g. sink -> sick; buckle -> bucket).
One possible explanation is in terms of a deficit at the level of the phonological
output buffer (Best, 1996). An impairment at this level results in the activation of
phonemes ‘decaying’ faster than usual. However, because of the feedback from the
buffer to the lexicon, patterns of phonemes that correspond to words in the
phonological output lexicon will receive more ‘support’ (refreshing of activation) than
those patterns corresponding to nonwords. Thus there will be a greater tendency for
errors to be words than nonwords (see Martin & Saffran, 1992 for a similar account).
Unrelated responses
Like phonologically related responses these may be words or nonwords but differ in
that they share no phonemes) with the target (or at least share fewer phonemes than
the criterion of 50% and/or share no more phonemes than would be expected by
chance).
Unrelated Nonwords: (e.g. microscope -> tugemum; spoon -> trizel).
These are often referred to as NEOLOGISMS (e.g. Buckingham, 1981, 1987;
Butterworth, 1979) although some caution is required regarding the scope of this
term. Some authors (e.g. Ellis, Miller and Sin, 1983) also include phonologicallyrelated errors that are nonwords under this category, while others also include
unrelated words (e.g. Goodglass & Kaplan, 1972). While many aphasic people
Nickels / 9
produce few, if any, of these responses others produce them in profusion. These
aphasic people are often referred to as having neologistic jargon aphasia (e.g.
Buckingham, 1987; and are also generally subsumed under the syndrome of
Wernicke’s aphasia).
There are many different theories regarding the origin of unrelated nonwords and
this most probably reflects the fact that they can arise for a number of different
reasons (see Buckingham, 1987, for a review). Some possible accounts are:
1) These unrelated nonwords could be phonological errors where so many
phonemes are erroneous that they no longer share any phonemes with the target.
This sometimes becomes apparent where a subject makes a sequence of errors
that progress towards the target (‘conduite d’approche’). The responses are all
clearly related to each other but only in the later responses is it apparent that they
are also related to the targets.
2) They may be the consequence of a semantic error which is then produced
erroneously - a ‘semantic-then-phonological’ error, which results in the ‘path’ of
the error being unrecognisable. For example, RK (Nickels, 1992b) named
‘monkey’ as /tlfnt/. This could be from a semantic error to ‘monkey’, resulting
in ‘elephant’ (/elfnt/), which was then erroneously produced as /tlfnt/ due
to a phonological error.
3) They may be an attempt to fill a ‘lexical gap’. If no word is retrieved from lexicon,
but the context requires a response (e.g. the linguistic context within in a sentence;
or the social context in a picture naming task), this ‘gap’ may be filled by generating
a random (but phonotactically legal) string of phonemes (see e.g. Butterworth, 1979).
For example, when required to name a picture of a piano, LMT (described above)
said “this is a /bredfd/”, and to a picture of a stable said “it’s a /ran/ for a /vlt/”.
Unrelated Words
These responses comprise words of the language that the speaker knows but that
are unrelated in both meaning and sound (and in any other detectable way) to the
target. They are possibly the most unusual responses, but once again they are a
feature of some ‘jargon’ aphasics (Wernicke’s aphasics) - described as ‘extended
English jargon’. For example, LMT responded ‘porridge’ to a picture of an elephant,
and responded ‘bump’ to a picture of a noose.
Once again the source of these errors is unclear and there may be many different
reasons why an word unrelated to the target might be produced. Possibly, as with
unrelated nonword responses, they may reflect an attempt to fill a lexical gap.
However, in this case one must assume that no activation of the target (or its
semantic neighbours) is occurring at the lexical level. As no related lexical item is
active a lexical item is retrieved at random from the output lexicon.
Further Response Types:
Although the discussion below will concentrate on the sources of semantically and
phonologically related responses, there are other response types that we should
briefly consider:
Multi-Word Descriptions Or Circumlocutions
Nickels / 10
When unable to retrieve the target word some aphasic people will instead describe
properties of the object to which it refers. For example, when attempting to name a
picture of a pair of pyjamas, RGB (Caramazza and Hillis, 1990) responded "what you
wear at night"; similarly, Caramazza and Hillis's other patient, HW, responded "for
going to school" to a picture of a bus. These ‘circumlocutions’ are often thought of as
quite distinct from the single-word semantically-related responses described above.
Circumlocutions are rarely described as ‘errors’ unlike single-word semantically
related responses. This distinction is clearly drawn by Goodglass and Kaplan (1972,
p8) who urge that "verbal paraphasias (single word semantic errors) which are
unintended, should be distinguished from "one-word circumlocutions" in which the
patient deliberately chooses an approximation to his intended idea because of his
word-finding difficulty". In other words they are suggesting a distinction between
automatically occurring error processes and strategic responses to a deficit.
Unfortunately they do not provide any criteria on which to base this distinction, in
practice this is a difficult discrimination to make.
In general, making inferences about an aphasic person’s deficit based on the
distinction between single-word and multi-word semantically-related responses may
be unwise. The different response types may reflect processing at the level of the
sentence (and beyond) more than the nature of the deficit underlying picture naming.
For example, the 'fluent' aphasic, who can produce multi-word utterances with ease,
may say 'it's like a radio' when attempting to name a picture of a record player (e.g.
EST, Kay and Ellis, 1987). In contrast the 'non-fluent', 'agrammatic', 'Broca's type'
aphasic who has difficulty assembling sentences and tends only to be able to say a
single word or short phrase, may produce 'radio'. Nevertheless, it could be that the
non-fluent aphasic was aiming at the same underlying utterance ('it's like a radio') but
due to his/her difficulty in sentence production could only produce the single word
utterance 'radio'. Thus, it would be a mistake in this case to label the single word
response an error whilst according the multi-word response a different status (such
as that of a description).
Morphologically-Related Responses: Some responses may be related to their
target by virtue of sharing the same morphological ‘stem’ e.g. fisherman -> fishing;
strawberry -> raspberry. One of the problems here is that these errors will very often
also be semantically and phonologically related to their targets (as in the examples
above). This can make establishing the source of these types of error difficult.
Phonetic Errors: Some aphasic people may produce errors where the sounds
produced are not those of the speaker’s language but may be ‘distorted’ versions of
these sounds. (In contrast to the phonological errors where the erroneous sounds
were always other sounds from the aphasic person’s language). In the case of
phonetic errors it is generally assumed that the sounds of the lexical item are
correctly retrieved but that there is difficulty in articulating the sounds (implementing
the motor programmes). This may be the result of physical problems with muscle
weakness or incoordination, as a consequence of damage to the neural pathways
controlling the muscles of face, lips, tongue, palate or larynx (a disorder known as
dysarthria). Alternatively the musculature itself may be intact but there is difficulty in
carrying out the sequences of motor commands - this is apraxia of speech.
Nickels / 11
In practice, dissociating phonetic and phonological errors is not straightforward. For
example, a speaker may have difficulty in coordinating timing of vocal cord vibration
(voicing) in speech (a phonetic problem). Thus, when trying to say the voiced
phoneme /d/ in the word dog, they may fail to initiate vocal cord vibration early
enough. This increased ‘voice onset time’ may lead the /d/ to be therefore produced
as a /t/ (the voiceless equivalent). In other words a phonetic level problem (control of
onset of vocal cord vibration) can lead to what is perceived as a phonological error
(that /d/ was substituted by /t/).
Visually-Related Responses
This refers to when a picture is named as something that looks visually similar to the
target7 e.g. pencil -> spear. These errors are generally attributed to problems with
elements of visual processing rather than a language deficit. Most aphasic people do
not have additional visual processing difficulties (Nickels & Howard, 1995b) and
hence visual errors are not common8. A preponderance of visual errors would
indicate a need for further testing of visual processing, using tests such as object
decision (deciding whether a picture is of a real object or a fake object made by
combining parts of other objects) and/or matching unusual views of objects (for a
comprehensive test battery for object recognition see Riddoch & Humphreys, 1993).
Perseverations
Errors are classified as perseverations when they are a repetition of an earlier
response. This may be a correct response or an error of some type. Some aphasic
people become more prone to errors the further they progress in the naming task. In
this case the problem may reflect activation persisting in the system longer than is
optimal (e.g. Best, Howard, Bruce et al., 1997).
No Responses
Some people with aphasia may be unable to produce a response in picture naming
(or in spontaneous speech may fail to complete an utterance). It is rare for them to
remain completely silent but they may say things like “I don’t know” or “its that thing”
or “I just can’t get it out”. These types of non-specific comments are often classified
as ‘no responses’. It is difficult to determine the cause of this failure to make an
attempt at the target. It could be because no single lexical item was sufficiently
activated. Alternatively, an erroneous lexical item may have been selected (or a
phonological error made) but the aphasic person monitored and ‘edited out’ this error
before it was produced (in the same way that we can detect and suppress errors
before we say them).
Stimulus attributes affecting word production
Every word has a number of different attributes associated with it, such as how many
syllables it has, how often it is used, how concrete or abstract an idea it represents.
Many of these attributes (often referred to as variables) have been associated with
different levels in models of language processing. In non-aphasic people these
variables often affect speed of response (how quickly a picture is named or a word is
read aloud). In people with aphasia, the same variables can affect response
accuracy. The particular pattern of variables that affect naming success can give an
indication as to what the level of deficit is for a particular aphasic9. We will briefly
Nickels / 12
mention here some of the more commonly investigated variables (for further
discussion see Nickels, 1997; Nickels and Howard, 1995b).
Word Length
This refers to the length of a word in terms of number of syllables or number of
phonemes (or in reading or writing, number of letters). If a length effect is shown,
most patients have more difficulty with longer words (although some patients
showing a REVERSE length effect have recently been reported, Best, 1995). For
example: WJ (Nickels, 1995) made 10% errors on naming pictures with 1 syllable
names but 43% errors when naming pictures with 3 syllable names. Difficulties with
longer words usually co-occur with predominantly speech sound (phonological and
phonetic) errors (Nickels, 1995). The length effects are usually attributed to deficits in
post-lexical phonological encoding procedures. In other words, problems after
retrieval of the phonological form, at the level of the phonological output buffer or
subsequent (phonological encoding) processes. The suggestion is that when a
procedure involving speech sounds and/or syllables is error prone, the more sounds
or syllables there are to process the more likely it is that an error will occur.
Frequency / Familiarity
‘Frequency’ refers to how often a word is used in speech or writing. It is measured
using objective counts of how often words occur in large corpora of spoken and/or
written language (e.g. Francis & Kucera, 1982; CELEX database, 1993). For some
aphasic people high frequency words may be better named than low frequency
words. For example, RD (Ellis, Miller and Sin, 1983) correctly named 22/25 high
frequency words (such as hospital, knife), but only 10/25 low frequency words (such
as crocodile, noose).
Frequency was once thought to be a powerful predictor of success for many
aphasics. However, it appears that many of the early studies confounded effects of
frequency with other variables with which it is correlated (such as length or age of
acquisition) leading to an overestimation of the strength of frequency effects (Nickels
and Howard, 1995b). Frequency is often taken to reflect accessibility of the lexical
form (e.g. Dell, 1986; Levelt et al., 1999; Morton, 1970). High frequency items are
considered to be easier to access from the phonological output lexicon and more
resistant to the effects of ‘noise’ or damage. Thus strong effects of frequency are
often attributed to deficits involving the phonological output lexicon or access to the
lexicon.
Rated familiarity is another variable that correlates highly with frequency of use (both
spoken and written word frequency; Brown and Watson, 1987) and may therefore
reflect subjective frequency10. Gernsbacher (1984) argues that it is 'experiential
familiarity' and not printed word frequency which is the important variable, particularly
given the unreliability of objective frequency measures for low frequency items.
Howard (1995) argues that for EE, familiarity is a better predictor of naming success
than frequency.
Age Of Acquisition:
This refers to the age at which children acquire words and can be a subjective rating
Nickels / 13
or an objective measure (the two measures are highly correlated; Morrison, Chappell
& Ellis, 1997). This measure correlates highly with frequency and word length
(children tend to acquire short, high frequency words early). However, Hirsh and Ellis
(1994) present a single case study of an aphasic, NP, whose naming was influenced
by the age of acquisition of the targets even when word frequency, imageability and
length were taken into account. Similarly, Nickels and Howard (1995b) found that
age of acquisition was a significant predictor of naming accuracy for eight of their
twenty-seven aphasics (even when frequency, word length, imageability etc. are
taken into account). For example, they would be poorer at naming words acquired
relatively late, such as ‘desk’, than words acquired relatively early, such as ‘table’.
Hirsh and Ellis favour an account which places age of acquisition effects at the level
of phonological word-form retrieval (like, for example, Gilhooly and Watson (1981)
for normal subjects). Nickels and Howard note that there are a variety factors which
may determine the age at which children acquire words (for a review, see Markman,
1989). For instance, names for parts of objects are harder to acquire than whole
discrete objects (Markman and Wachtel, 1988), and young children avoid using
names for items that are difficult to articulate (Schwartz and Leonard, 1982). Some
or all of these factors may also be important for aphasic word retrieval and Nickels
and Howard suggest that under this account it would not seem sensible to seek a
single locus for age of acquisition effects.
Imageability
This variable refers to ratings of how easy it is to conjure up an image of an item. It is
closely related to how concrete or abstract an idea is. For example: ‘fire’ is more
imageable and concrete than ‘heat’.
Differences in imageability and/or concreteness are also known to affect aphasic
performance in a number of tasks. Many aphasic patients perform worse on
comprehension tasks with abstract/low imageability words than concrete/high
imageability words (e.g. Franklin, 1989). Goodglass, Hyde & Blumstein (1969)
demonstrated that some aphasic patients show an unusual reliance on concrete
words in their spontaneous speech. In picture naming the range of
imageability/concreteness that can occur is inevitably restricted, since picturable
items are necessarily highly imageable. Nevertheless, Nickels and Howard (1995b)
found significant effects of imageability for three aphasic individuals. In every case
these individuals were better able to name pictures higher in
imageability/concreteness, such as boat, mirror and knife, than those lower in
imageability such as mat, path and king. These effects are generally attributed to
properties of the semantic system or access to that system possibly due to the
‘richer’ semantic information that is available for high imageability/concrete items
(e.g. Franklin, Howard & Patterson, 1995).
The majority of aphasics who are affected by this variable are more impaired with
low imageability/abstract items than high imageability/concrete items. However,
Breedin, Saffran and Coslett (1994) describe a man, DM, with a progressive
semantic disorder who performed more poorly with concrete words than abstract
words across a range of tasks (see also, Warrington, 1975,1981; Warrington and
Shallice, 1984). For example, when asked to define words he produced more
accurate definitions for abstract words than concrete words. Thus he defined
Nickels / 14
abstract word ‘dead’ as ‘no longer alive’ and ‘opinion’ as ‘your concept or
perspective’; but for concrete item, ‘ram’ he said ‘a little animal’ and for ‘ink’ he said
‘something that covers’. Breedin et al. suggest this reverse concreteness effect was
due to a selective difficulty with perceptual features, which are more important for the
representation of concrete concepts.
Semantic Category
Some people with aphasia have been reported who show particular difficulty in
naming (and sometimes also in comprehending) items from certain categories. For
example, Warrington & Shallice (1984) report two aphasics, JBR & SBY, who had
trouble with living things (animals, plants and foods) but not non-living things (tools
and household objects). For example, when asked to define words, JBR defined
non-living things succinctly and accurately: BRIEFCASE “small case used by
students to carry papers”; COMPASS “tools for telling direction you are going”.
These definitions are in marked contrast to those for living things: DAFFODIL “plant”;
SNAIL “an insect animal”.
While these perceived category effects can be due to confounds with other variables
(e.g. familiarity, Funnell and Sheridan, 1992; visual complexity, Stewart, Parkin and
Hunkin, 1992; word length, imageability, age of acquisition and operativity, Howard,
Best, Bruce et al., 1995), some aphasic individuals do show better naming (and
comprehension) of inanimate items than animate (e.g. Farah, McMullen and Meyer,
1991) while others show the reverse (e.g. Sacchett and Humphreys, 1992; but see
Howard et al, 1995). Some authors suggest that these category specific deficits
reflect the different emphasis on perceptual and functional information for the
representation of the concepts - perceptual attributes are held to be more important
for living things and functional for non-living things (e.g. Warrington & Shallice,
1984; Farah & McClelland, 1991). Thus, for example, a deficit affecting perceptual
features would produce more difficulty with living things. However, recent studies
question the accuracy of account (e.g. Caramazza, 1998; Lambon-Ralph, Howard,
Nightingale et al., 1998).
Converging evidence for level of breakdown
So far we have described two features of an aphasic person’s word production that
can help identify the level of breakdown in naming: the type of errors they produce
and the stimulus attributes that affect their accuracy and likelihood of error. However,
in order to confirm the precise level of difficulty, additional information is usually
required. This is found by examining performance on other tasks that also use some
of the processing components involved in word production11. If a person with aphasia
can perform a task that utilises one of these components as accurately and as fast
as a non-brain damaged person of the same age, education and culture, then it can
be assumed that that component is not the source of the difficulty in word production.
Figure 3 puts the word production components of figure 1 into the context of other
aspects of word processing such as (written and spoken) word comprehension and
writing.
Thus, for example, in order to accurately perform comprehension tasks such as
word-picture matching (where one word has to be correctly matched to one of a
Nickels / 15
choice of pictures) access to the stored representations of the meanings of both
words and pictures must be achieved. It follows then, that if the aphasic person can
do this task it can be concluded that access to conceptual representations in the
conceptual-semantic system is possible and that these representations are intact (or
at least sufficiently intact to allow discrimination between the target and the
distractors; Hence the need for closely semantically-related items as distractors).
Figure 3: A sketch of how other modalities of input and output are related to spoken
word production.
Pictures
Spoken
input
Conceptual
Semantics
Written
input
Lexical
Semantics
Phonological
Output Lexicon
Writing
Phonological
Output Buffer
Phonological
encoding
Speech
Nickels / 16
However, if the aphasic person makes errors on the task it could be for a number of
different reasons. It could be due to an impairment at the conceptual level. But
equally it could be because, for example, of a difficulty in interpreting the letters in
the word or a problem in accessing the stored form of the word in the lexicon, or a
problem interpreting the pictures, or a problem in the process of holding and
comparing the representations in order to choose the correct response. Thus, while
accurate performance on a task can confirm a hypothesis regarding level of
breakdown, error-prone performance rarely allows one to do the same.
Levels Of Breakdown In Spoken Word Production
We will now discuss the symptoms of an impairment to processing at each level of
spoken word production outlined earlier. We will identify the most likely symptoms of
each level of breakdown and give examples of aphasic people who appear to
demonstrate these impairments. However, it is important to note that the accuracy of
these summaries is dependent on the accuracy of the underlying theoretical account.
As this theoretical account evolves so will the analysis of the impairments. It is most
probable that the impairments presented here will be subdivided as the component
processes of the theoretical model are specified in further detail. Nevertheless, the
level of detail provided by this model surpasses earlier descriptive models and
discriminates impairments at a level of detail useful for planning therapy.
These summaries represent only a selection of the possible deficits that can occur.
We do not, for instance discuss deficits involving loss of stored lexical
representations (see for example, Howard, 1995) or those individuals who produce
large numbers of phonologically related words (formal paraphasias - see above) both of these are considered relatively infrequent disorders. Those that are
described are chosen to give a ‘feel’ for the nature of the deficits that can be
identified and the tools used to identify them.
In addition, brain-damage is often not as selective as these summaries (and the case
studies that appear in the literature) might imply - while people can and do have
‘pure’ deficits it is far more common for aphasic people to have combinations of
impairments (either within spoken word production or across spoken word production
and other language processes).
Conceptual-Semantic impairments
Primary error type: Semantic
Performance of other skills and tasks: Any task necessitating access to the
meaning of a word or thing will also be impaired. This will include writing, reading for
meaning (but not necessarily reading aloud), understanding speech, and nonverbal
comprehension tasks. People with this level of difficulty will be impaired not only in
linguistic tasks (such as word-picture matching, or sorting words into their
categories) but also similar tasks involving purely non-linguistic materials (e.g.
sorting pictures into categories; picture association tasks such as Pyramids and
Palm Trees, Howard & Patterson, 1992). They may nevertheless be unimpaired on
tests of nonverbal reasoning that do not demand access to stored conceptual
knowledge (e.g. Raven’s progressive matrices, Raven, Raven & Court 1998).
Imageability effects are common in comprehension and production tasks with this
level of deficit.
Nickels / 17
Nature of deficit:
This level of deficit is probably the most common reason for the production of
semantic errors by people with aphasia. People with this level of impairment will
make semantic errors in all modalities of input and output. Nickels and Howard
(1994) found an exact correspondence between those aphasic patients (of a group
of 15) who performed outside the normal range on tests of comprehension of high
imageability words (including word-picture matching, synonym judgements) and
those who produced more semantic errors than normal elderly subjects in picture
naming. Despite the strong relationship between comprehension impairment and
naming failure, Butterworth, Howard and McLoughlin (1984) found that pictures that
had elicited semantic errors in comprehension were no less likely to be named than
those that had not. Therefore, at least for their group of patients, the semantic deficit
is unlikely to be the permanent loss of particular lexical items or of the semantic
information related to these items.
Butterworth et al explain these results in terms of a variable inability to retrieve or
use a full semantic specification in both comprehension or naming. They
hypothesise that for each word, there is a semantic representation which consists of
a set of "items of information" and that the patients are sometimes forced to operate
on the basis of some of these items of information but not others12. For example,
when attempting to name the picture of a DOG, only a subset of the semantic
features of the concept may be available at any time e.g. [four legs] [pet] [has fur]
but not [barks]. Thus, at the lexical semantic level, [dog] will be activated, but [cat]
and [rabbit] will be equally active. However, [fish], [robin], [house] will not be as
highly activated. Therefore, when a word is to be produced it could be the correct
response ‘dog’ or it could be a semantic error ‘cat’ or ‘rabbit’.
A severe semantic deficit can result in more distantly related semantic errors, use of
superordinate or general terms (e.g. animal, thing) or unrelated errors as even less
semantic information would be available regarding the target. The most severe
semantic deficits are often observed in people with progressive semantic
deterioration - Semantic dementia (e.g. Hodges, Graham and Patterson, 1995). In
these people the degree of semantic impairment becomes increasingly severe and
the errors show a progression from semantically related responses to unrelated
responses or no responses, as semantic information is lost (or access to that
information becomes impossible).
Example of a case with a conceptual-semantic deficit.
AER (Nickels, 1992b, 1995; Nickels & Howard, 1994, 1995ab) had just retired from
his job as a building surveyor at the age of 65 when he had a left CVA. He had a
dense right-sided hemiplegia and severe aphasia. His spontaneous speech was
extremely limited, mostly comprising single words and stereotyped utterances. His
comprehension seemed adequate for understanding simple conversation in context.
He was completely unable to write or spell aloud.
When asked to name pictures, he performed better than would have been predicted
from his spontaneous speech, correctly naming 54% of a set of pictures whose
names varied in word frequency and length. However, neither of these variables
Nickels / 18
affected his accuracy. When unable to name a picture, AER almost exclusively
produced semantic errors (over 30% of responses). For example, for a picture of a
SPIDER he said “ant”, for a picture of a POCKET he said “sleeve”, and for
TWEEZERS he said “pinchers no nails …. scissors no pliers no”.
When assessed on tests of semantic processing he made errors whatever the task
and whatever modality. He was given the Pyramids and Palm Trees test (Howard &
Patterson, 1992). This task involves matching a stimulus picture (e.g. pyramid) to the
one of two co-ordinates that is associatively related to it (e.g. palm tree or fir tree).
AER scored 87% correct (non-aphasic elderly controls all score 94% correct or
more). For example, he matched a picture of a bat to a picture of a woodpecker
rather than a picture of an owl. He obtained the same score when a spoken word
was the stimulus, and when a written word was used he scored 85% correct.
Similarly, when asked to match written or spoken words to a choice of pictures, he
made errors, selecting a semantically related item instead of the target. For example,
asked to point to a paintbrush, he selected a picture of a painter’s palette rather than
that of the paintbrush. Likewise, he pointed to a cigar rather than a pipe. He also
made more errors than non-aphasic controls on a synonym judgements task, where
pairs of spoken or written words have to be judged for similarity of meaning13. For
example, he judged ‘harvest’ and ‘ ‘smile’ as having similar meanings, and ‘throng’
and ‘crowd’ as having dissimilar meanings. When the pairs of words were abstract
he often had more difficulty than when they were concrete: with spoken pairs of
words he correctly judged 87% of high imageability/concrete words, but only 63% of
low imageability/abstract words.
Thus, AER makes semantic errors in every modality, speech production,
understanding written and spoken words and understanding non-verbal material
(pictures). This allows us to localise the level of his breakdown as within the
conceptual semantic system.
Lexical-Semantic impairments
Primary error type(s): Semantic
Performance of other skills and tasks: Aphasic people with this level of deficit will
be unimpaired on non-verbal tests of semantics (e.g. picture categorisation;
Pyramids and Palm trees test with 3 pictures, Howard and Patterson, 1992) as these
tasks can be performed purely by recourse to non-verbal, conceptual semantics.
Gesture and spontaneous drawing may also be unimpaired. However, the
impairment to the lexical-semantic level will affect performance on any linguistic task
requiring access to meaning. Thus, there will be semantic errors in written and
spoken naming, and semantic errors in comprehension tasks involving words
(including spoken and written versions of the tasks that could be performed
accurately with pictures).
Nature of the deficit: With impairment at the lexical-semantic level, the lexicalsemantic node for a particular lexical item will not be successfully retrieved (despite
the conceptual semantic representation being intact and able to address the lexicalsemantic system). Thus, despite all of the semantic features of the target ‘dog’ being
active at the conceptual-semantic level, the lexical-semantic node for ‘dog’ is no
Nickels / 19
more active than that of its semantic neighbour ‘cat’. This could be thought of as
being due to a problem with the strength of connections between the conceptualsemantic and lexical-semantic levels. Weakened connections affect the transmission
of activation from one level to another. This means that the target (dog) may not be
activated as highly as would be the case in the non-impaired system. This reduced
activation is combined with the normal levels of randomly fluctuating activation
(noise) within the lexical-semantic level (which results in random addition of
activation to different nodes). This could therefore result in a non-target being more
active than the target lexical-semantic node. This is most likely to be a semantically
related item as they will also be receiving activation from the conceptual-semantic
level (and unlikely to be an unrelated item as they will have no other source of
activation). The combination of partial activation from the semantic system and
random activation fluctuations (noise) may therefore cause the semantically-related
node (cat) to become more active than the target (dog), resulting in a semantic error
(cat being produced in response to the picture of a dog).
Example of a case with lexical-semantic deficit:
TRC (Nickels, 1992a b, 1995; Nickels & Howard, 1994, 1995ab) was 43 when he
had the sudden onset of aphasia and right sided weakness. A CT scan confirmed a
stroke noting “a very substantial left middle cerebral artery infarct”. Although initially
severely impaired with almost no speech output his aphasia quickly improved. Five
months after his CVA, a clinical psychologist’s assessment was that his perceptual,
visuo-spatial and nonverbal reasoning skills were intact, as was memory when
assessed non-verbally. He returned to managing his wholesale and retail
fishmongers 3 months after his CVA. Approximately a year later, he used a creative
combination of speech, drawing, writing and gesture to communicate. His speech
output while fluent at times was frequently interrupted by difficulties in word finding
(see table 3).
Table 3: Examples of TRC’s connected speech and errors in written and spoken
naming.
A. Sample of conversation – what he did at the weekend.
TRC: Um .. right… it was .. I was.. I was in /l/ /l/ London on.. on Friday.. I went
with Rose, not Rose, Jo. It was .. um … um …um …marriage. It wasn’t.. it was a
marriage but I didn’t go to church, I comed later.
LN: So what bit did you go to then?
TRC: I came for the champagne
LN: Seems like a good time to arrive! Whose wedding was it ?
TRC: Some one called XXXX YYYY. And there was a lot of people, and the thing is,
I was going to drive so not too much champagne. So I had one two champagne and
that was enough.
B. Examples of picture naming responses
Target
Spoken naming response
picture
name
Magazine papers, its not called papers
Written naming
response
PAPER
Nickels / 20
Bottle
Kennel
King
devil
desk
Pocket
Grave
whisky, wine, bottle
It’s the dogs…
The queens.. the queens? queen
mother’s.
I know exactly what it is um… things like
god and all the rest of it but its not called
god, its downstairs, so but I k.. I don’t I
s’pose it would be..I s’pose.
s…school, no it’s not a schoo, well it is a
school…
Coat but its not called coat
They’re dead, dead
WINE
KENNEL (correct)
CROWN
DEVIL (correct)
No response
No response
CEMETRY
TRC’s spoken picture naming was extremely poor. On a set of 140 pictures of
common objects he scored only 8% correct, 37% of his responses were semantically
related to the targets (see table 3 for examples). These were both multi-word
circumlocutions (26% of total responses) and single word semantic errors (11% of
total responses). A further 44% of spoken naming attempts were “no responses” or
non-specific comments such as “I know him, he’s called…” (to a picture of a
monkey), these responses were often accompanied by (accurate) gestures of the
use of objects and he would attempt to write the name of the picture. Although
written picture naming was far superior to spoken naming (49% correct) he still
produced many semantic errors (11% of responses) and no responses (15%), but
also made spelling errors and incomplete attempts at the target (17%).
On comprehension testing he scored within normal limits when given the three
pictures version of the Pyramids and Palm Trees task (98% correct), but was
impaired compared to non-aphasic controls on both the versions where the stimulus
picture is replaced with a word (1 spoken word-2 pictures, 75%; 1 written word-2
pictures, 90%). Thus, for example, when given a picture of a tent and asked to select
which of a fire or a radiator ‘went with it’ he correctly selected the fire. However,
when given the word ‘TENT’ rather than the picture, he incorrectly matched it with
the radiator. He also made errors on other comprehension tasks using words
including synonym judgements.
TRC also made semantic errors when reading aloud (which was performed using a
semantic route, due to the unavailability of a sublexical route, as indicated by is
inability to read aloud nonwords). In contrast his repetition of words was good (90%
correct), due to the availability of phonological information via sublexical routes (he
could repeat some nonwords).
Thus, TRC shows the pattern of semantic errors in all modalities of input and output
that involve linguistic material. This is clearly indicative of a problem with conceptual
or lexical semantics. However, as he performs accurately on any test that is purely
non-linguistic, we can rule out a problem at the conceptual -semantic level and
localise his deficit as one at the level of lexical-semantics. TRC also shows his nonlinguistic skills in his ability to use gesture and drawing even when unable to find the
name of an item he wants to say. Furthermore, he copes extremely well with the
cognitive demands of running a business and organising a busy social life.
Nickels / 21
Further cases in the literature include JCU (Howard and Orchard Lisle, 1984) and KE
(Hillis and Caramazza, 1995; Hillis, Rapp, Romani and Caramazza, 1990).
Impairments of lexical access - 'OUTPUT' SEMANTIC ERRORS
Primary error type(s): semantic.
Performance of other skills and tasks: The primary feature of this level of
breakdown is that semantic errors occur only in spoken naming - no semantic errors
will be found in written picture naming (which will be accurate unless there is a
separate, co-occurring deficit). In addition, there will be no impairment on
comprehension tasks or tasks involving semantic access for either pictures or words.
Nature of the deficit:
Caramazza and Hillis (1990) describe two people with aphasia who produced
semantic errors only in spoken naming. They proposed that the locus of the deficit
for these individuals was at the level of activating the representation of the target
word in the phonological output lexicon. They suggest that semantic errors occur
when the target phonological representation is inaccessible and the most highly
activated semantically related item will be output instead14. In other words, the target
is the most active at the lexical-semantic level. However, due to weakened strength
of connections between the lexical-semantic and the phonological levels, the node
for the target at the level of the phonological output lexicon may not be activated (or
not activated to the usual level). In this situation, the random fluctuations in activation
that occur may lead another node to be the most active and be selected - a
semantically-related neighbour of the target15.
Some theoretical models would suggest that the chance of a semantic error
occurring should be related to the frequency of the target (e.g. logogen model,
Morton, 1970). High frequency targets would have higher resting levels of activation
and therefore be more likely to be more highly activated than semantically related
competitors. In contrast, in a ‘noisy’ system, low frequency items would be more
vulnerable to error if they have semantically related competitors which are high
frequency. The higher resting level of activation makes it more likely that the overall
activation level of the semantic competitor will exceed that of the target and it will be
selected However, this prediction depends crucially on the locus of frequency within
a model and more general processing assumptions (e.g. the nature of the spread of
activation).
Example of a case with lexical access deficit:
RGB (Caramazza and Hillis, 1990), a retired personnel manager, was 58 when he
suffered a stroke. CT scan showed a large left hemisphere fronto-parietal infarct (the
result of a left middle cerebral artery occlusion). When studied 4 years later he had a
dense hemiplegia and fluent, grammatical speech. He made frequent semantic
errors and circumlocutions in spontaneous speech. When he was asked to name a
set of 144 pictures, he correctly named 68%. Almost all of his errors were
semantically-related words or descriptions. For example, for a picture of celery he
said ‘lettuce’, for a picture of a banana ‘you peel it’. In contrast when asked to write
the names of the same pictures, he made no semantic errors. Although there were
Nickels / 22
many spelling errors (e.g. ‘celery’ written as ‘celey’, banana as ‘bana’), it was clear
the target word had been accessed.
RGB’s comprehension was assessed using a word-picture verification task: he was
given an object name accompanied by a picture. This picture could be either that
corresponding to the word, or a picture of a semantically related object or of an
unrelated object. RGB was required to say whether the word and the picture
matched or not. For example, given the word “tiger” he might either see a picture of a
tiger, or of a lion, or of a house. RGB made no errors on this task, with either written
or spoken words. The fact that he can do this task faultlessly therefore suggests that
he is able to access the concepts associated with these items and that they are
intact. In other words, he does not have an impairment at either conceptual-semantic
or lexical-semantic levels. This is confirmed by the fact that he does not make
semantic errors in written naming.
It follows then that RGB’s deficit in spoken naming is localised after the level of
lexical-semantics. RGB could repeat both words and non-words accurately, which
suggests that there is no problem with phonological encoding and articulatory
processes. This is confirmed by the fact that the naming errors RGB produced are
semantic and not phonological. Thus, we can localise his impairment at the level of
the phonological output lexicon or access to this lexicon.
Post-lexical access impairments - deficits of the phonological output buffer
and beyond
Primary error type: Phonologically related responses; severe impairments may
result in unrelated nonwords (neologisms)
Performance of other skills and tasks: Equivalent error types should be produced
in all modalities of spoken word production (naming, repetition and reading aloud)
and with all types of stimuli (e.g. words, nonwords, function words). However, the
relative level of accuracy may vary depending on both task (due to the effects of cooccurring deficits on performance) and stimulus type.
The aphasic person with this level of breakdown in naming should be able to perform
accurately on comprehension tasks (unless they have an additional impairment), and
written naming should be unimpaired. Furthermore, so called ‘silent phonology’ tasks
should be completed accurately. These are tasks which require judgements
regarding the sounds of a word without producing the words themselves. For
example, in homophone judgements, two written words are presented together (e.g.
BARE BEAR; BORE BEAR) and the subject is required to say whether they are
pronounced the same WITHOUT saying them aloud. If these tasks can be
completed accurately it indicates that they can access the phonological form of the
word even if they are unable to say the words aloud accurately. In other words, the
problem in word production is subsequent to the level of the phonological output
lexicon. (As discussed above, if the task cannot be performed accurately,
conclusions are limited as there may be many different reasons for failure).
Nature of the deficit: After the phonological form of the word is retrieved, there are
several further processes that are needed before it can be produced. If any of these
processes become error-prone then phonological errors will result. For example, if
the phonological output buffer is damaged the memory trace of the phonemes of a
word held in this buffer may fade or ‘decay’ more quickly than usual. Thus, when the
Nickels / 23
phonemes are to be retrieved some of them may no longer be available. This would
lead to ‘gaps’ in the phonological form of the word which may either be realised as
phoneme omissions or maybe filled by other phonemes chosen at random. For
example, when trying to name a picture of a ‘carpet’, the phonemes /k a: p  t/ may
all be retrieved from the lexicon and held in the buffer until they are required for
phonological encoding. However, while they are being held, they rapidly decay so
that when they are retrieved from the buffer the /t/ is no longer available. The word
may therefore be produced as /ka:p/ or a phoneme selected to fill the final position
resulting in for example /kapn/ or /kapk/. It is often suggested that these kinds of
impairments will be more severe when the buffer is required to maintain more
information - in other words, errors will be more likely to occur with longer words.
Deficits of phonological encoding procedures will also result in phonological errors.
For instance, if there is an error when phonemes are associated with their position in
the syllable, then a phoneme may appear in the wrong syllable position. For
example, when saying the word ‘carpet’, if the /p/ is incorrectly associated with the
first position of the first syllable rather than of the second then ‘parpet’ or ‘parket’
may be said. Once again, if these processes are error prone then the more
phonemes that there are to process, the more errors will result. Another possible
source of error is when the wrong motor plan is retrieved for a syllable. For example,
the syllable /ka:/ has been assembled however an error in the retrieval process
results in retrieval of the motor plan for /pa:/. Thus, once again ‘parpet’ may be
produced. As before, the longer the word the more likely it is an error will result (this
time with length in syllables rather than phonemes being the important variable).
Problems in executing the retrieved motor plans may result in errors that are less
fluently articulated and may equate with apraxia of speech. Detailed discussion of
apraxia of speech is beyond the scope of this paper (see Nickels, 1997, for further
discussion).
It is clear there are many deficits subsequent to the level of retrieval of the
phonological form, however, the characteristics of these deficits are all very similar
(phonological errors and word length effects) and at present are difficult to dissociate
from each other.
Example of a case with a phonological encoding deficit:
CI (Nickels, 1992b, 1995; Nickels & Howard, 1994, 1995ab) had a left temporoparietal infarct when she was in her late thirties. She had fluent, well articulated
speech with no signs of slurring or groping for sounds as might be characteristic of a
dysarthria or dyspraxia. However, she made large numbers of phonological errors, of
which she was very conscious. Table 4 gives an example of her speech when
describing a picture.
On picture naming she made no more semantic errors than control subjects, but
20% of responses were phonologically related errors. There was a marked effect of
word length on her accuracy: she correctly named 86% of one syllable words, but
only 23% of three syllable words. She usually made several attempts at a response
and for one and two syllable words this often resulted in successful production of the
target. However, for three syllable words she was rarely able to correct her attempts
(see table 4 for examples).
Nickels / 24
On tests of comprehension she scored within the range of non-aphasic controls even
with abstract words. For example, she made no errors on a synonym judgements
task, where she was required to judge whether pairs of (spoken or written) abstract
words were similar in meaning (e.g. impotence-weakness, impotence-truth). This
suggests that she has no semantic impairment (as does the fact that she makes no
semantic errors in picture naming).
Repetition and reading of the names of the pictures produced very similar levels of
accuracy to naming for one syllable words (88% for repetition, 94% for reading).
However, for three syllable words while reading was roughly equivalent to naming
(30% correct),repetition was substantially better (57%). Nevertheless the quality of
her errors were similar across all three modalities (see table 4). Reading and
repetition of nonwords were both poor, but once again she produced similar error
types as for real words and was strongly affected by the length of the word.
Table 4: Examples of CI’s connected speech and word production errors.
A. Description of a picture of a supermarket scene.
Target words are in square brackets.
It’s the /su:ppa:rt/ /su:ppa:rt/ /su:pba:gt/ [supermarket] and there’s a lot of
food there. There’s the veg, this one’s on the side. There’s some fruit and there’s lots
of .. er.. jars and things. And a /le/ - an old lady was putting into a trolley. And at the
erm .. the /tr/ .. cash /redstr/ [register] bit on /s/ the counter they, the mother
-woman - was with her children, together, but they were paying the man. The man
looks Greek to me! He’s got /mstr/ /mstr/ [moustache] … he’s got hair on
his face! And um then there’s eggs are on there and packets. And the little boy’s
goin’a pay money and the little girl was reading some cards by her mother.
B. Word production errors
‘.’ represents a separation between adjacent syllables – a syllabified response.
All responses are transcribed using a broad phonological transcription.
Target
submarine
Picture naming
su:pbnn sbbri:
sb.mri:n
hospital
elephant

efl .. efltn
lfnnt lfnnt
lftn lfnt elfn
eflnt
prmnt no pr..
pr.. prmnt prmnt
Pyramid
Reading aloud
sbrli:n strelen
s’mrli:n sbrmi:n
sbri:n
hsptl t .. 

Repetition
sbmn ….
prmdd prmdd
prmdd
prmmm
prmmn pr..
prmmem
hs …. 
elfmnt .. el … 
Nickels / 25
prmmtd
The fact that similar rates of phonological errors are produced across a number of
tasks and stimuli suggests a common locus to the deficit causing these errors.
It seems unlikely that she has an impairment to the stored phonological forms as i)
she is more accurate with words than nonwords; ii) she often manages to self-correct
her errors; and iii) she is unimpaired on ‘silent phonology tasks’ - tasks that require
judgements of phonology from the written word without overt pronunciation. In other
words, we can localise her impairment at the level of the phonological output buffer
or subsequent phonological encoding processes.
Summary
The production of spoken words is complex and we are only beginning to be able to
specify some of the processes involved. However, we can see that by applying even
an emergent cognitive model of word production to aphasic naming disorders,
differential diagnosis of level of breakdown can be made. The advantage of this
differential diagnosis is that having pinpointed the deficit in naming (and the cooccurring deficits in other areas) treatment may then be targeted more accurately
and the effects of treatment monitored with precision (for further discussion see for
example, Berndt & Mitchum ,1995; Nickels, 2000; Nickels & Best, 1996).
Furthermore, it provides a principled and clear structure to convey to the aphasic
individual and their carer(s) the nature of their impairment in word production.
Nickels / 26
FOOTNOTES
1 We deliberately use a model which is not that advocated by any one author in
particular but rather takes the commonalities from as many as possible. Clearly there
will be those individuals who disagree with some of the assumptions made here.
However, for clinical diagnosis a model that is broadly based fares well. The most
complete model of spoken language processing is that of Levelt et al (1999).
Unfortunately this model has yet to be applied to aphasic data (or even to ‘normal’
speech errors) in any systematic way and it is therefore difficult to predict precisely
how it will fare under conditions of damage. Dell et al (1997), use an interactive
activation approach, and simulate the naming performance of a number of aphasic
people. However, it is doubtful that this model can account for the full range of
aphasic naming (see for example, Rapp and Goldrick, 2000).
2
This is simply one way of representing concepts - I am not suggesting that this is
the definitive account. How semantics is represented is a hotly debated issue and for
the model described here nothing rests on the nature of the conceptual
representation; it is a descriptive device to enable the reader to gain a ‘feel’ for the
nature of processing in a model of this sort. This is a decompositional account of
conceptual representation, see Levelt et al (1999) for a nondecompositional account,
and (for example) Fodor (1998), Roelofs (1997) and Jakendoff (1992) for arguments
regarding the nature of conceptual representation.
3
This level corresponds to the lemma as described in Levelt (1989) - this is referred
to as the ‘old lemma’ by Levelt et al. (1999); The ‘new lemma’ (Levelt et al 1999)
represents only part of the information contained in the ‘old lemma’ (the syntactic
part). Levelt et al. make clear that the old lemma is a combination of lexical semantic
information (at the level of lexical concepts) and syntactic information (at the lemma
level) in the more recent formulation of their theory (Levelt et al., 1999).
4
Here we have illustrated the phonological representation as a single node. Some
models (e.g. Dell et al, 1997) use the pattern of activation across a set of phonemes
instead.
5
Dell et al. (1997) argue that it is not necessary to advocate different levels of
breakdown within a model of word production in order to account for the different
patterns of errors found in aphasic naming. Instead they suggest that within their
interactive activation model, varying global parameters such as decay rate and
connection strength can reproduce the variability found. However, data from Rapp
and Goldrick (2000) suggest this is unlikely to be the case and local lesioning is
required (and a model with less feedback).
6
Throughout this paper I shall refer to errors in terms of the nature of their
relationship to their targets, rather than use some of the alternative labels that have
been used elsewhere in error classifications (e.g. Goodglass and Kaplan, 1972).
This is the most transparent system of labelling and hence is least likely to lead to
confusion (see, for example, the discussion later regarding the term ‘neologism’ ).
Nickels / 27
7
These are not the same as visual errors in reading where visual more precisely
means ORTHOGRAPHIC - ie the target and response have letters in common.
8
At least no more common than when non-brain damaged people of the same age,
education and culture are asked to name the same pictures. For example, in the
Boston Naming Test (Kaplan, Goodglass & Weintraub, 1983) there is a picture of a
pretzel. These are unfamiliar to many British and Australian people and hence both
aphasics and non-aphasics will tend to instead name it as something visually similar
e.g. rope, or snake.
9
This is certainly true of strictly serial models of language processing where
processing at one level is completed before processing begins at a subsequent level.
However, in interactive models and those where there is cascading of information
throughout the language system the situation becomes more complex. It is possible
that for these models a variable might have its effects throughout the system (for a
debate regarding this see Nickels, 1995; Harley, 1995).
10
It is important to note, however, that the nature of 'familiarity' differs between
studies. For example, Funnell and Sheridan (1992) used the ratings of Snodgrass
and Vanderwart (1980) who define familiarity as "the degree to which you come into
contact with or think about the concept". They, therefore, require subjects to rate
pictures on the basis of "how usual or unusual the object is in your realm of
experience". In contrast, Brown and Watson (1987) use the ratings from Gilhooly and
Logie (1980) which are rated on the basis of how often a subject believes s/he sees,
hears or uses that word.
11 An example of a reasonably comprehensive set of tasks that can be used for this
purpose can be found in ‘PALPA’ (Kay, Lesser & Coltheart, 1992).
12 This semantic representation does not have to be in terms of a hierarchical set of
features.
13In this task, two (written or spoken) words are presented which may be similar in
meaning or different, the task is to judge whether they are related in meaning or not.
Half the stimuli are of high imageability, half of low imageability (e.g. ship-boat; shipflower; idea-notion; idea-security). The version used with AER was an early version
by Coltheart (unpublished) of the test incorporated into PALPA (Kay, Lesser and
Coltheart, 1992).
14This argument is similar to the 'response blocking' explanation for semantic errors
suggested by Morton and Patterson (1980). In their account, the output logogens for
certain items have outputs which are blocked, or at least have greatly raised
thresholds (although which items are blocked may fluctuate). The correct and full
semantic code is sent from the cognitive system to the output logogen system. The
appropriate output not being forthcoming, the logogen nearest to threshold activation
is selected which would be semantically (but not phonologically) related to the target.
Morton and Patterson also put forward another possible explanation, where the
correct code is produced by the semantic system but is distorted in transmission to
the output logogen system. This account would be difficult in practice to discriminate
from response blocking.
Nickels / 28
15
The type of error that will occur critically depends on the processing assumptions
of the theoretical model. The account described requires a model where every item
that is active at the lexical-semantic level also activates its corresponding node at the
level of the phonological output lexicon. If, alternatively, only the most highly
activated node at the lexical-semantic level activates its corresponding phonological
node, then there are two possibilities.
a) If the model is ‘strictly feed forward’ with no interaction from later levels to earlier
levels, then in order to get an overt response some ‘strategic’ factors must come
into play. In other words when no one phonological node is sufficiently active to be
produced, then the next most activated node at the lexical semantic level may be
“allowed” to activate its phonological form. If this is successful then a semantic
error will result.
b) If the model incorporates limited feedback from the levels subsequent to the
phonological output lexicon (phoneme levels) then phonologically related words
will become activated. Hence a phonological error may result.
The nature of the flow of activation in models of spoken word production is a matter
of great debate, see for example Dell et al., 1997; Levelt et al., 1999, Rapp and
Goldrick, 2000.
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