Neural Basis of Cognition
Lecture 8
Attention
Attention
• What is attention?
– There is a limit to how much information the
human brain can process
– The process of choosing what information to
further process is “attention.”
Attention
• How can we characterize attention?
– Alertness and arousal: general awareness of the
world (decreases during sleep)
– Vigilance (“sustained attention”): the ability to
maintain alertness continuously over time, important
when a task must be performed in a nonstop matter
– Selective attention: the selection of information
essential to a task, such as ignoring background noise
during a conversation in a crowded restaurant
Attention
• Are these “attention systems” distinct? Do
they use the same resources?
– Evidence to the contrary: the brain’s processing
capacity is larger when tasks draw from different
resource pools (e.g. spatial, auditory) than from
the same one
Attention
• What brain structures are involved in
attention?
– Reticular activating system
– Superior colliculus
– Thalamus
– Parietal lobe
– Anterior cingulate cortex
– Frontal lobe
Reticular Activating System (RAS)
• The RAS is involved in alertness and arousal
and is responsible for controlling sleep-wake
cycles
• Cell bodies in the RAS have diffuse
connections to most regions of the cortex,
allowing them to modulate the arousal and
alertness of the entire brain
• Damage to the RAS or its functioning results in
coma
Superior colliculus
• Responsible for saccades (to quickly bring
peripheral visual stimuli into foveal vision)
– Express saccades: fast (~120ms), triggered by
novel visual stimulus in the periphery, disappear
upon destruction of the superior colliculus
– Regular saccades: voluntary, take about 200300ms
Thalamus
• Some nuclei (medial dorsal, intralaminar, reticular nuclei) in
the thalamus modulate the level of arousal of the cortex
• Sensory information is relayed to the brain through the
thalamus, so it is in a logical position to have a role in
selective attention
– The pulvinar seems to play an important role in selective
attention
• PET studies indicate that the thalamus is more engaged
when filtering of sensory information is required, such as in
a task where an item must be detected among eight other
items as opposed to when it is shown alone
• ERP and MEG studies indicate that such filtering occurs very
early after the receipt of the stimulus
Parietal Lobe
• The parietal lobe, important for visual and
spatial aspects of attention, is thought to be
involved in more fine-grained selection of
sensory information
• The parietal lobe is also taught to be
responsible for the overall allocation of
attentional resources to a particular stimulus
or task
Parietal Lobe
• Evidence:
– Single-cell recordings in monkeys show that the firing
rate of some parietal neurons is enhanced any time
attention is directed to a visual object (independently
of motor actions). In particular, the lateral
intraparietal region is important for the
representation of attended or salient spatial locations,
responding regardless of the modality of information
about a location or whether there is a motor response
– Neuroimaging in humans has provided converging
evidence: increased activation in parietal regions
across a variety of tasks that involve increased visual
attention but not by highly demanding tasks in general
Parietal Lobe
• The parietal lobe is thought to play a role in binging together visual
attributes with their position in space
• Feature integration theory: basic visual features such as color are detected
relatively automatically, but we cannot know which features go together
unless we direct our attention to a particular location
– Attention is the “glue” that binds this information
• Classic example:
– Finding a single red X in a table of green Xs happens at the same speed
regardless of the number of green Xs (preattentive processing)
– Finding a single red X in a table of green Xs and red Os takes longer with larger
tables
– This increase in time occurs because attention can be directed to only one
point at a time; directing attention to a point in space precedes the
identification of information, which means that directing attention to a
particular spatial location allows the features at that location to be bound
together so that an item can be identified
Parietal Lobe
• Bilateral damage to the parietal regions
disrupts the ability to bind together features
• Individuals with such deficits cannot detect
conjunction of features, whereas their ability
to detect a single attribute remains intact
Anterior Cingulate Cortex
• Once the brain has filtered sensory information, it must choose a
response; the region responsible for that is the cingulate cortex, which can
be thought of as an interface between subcortical and cortical regions
• Recall from lecture 3 that the anterior cingulate cortex is involved in
choosing a novel response
• Activity in the anterior cingulate cortex during conflicting responses but
not during noncongruent responses in the Stroop task supports that the
cingulate is critically involved in response selection
• Cingulate activity is observed when there is a need to select between
directly conflicting responses, and when selecting the correct response is
demanding or complicated (such as wel there are multiple possible
responses)
• Greater cingulate activity is also found when the determination of a
response is complicated because it relies on multiple attributes of a
stimulus (e.g. color, form speed) rather than a single one (e.g. color)
• Also, cingulate activity is thought to be correlated with task difficulty
Frontal Lobe
• The frontal lobe is involved in selection of
information for more abstract characteristics,
such as selecting words that have particular
meanings or selecting information that must
be held in working memory
• Discussion of the frontal lobe’s role in
attention falls more in the realm of
discussions of executive function
Attentional selection
• Early selection (early stage of processing, before items are identified) or
late selection (after sensory processing is complete and items have been
identified)?
• Classic example of an experiment designed to answer this question:
– Individuals are instructed to listen and count the number of target tones, such
as long tones, interspersed within more frequent non-targets, such as short
tones
– They are told to attend only to information in one ear
– Responses are compared for targets when they are attended for as compared
to when they are unattended
– The point in time as which the amplitude of the ERP to the attended stimulus
begins to diverge from that of the unattended stimulus is noted
– This happens approximately 80ms after stimulus presentation, suggesting that
attention can happen early
• However, the P300 component (mentioned in lecture 2) occurs only in
response to stimuli being attended to
• So, attentional selection can happen early or late
Selection by features
• What aspect of the sensory world is used
when selecting information?
– Space-based viewpoint of attention
– Object-based viewpoint of attention
Space-based viewpoint of attention
• Neuroimaging studies have provided evidence
that regions of both visual and parietal cortex
mediate space-based attentional deficits
– The mapping of the visual world in early visual
processing areas (V1-V4) is retinotopic
– Attending to information in one visual field increases
activation over V2-V4 regions of the opposite
hemisphere
– ERP and MRI studies show that this space-based
attentional modulation occurs early in processing,
~100ms after stimulus presentation, in the secondary
visual cortex
Object-based viewpoint of attention
• In a task where attention is directed to a spacially
constant attribute associated with faces, increased
activation was observed in the fusiform face area
(which recognizes faces); in contrast, when attention
was directed to an attribute associated with houses,
increased activation was observed in the
parahippocampal place area (which recognizes scenes)
• This modulation of attention appeared to occur
relatively early on in processing, when visual features
are first recognized as forming a particular object
Attention
• The conclusion of all the evidence mentioned
in this lecture is that attention manifests as
increased activation of the areas of the brain
involved in processing the type of information
being attended to
• There is also some evidence that taskirrelevant information undergoes decreased
processing
Hemineglect
• Hemineglect (sometimes referred to as hemi-inattention) is
a syndrome in which an individual ifnotes, or does not pat
attention to, the side of space contralteral to a lesion
• The side of space is usually defined with reference to the
body midline but may occur with regard to other spatial
reference ftames as well
• This inattention is seen regardless of the modality in which
information is presented
• Depending on the severity, an individual might fail to eat
food on the left side of a plate, draw the left side of objects,
read the left side of words, or use the left side of the body
• In severe cases, an individual may even deny that the left
side of this body belongs to him
Hemineglect
• One severr case:
– A patient with hemineglect complained to a nurse that a staff member had
played a cruel practical joke on him by placing a severed leg in his bed.
– The patient then attempted to throw the leg out of his bed, hurling himself
onto the floor.
• Another:
– A woman had a stroke and fell in her bathroom.
– While being examined, she insisted that her left arm was not hers but the
examiner’s. When the examiner brought the patient’s left arm into view and
asked whose it was, she answered, “It’s not mine. I found it in the bathroom,
when I fell. It’s not mine because it’s too heavy; it should be yours. It can move
and do everything; when I feel it too heavy, I put it on my stomach. It doesn’t
hurt me, it’s kind.”
– When she was asked where her own arm was, she replied, “behind the door.”
• Denying ownership of a limb and claiming it belongs to someone else
without any other deficit in reasoning is called somatoparaphrenia.
Hemineglect
• Neglect is usually observed after vascular damage to
the parietal regions extending into subcortical regions.
• Neglect is observed more commonly and is more
severe after right then left hemisphere lesions, and so
neglect is observed more often for the left side of
space.
• Neglect can also occur after damage to fronta regions,
the basal ganglia, and the thalamus
• Usually, neglect is severe at first (all items on one side
of space are ignored), but, with weeks to months,
some information on the neglected side is processed.
However, it rarely, if ever, disappears completely.
Hemineglect
• Hemineglect does not occur from low-level sensory
processing deficits; patients with hemineglect can still
perform motor acts such as showering, dressing, and eating
with both sides of the body (though one side may be
preferred).
• Lack of information from one half of the visual field cannot
account for neglect because re-orienting the head would
bring that side of space into view, but patients with
hemineglect ignore it.
• Sensory processing deficits cannot account for hemineglect
because information from the contralateral side of space is
generally ignored regardless of modality, but ipsilateral and
contralateral projections in the auditory system, for
example, make that unlikely.
Hemineglect
• Drawing and line bisection tasks:
Hemineglect
• Bringing attention to the left side of the line in the line
bisection task can improve performance
• If information on the neglected side is critical for
understanding or compregension, it tends to receive
attention
– “antiballistic” might be read as “ballistic” though only the letters
“llistic” are past the midline of the word
• Performance on a task can improve if a greater reward is
promised
• Hemineglect seems to be associated with attention
• “The patient with hemineglect treats one side of space the
way you normally treat the space behind your head.”
Hemineglect
• Perhaps hemineglect is the result of the lack of an
internal mental representation of the neglected side of
space
• Bisiach and Luzzatti, 1978:
– Two patients with hemineglect for the left side of space
were asked to imagine standing at one end of the Piazza
del Duomo of Milan and to describe what they saw. They
described landmarks on the right.
– The patients were then asked to imagine standing at the
opposite end of the plaza, facing the opposite direction,
and to describe what they saw. The described landmarks
on the remaining half of the plaza
• This rules out the theory of lack of a full mental map
Hemineglect
• One theory says that hemineglect is due to
preference for stimuli on one side of space,
which can be interpreted as an uneven
competition between hemispheres for
controlling the direction of attention
• Oliveri et al. 1999:
– Using TMS to disrupt the processing of the intact
hemisphere briefly eliminated hemineglect
Hemineglect
• Actively or passively moving the limb on the neglected
side of space to make the neglected side of space more
salient
• Caloric stimulation:
– Water at least 7 degrees Celsius cooler than body
temperatere is poured into the ear canal, inducing motion
in the semicircular canals of the vestibular system
– Can result in vertigo
• Neck-proprioceptic stimulation (vibration in the left
neck muscles)
• No permanent treatment
Attentional Bias
• Each hemisphere appears to have an attentional
bias for the contralateral side of space.
• Damage to the right hemisphere appears to slow
response times to simple stimuli more than
damage to the left hemisphere, suggesting that
the right hemisphere had a larger role in alerting
and arousal.
• PET studies indication that the right hemisphere
is important in sustaining overall attention, such
as in vigilant tasks.
Attentional Bias
• Which face looks happier?
Attentional Bias
• Most right-handed individuals perceive the
face on the left as happier.
• It is suggested that the left half-face is
perceived as more expressive because the
right hemisphere is more adept at processing
emotional and facial information.