INVESTIGATING THE CWH: AN N400 MICROSUMMARY
Ellen Lau
LING646
May 23, 2005
I. Introduction
The N400 effect was introduced to the language literature with a within-sentence
implausibility paradigm (Kutas & Hillyard, 1980), and so we often tend to think of the
effect as one which indexes ease of semantic integration, at some level. A provocative
alternative hypothesis recently argued by our colleague, Carol Whitney, is that the N400
effect indexes simple lexical activation—in other words, “it’s just priming” and doesn’t
relate to any higher level integration processes. She suggests that it only looks like it
does because, in many of the paradigms used, words within the sentence associatively
prime the lexical item involved in plausible and implausible continuations differentially.
This hypothesis is supported by studies which have shown the same level of N400
amplitude reduction for sentences incorporating strong semantic category relationships (A
carrot is a vegetable) as false sentences incorporating the same type of relationships at
the lexical level (A carrot is not a vegetable) (Fischler, Bloom, Childers, Roucos, & Perry,
1983). Similarly suggestive are recent studies which fail to show an N400 effect for
sentences with semantically compatible arguments but whose argument structures are
completely implausible (e.g. van Herten, Kolk, & Chwilla, 2005; Kim & Osterhout,
2005; Hoeks, Stowe, & Doedens, 2004).
In this paper, I investigate some selected work from the massive N400 literature which
can bear on Whitney’s hypothesis (henceforth, CWH). For this review, I chose to focus
on findings regarding the N400 effect to priming at the lexical vs the sentence-level;
however, another important area of work that speaks to this topic is that which has shown
N400 effects modulated by changes at the discourse level, which presumably will be
somewhat harder to capture with a lexically-based story (e.g. van Berkum, Hagoort, &
Brown, 1999; Nieuwland & van Berkum, submitted; Hagoort, Hald, Bastiaansen, &
Petersson, 2004).
II. Automatic vs Conscious/Strategic Priming
Because CWH reduces sentence-level N400 effects to lexical-level ones, it makes sense
to take a look at the N400 work on lexical priming. Within the lexical priming literature
itself, an influential distinction is made which is somewhat analogous to CWH’s
integrative-vs-associative one: this is the distinction between ‘automatic’ and ‘strategic’
priming effects. By ‘automatic’, researchers are usually referring to the spreading
activation theory of priming (Collins & Loftus, 1975). On this theory, lexical
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representations are imagined as nodes in a network, and lexical activations that arise in
response to prime perception spread through the “nearby” nodes of the network, such that
full activation of “primed” nodes is faster subsequently. ‘Strategic’ in general covers all
processes which require some extra step besides simply prime perception for priming to
occur. For example, one strategic process thought to be responsible for semantic priming
in lexical decision tasks is semantic matching (e.g. Neely & Keefe, 1989; De Groot,
1985). Here, in order to make a rapid lexical decision, the target is quickly compared to
the prime, and if they share some amount of features, an affirmative decision can be
made. Another example is expectancy priming (e.g. Becker, 1980; De Groot, 1984), in
which a prediction (or predictions) are made for the target based on the prime, and the
lexical decision is speeded to the extent that the target matches the predictions.
Automatic priming has traditionally been thought to be fast and ‘early’, and strategic
priming to be slower or ‘late’, for obvious reasons—strategic priming by definition
requires more processing steps.
Several manipulations have been used in the behavioral literature to try to dissociate
automatic from strategic effects. Word naming tasks are thought to be less sensitive to
strategic effects than lexical decision tasks (Balota & Lorch, 1986; de Groot, 1984;
Seidenberg, M., Waters, G. S., Sanders, M. & Langer, P.,1984). Using a double lexical
decision task rather than a non-response prime followed by a lexical decision target has
also been argued to reduce strategic effects (McNamara & Altarriba, 1988). However,
the main technique used within the lexical decision paradigm is masked priming. In
masked priming, a briefly presented prime is immediately followed by a mask (e.g.
#####), and then the target. Using this technique, researchers can present primes without
conscious perception by participants (Marcel, 1983; Cheesman & Merikle, 1984; Forster
& Davis, 1984).
It’s not completely clear how the CWH aligns with the automatic-strategic distinction
with respect to N400 effects—namely, I’m not sure whether CWH would lump processes
like expectancy priming together with sentence-level integrative processes, or whether
these would still be considered within basic lexical priming processes. However, I think
that CWH would at the least expect the automatic, spreading activation priming to be
underlying some of the N400 effect. What is interesting is that, in the early literature on
N400 priming, the results appeared to be exactly the opposite—it looked like the N400
was only sensitive to strategic effects, while not showing any sensitivity to automatic,
spreading activation priming.
Once the N400 was shown to be sensitive to lexical priming effects (McCarthy & Nobre,
1993), the question naturally arose whether the N400 amplitude was reflecting strategic
processes, automatic processes, or both. Early work relevant to this question showed that
the N400 effect was larger when participants were encouraged to use strategic processing
within a lexical decision priming experiment (Holcomb 1988), a finding that suggested
that the N400 did reflect strategic processing, but that didn’t speak directly to the
relevance of the automatic priming component. This question was addressed by a very
influential paper by Brown and Hagoort (1993), who tested masked priming effects in
ERP. Brown and Hagoort reported no N400 effect when primes were masked below the
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level of conscious perception, in contrast to the conditions in which primes were
unmasked. This seemed to settle the issue; the N400 reflects ‘strategic’, ‘late’ effects
only, not early ‘automatic’ effects. Furthermore, this was actually a promising finding
for the lexical priming field, as until this point there were no paradigms that were able to
isolate the ‘strategic’ stage, in contrast with the ‘automatic’ stage. Indeed, Brown and
Hagoort followed these results with several studies that took the ‘strategic’ nature of the
N400 effect as a given and used the presence vs absence of the N400 effect as a
dependent measure to determine what kinds of processes were underlying various types
of priming manipulations (e.g. mediated priming, Chwilla, Kolk, & Mulder, 2000;
backwards priming, Chwilla, Hagoort, & Brown, 1998). As I suggested above, these
results would seem to be at odds with at least the spirit of the CWH, which claims that all
N400 effects, even sentence-level, arise from manipulations of relative lexical activation;
although perhaps we could argue that some ‘strategic’ processes like expectancy priming
or semantic matching could directly lead to differences in relative ease of activation.
However, in recent years, more and more studies have begun to cast doubt on Brown and
Hagoort’s (1993) seminal finding. Firstly, Rolke, Heil, Streb, and Henninghausen (2001)
showed that N400 effects showed up for prime words presented during the attentional
blink, suggesting that conscious perception of the prime was not a necessary component.
More directly contradictory, Kiefer (2002) showed N400 modulation by masked primes
presented with a very short SOA, and showed with various behavioral measures that
participants seemed to show no conscious perception of the prime. Interestingly,
however, the latency of the N400 difference was delayed by about 80 ms in the masked
prime case relative to the unmasked case. Kiefer argued that Brown and Hagoort failed
to find the effect because they used too long of an SOA (500 ms) to capture the shortlived masked priming effect. If this account is correct, then Kiefer’s finding may not
make much different for CWH regarding sentence-level effects, because crucially, if
lexical priming is causing the effects at the sentence-level, it is really going to need to be
long-lasting enough to exert influences over at least several words.
Most recently however, Misra and colleagues using masked primes have showed
repetition priming effects on the N400 which span over as many as four trials, casting
doubt on this hypothesis (Misra & Holcomb, 2003; Holcomb, Reder, Misra, & Grainger,
2005). They suggest that the inconsistency with Brown and Hagoort (1993) because their
task—semantic monitoring—may have made processing deep enough that the effect was
measurable, in contrast to a task like lexical decision. The results suggest that it is at
least plausible that the N400 effect seen in sentence-level paradigms is driven in part by
spreading activation/associative priming. However, Misra and colleagues do find that the
N400 effect is larger in the unmasked cases, which is also consistent with the earlier work
by Holcomb (1988) showing a larger effect with more strategic processing. Therefore, I
think that the CWH is going to have to admit that at least some other mechanism besides
simple associative priming feeds into the N400. At the same time, however, it is still
quite possible that this other mechanism is much more basic than integrating the
incoming word with the entire semantic structure of the preceding sentence fragment.
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III. Associatively based priming vs other relationships
If, as CWH postulates, the N400 effect is fundamentally a lexical priming effect, we
would like to get a better idea of exactly what mechanisms are underlying this priming
effect. Therefore, is there any work which could speak to what ‘strategic’ mechanisms
are behind the increased size of the effect in the unmasked case? In the priming literature,
some work has tried filtering out associative priming effects by using word pairs which
are not semantically related, but are not strictly speaking “associates” of each other, in the
sense that they are not elicited by participants in free-association norming tasks. Priming
effects between such pairs is thought to be more likely to be caused by a strategy like
semantic matching than either expectancy-based or spreading activation, both of which
would presumably be correlated with free-association responses. In principle, then, we
could look at parallel N400 studies in which the N400 effect was compared for
associative priming and purely semantically-related priming.
An early paper by Hagoort, Brown, and Swaab (1996) compared N400 effects to
associatively and semantically related prime-target pairs and purely semantically related
pairs in aphasics and elderly controls. In the controls, they showed equal-sized N400
priming effects for the two kinds of pairs. More recently, Kovisto and Revonsuo (2001)
made a more direct comparison, in which they compared the N400 response to purely
semantically related primes (sofa-bed) and purely ‘associative’ primes, for which they
used opaque compound pairs (wind-mill). They also showed a similarly sized decrease in
N400 amplitude for both pairs. Interestingly, however, the purely associative pairs
showed an effect of a longer duration than the semantically related pairs, and this effect
in the later part of the N400 window seemed to have a different topography than earlier.
Therefore, Kovisto and Revonsuo hypothesize that the N400 in the associative case is a
combined effect of several different processes. This is an intriguing idea, and, were it to
hold up, it could provide an interesting diagnostic of the mechanisms underlying the
sentence-level effects that CWH is questioning—however, this is the only work I could
find making such a comparison so far, so it is quite preliminary.
Several other papers which speak to the mechanism underlying the N400 lexical priming
effect are a pair of papers by Chwilla and colleagues (Chwilla, Hagoort, & Brown, 1998;
Chwilla, Kolk & Mulder; 2000). As mentioned above, these papers assumed based on
Hagoort and Brown (1993) that the N400 did not reflect automatic spreading activation
priming, and so they used it as a dependent measure to test the contribution of automatic
priming to specific types of priming relationships. Although the base assumption turned
out to be wrong, the effects that they find can still tell us something about the
mechanisms that the N400 reflects.
Chwilla, Kolk, and Mulder (2000) investigate the N400 in conjunction with two-step or
mediated priming. Mediated prime pairs are not associated with each other, but rather are
linked by a shared associate (e.g. lion-stripes, where tiger is the mediator). Mediated
priming can be explained either by spreading activation or by certain versions of
semantic matching (Gillund & Shiffrin, 1984). The authors show N400 effects for both
regular (one-step) and mediated (two-step) priming, but the effect for the mediated
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primes is much smaller. Given that we know from later work that the N400 reflects both
automatic and strategic effects, this difference between one- and two-step priming could
mean that the N400 varies parametrically with semantic-network distance (which is
assumed to be greater in the two-step case) or that the N400 is a summed contribution of
the automatic and strategic effects, only one of which (automatic) is present in the
mediated case.
The other paper in the pair lends more support to the second idea. In Chwilla, Hagoort,
and Brown (1998), the authors examine a priming effect known as backwards priming
between word pairs that are associated in one direction but not the other (stork-baby vs
baby-stork) (Koriat, 1981). Priming effects on RTs have been found in both directions,
although sometimes they are found to be larger in the forwards cases (Thompson-Schill,
Kurtz, & Gabrieli, 1998; Peterson & Simpson, 1989; Seidenberg, Waters, Sanders, &
Langer, 1984; Koriat, 1981). Chwilla and colleagues showed N400 effects for
bidirectional, forwards-only, and backwards-only pairs, although the effect was larger for
bidirectional pairs; additionally, they showed that these effects were robust at both short
(0 ms) and longer (500 ms) ISIs. How can we understand the difference in the size of the
effect in the bidirectional vs the unidirectional cases, when both the bidirectional and
forwards cases should have been equally associated? The authors don’t explain this
difference. However, the forwards and backwards pairs were largely made up of partially
opaque compounds, which means they may have been mainly associatively related, rather
than semantically related1. In contrast, the bidirectional pairs were made up from first or
second associates from free association norms, which probably resulted in an increased
amount of semantic relatedness in this condition. Therefore, if we take the Kovisto and
Revonsuo result to indicate that both associative and semantic relatedness contribute to
the N400 effect, then we can predict the result that the bidirectional condition shows a
larger effect, if we assume that these contributions are at least partially additive.
IV. Direct comparisons of word and sentence level effects
CWH argues that effects of sentence-level contextual fit of words on the N400 can be
explained by lexical-level priming. A few studies have looked at the similarity between
lexical- and sentence- level effects; for example, Kutas (1993) compared the profile of
N400 effects for lexical priming and sentential anomaly within the same experiment, and
found their topography, amplitude, and timing to be almost indistinguishable. But,
perhaps most relevant to the CWH question, Van Petten (1993) actually directly
manipulated the association between words within a sentential context. She examined
sentences like the following:
1
Partially opaque compound pairs actually seems in retrospect like a strange choice for the materials, given
that the authors thought that the N400 reflected a semantic matching strategy!
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In the congruent associated case, the critical word was both congruent with the context
and strongly associated with a word in the previous context (moon-stars). In the
congruent unassociated case, the critical word was congruent with the context, but did not
have any strong associations with other words in the sentence. To complete the design,
van Petten also compared cases in which the critical word was incongruent with the
context in either the presence or absence of lexical association.
For each condition, van Petten compared the waveforms of the first italicized word (the
associate or control) and the second, critical word. Under CWH, N400 reduction should
take place solely on the basis of lexical association. Therefore, the simplest prediction of
the hypothesis would be that the associated conditions would show an N400 reduction,
and the unassociated ones would not. On the other hand, if the N400 also reflects ease of
contextual-sentential integration of the lexical item, N400 reduction should also take
place in the congruent unassociated condition, as the increased amount of preceding
context for the second word of the pair should make integration faster.
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The results showed that all conditions except the anomalous unassociated condition
showed a reduction in N400, with the same latency. This would seem to support the
hypothesis that the N400 is sensitive to contextual integration, contra CWH. However,
the relative amplitudes of the effects are suggestive; the congruent unassociated N400
effect looks smaller than in either the congruent associated or the anomalous associated
conditions. Furthermore, the effect in the anomalous associated condition does not look
smaller than the effect in the congruent associated condition, although we would expect a
difference in the relative size of effect if the associative and contextual effects were
somewhat additive as we saw in the case of associative and strategic effects in the lexical
priming N400 work.
These facts suggest a way in which the spirit of the CWH could perhaps be reconciled
with these results. We can maintain that the N400 crucially reflects lexical activation, not
integration, if we can describe all the sentence-level effects as expectancy based, by
which the sentence-level effect becomes a special case of expectancy-based priming. In
the congruent associated condition, the critical word is within a small field of lexical
items being weakly predicted in that position, and this increases the base activation on
this item so as to create a priming effect when it is encountered, similarly to the
expectancy-based priming effect in lexical decision (de Groot, 1984). The only caveat is
that since expectancy-based priming is seen as a strategic, ‘post-lexical’ effect, we may
be forced to add the extra assumption that reading comprehension involves something
analogous to the overt ‘decision’ that differentiates access from decision stages in lexical
access.
The idea that the N400 is at least partly expectancy-driven has a long history in the
literature, which I won’t delve into here (see, for example, Kutas, Lindamood, & Hillyard,
1984). In particular, recent work by Federmeier and colleagues has demonstrated N400
reduction for semantically anomalous continuations which belong to the same semantic
category as a highly probable continuation (Federmeier & Kutas, 1999; Coulson,
Federmeier, Van Petten, & Kutas, 2005). Thus, this version of CWH has reasonable
empirical support.
V. Conclusion
In this paper, I have examined if and how a version of the hypothesis that the N400
indexes solely lexical-level effects (i.e., lexical activation ‘effort’) could be implemented
given existing findings. Based on the literature on lexical priming and the N400, I
conclude that, in such an implementation, we must define the relevant ‘lexical-level
effects’ that the N400 indexes so as to include so-called ‘late’ priming processes such as
semantic matching or expectancy, as well as ‘early’ processes, namely spreading
activation. Based on Van Petten’s (1993) study, I conclude that we must adopt a
completely expectancy-based explanation of sentence-level effects in order to maintain
the idea that the N400 indexes only effects of lexical activation, not integration.
However, the hypothesis thus implemented seems to be compatible with the areas of the
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literature we have reviewed here. However, as I alluded to in the introduction, the
hypothesis may still face a challenge from the research on discourse-level effects on the
N400.
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