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Transcript
Visual Attention
• Attention is the ability to select objects of interest from the
surrounding environment
• A reliable measure of attention is eye movement during
object (target) selection
• Early studies show that there are specific brain regions that
are involved in the process of target selection
Visual Attention
• Attention is the ability to select objects of interest from the
surrounding environment
• A reliable measure of attention is eye movement during
object (target) selection
• Early studies show that there are specific brain regions that
are involved in the process of target selection
– Superior Colliculus
Visual Attention
• Attention is the ability to select objects of interest from the
surrounding environment
• A reliable measure of attention is eye movement during
object (target) selection
• Early studies show that there are specific brain regions that
are involved in the process of target selection
– Superior Colliculus
– Frontal Eye Field
Visual Attention
• Attention is the ability to select objects of interest from the
surrounding environment
• A reliable measure of attention is eye movement during
object (target) selection
• Early studies show that there are specific brain regions that
are involved in the process of target selection
– Superior Colliculus
– Frontal Eye Field
– Extrastriate Areas (V1, V4, IT)
Superior Colliculus & Target
Selection: Early Studies
• Cells in the superficial layers of the SC respond to light
falling on a specific area of the retina
• Visual response occurs 50ms after stimulus onset even
when the stimulus appears in the RF and the monkey
maintains fixation
Superior Colliculus & Target
Selection: Early Studies
• Enhanced visual response when the stimulus appears in the
cell’s receptive field AND the monkey selects it as a target
and saccades (overtly attends) to it. Latency of saccade is
about 200ms
• Activity is greater for target + saccade than for target
Superior Colliculus & Target
Selection: Early Studies
• No enhanced response when the monkey saccades to a
stimulus outside the receptive field (or in total darkness)
Superior Colliculus & Target
Selection: Early Studies
• No enhanced response when the monkey covertly attends
to the stimulus in the receptive field
• At least this is what they found back then
Conclusions from Early Studies
• Early studies suggest that the superior colliculus is involved
in the selection of targets for eye movements
– Not just a visual response to stimulus
– Not just a motor response to saccade
• Greatest level of activity is found when both a target is
presented and a subsequent saccade is made to that target
Target Selection and Uncertainty
• Behavioral Task
– Uncertainty period: array of
possible targets presented
– Selection period: one of the
targets is dimmed
– Removal of fixation signaled
saccade initiation
• Manipulated uncertainty by
varying the # of possible
targets (distractors)
• Recorded activity in SC
intermediate layer neurons
Visual Response Modulated by
Uncertainty / # of Distractors
• Response during stimulus
onset in the uncertainty
period decreases as the #
of distractors (uncertainty
level) increases
• Response during selection
and saccade initiation were
not significantly different
regardless of uncertainty
The Frontal Eye Field & Target
Selection
• FEF cells respond to visual stimuli like the SC
– Greater activity for spots of light in the receptive field
– RF is larger: as big as a ¼ of an entire visual field
Decreased Activity with Increased
Uncertainty / # of Distractors
Major Connections of the Superior
Colliculus
Striate Cortex (V1)
SC
Retina
Superficial Layers
Extrastriate Cortex
(e.g. V4, MT)
Parietal Cortex (LIP)
Frontal Eye Field
Intermediate and
Deep Layers
Brainstem Saccade
Generator
Medio-Dorsal Thalamus
Major Connections of the Superior
Colliculus
Striate Cortex (V1)
SC
Retina
Superficial Layers
Extrastriate Cortex
(e.g. V4, MT)
Parietal Cortex (LIP)
Frontal Eye Field
Intermediate and
Deep Layers
Brainstem Saccade
Generator
Medio-Dorsal Thalamus
Early Studies of Target Selection
Activity in FEF
• Detection Task
– Target presented alone
– Goal: saccade to target
• Visual Search Task
– Target with distractors
– Goal: saccade to target
• Recorded neural activity in
FEF at two time intervals:
– 100ms after stimulus onset
– 100ms before saccade
Selection-Dependent Visual Activity
• Enhanced response when
the target is in the RF
• Greater response for target
alone (detection) than with
distractors (search)
• Suppressed activity when
the target is near the RF
only in search trials
Target Selection in FEF Neuron
• Detection Trials
– Activity decreased as distance
from RF increased
• Search Trials
– Initial activity is the same,
since a stimulus (distractor) is
always present
– Enhanced activity for target in
RF
– Suppressed activity for target
near RF
Go / No-go Task
• Monkeys fixated on white spot
• Trigger signal: change of color
– Go trial (solid square): shift gaze to the target – saccade
– Nogo trial (asterisk): maintain fixation – no saccade
• Color change can occur before, during or after target onset
Neural Response for Go & No-go Trials
Evidence for Center-Surround Activity
FEF Connects to Extrastriate Cortex
Target Selection in V4
Visual Properties of IT Neurons
• Neurons respond selectively to complex objects
– Input: V1 and V4 (feature detection)
– Output: entorhinal cortex  hippocampus (memory)
• Large receptive fields that always include the fovea
• Retinotopic organization is absent
• Invariant Properties
– Scale invariance: insensitive to changes in stimulus size
– Translation invariance: insensitive to changes in position
• Columnar organization
• IT neurons are expected to be involved in more complex
aspects of target selection
Experimental Design
• Good (square) and poor
(triangle) stimuli were
selected prior to task
• Sequence of Events
– cue  delay  target
– Saccade to target that
matches the cue
• Block trial: used one cue
for 20 – 30 trials
• Recorded activity in IT
IT Neurons Carry Target Information
Next Week’s Readings
• McPeek & Keller: effects of superior colliculus inactivation
on target selection
• Bichot & Schall: responsiveness of FEF to target similarity
and history
• Sheinberg & Logothetis: inferotemporal cortex and natural
target selection of familiar objects