Visual Attention

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) Extrastriate Cortex (e.g. V4, MT) SC Parietal Cortex (LIP) Superficial Layers Retina 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

Visual Attention