Transcript What is attention for?

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Attention and Consciousness
The breath of the mind is attention
– Joseph Joubert
Auditory attention – Cocktail party effect
Auditory attention – Cocktail party effect
Selective attention
Focusing on a particular conversation while ignoring others
Keyword detection
Immediately capture “important” words originated from unattended stimuli
Binaural processing
Audio attention can be allocated to a sound source in space.
What is attention for?
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Our brains have limitations on the ability to simultaneously carry out multiple
cognitive or perceptual tasks
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The limitations reflect the limited capacity of some stage or stages of sensory
processing, decision-making, or behavioral control.
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As a result of such computational bottlenecks, it is necessary to have neural
mechanisms in place to ensure the selection of stimuli, or tasks, that are
immediately relevant to behavior.
“Selective attention” is a broad term denoting the mechanisms that
mediate this selection.
It is possible to focus on selected stimuli in any sensory modality—sights, sounds,
smells, and touch.
Broadbent’s model of selective attention
Early versus late selection of information processing
Perhaps unattended channel information was not completely gated from
higher analysis but was merely degraded or attenuated
Varieties of attention
Overt vs. Covert
We look at an object when we overtly direct our attention to it.
However, it is also possible to attend covertly to objects without looking directly at
them.
Covert attention improve peripheral visual acuity, thereby extending the functional field
of view.
Top-down vs bottom-up
Bottom-up attention: exogenous or stimulus driven
Top-down attention: based on internally defined goals and against potential
external distractions
Voluntary attention (Top-down)
• Endogenous cuing
• “Spotlight”
Reflexive attention (bottom-up)
• Exogenous cuing
• Responses are faster to targets at the cue location, but only for a short time
(50-200 ms)
• The effect is reversed when the time between the task-irrelevant cue and the
target is more than 300 ms -> Inhibitory aftereffect or inhibition of return
Pop-out search and conjunction search
The computational bottleneck in search tasks
Many studies supported the theory that limited capacity results from information
processing bottlenecks that occur relatively late in processing, after features are
already integrated into wholes. One possibility is that the bottleneck occurs when
objects enter a limited capacity working memory. Under this view, attention plays
the role of selecting which objects pass through the bottleneck.
Balint’s syndrome
• A severe disturbance of visual attention and awareness
• Only one or a small subset of available objects being perceived at any one
time
• Balint’s syndrome informs us about the nature of the attention and
awaremess
http://www.youtube.com/watch?v=4odhSq46vtU
Steven Hillyard et al 1973 experiment
The latency of the peak < 90 ms
-> Supporting the earlyselection models
• Woldorff and Hillyard (1991) experiment: P20-50 effect
• Woldorff and Hillyard (1993) experiment: MEG (magnetoencephalogram)
The M20-50 effect was localized to the auditory cortex in Heschl’s gyri
Neurophysiology of voluntary visual attention
• This P1 attention effect (70-90 ms) is only reliably found in spatial attention, not on
other visual features (color, shape, etc).
• The attentional effect of more complex features were observed later in the ERPs
(>120 ms)
• The first volleys of afferent inputs into striate cortex (V1) take place
with a latency longer than 35 ms.
• Numerous studies showed that the P1 attention effect are
generated in extrastriate cortex
• As for the auditory selective attention, selective visual attention
effect takes place early in visual cortex.
Reflexive cuing task
Spatial attention and visual search
Question:
Spatial attention precede
feature/object attention
Or
Feature attention leads
spatial attention?
Feature and spatial attention
• In general, spatial attention produces
the shortest-latency ERP
• A host of feature-based ERP comes
with slightly longer latencies.
• N2pc -> a response indexes how
attention zooms down to focus on
objects in visual space in a search task
• Max Hopf et al 2004 experiment
showed that 1. feature selection ERP
in ventral occipitotemporal cortex with
140 ms latency. 2. N2pc follows in
30ms in more anterior regions
• We now understand that spatial attention influences the processing of visual
inputs
• Attended stimuli produce greater neural responses than do ignored stimuli
• The difference is observed in multiple visual cortical areas
• But, two questions come to mind:
1. Are such effects limited to visual cortical processing, or might they begin
earlier in the subcortical relays?
2. What does attention do for the brain to permit the attended signal to
exert a greater control on perception and awareness
• John Duncan and Robert Desimone proposed a biased competition model for
selective attention.
Summary of early neuroimaging attention studies using position emission
tomography (PET) by Corbetta and colleagues (1991), Heinze and colleagues
(1994), and Mangun and colleagues (1997).
Nonspatial attention modulates sensory cortices
Studies found:
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Attention to shape and color led to response enhancement in regions of
the posterior portion of the fusiform gyrus, including area V4.
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Attention to speed led to response enhancement in areas MT/MST.
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Attention to faces or houses led to response enhancement in areas of the
mid anterior portion of the fusiform gyrus, areas responsive to the
processing of faces and objects.
Desimone and associates studied the effect of selective attention on the
responses of a neuron in area V4 of macaque monkeys
V1 and V2 neurons showed similar effects
Frontal and parietal cortices play roles in visual attention
LIP: lateral intraparietal area
FEF: frontal eye field
SEF: supplementary eye field
DLPFC: dorsal lateral prefrontal cortex
Fecteau, J. H. and Munoz, D. P. Nat.
Rev.
Neurosci. 4 (1-9), 2003
Role of frontal and parietal cortices – fMRI studies
Brain areas in the parietal, frontal, and cingulate cortices are especially active
in relation to spatially directed attention.
SPL: superior parietal lobule
FEF: frontal eye field
SEF: supplementary eye field
Neuronal receptive fields – Possible source of limitation?
The sizes of receptive fields increase from less than a degree of visual arc in primary
visual cortex (area V1) to about 20° of visual arc in area TE, the last purely visual area
in the ventral visual processing stream
Therefore, a likely explanation for why one cannot process many different objects in a
scene simultaneously is that neurons, whose signals are limited in bandwidth, cannot
simultaneously send signals about all the stimuli inside their receptive fields. This idea
implies that processing limitations exist at all levels of processing but that they
become more pronounced in higher order visual areas, where receptive field sizes are
larger.
Think of a corporation again!
Neglect syndrome: A deficit of spatial attention
Unilateral lesions in the parietal lobe, the frontal lobe, and the anterior cingulate cortex
(Heilman, 1979; Vallar, 1993) in humans may cause a profound inability to attend to
certain spatial regions, a syndrome known as spatial neglect.
Neglect patients may be reluctant to initiate
movement in contralesional space, with or
without external sensory stimulation.
Neglect patients have normal vision in the contralesional
visual field once their attention has been directed there,
and they have no hemiparesis that could account for their
reluctance to move.
Neglect syndrome: A deficit of spatial attention
In severe cases, patients with neglect behave as if the world contralateral to their
lesioned hemisphere (the contralesional world) has ceased to exist.
Different forms of neglect correlate with dysfunction in different brain regions
• Lesion in inferior parietal lobule and temporoparietal junction -> neglect for
extrapersonal space
• Lession in superior temporal gyrus -> object-based neglect
Visual recollections of two ends of an Italian piazza by a neglect patient