electrophysiology of visual attention

Electrophysiology of Visual Attention

Does Visual Attention Modulate Visual Evoked Potentials?

• The theory is that Visual Attention modulates visual information at the level of visual cortex

• How would you design an experiment to test this theory?


• Sustained visual selective attention based on location (attend left or attend right)

• Eyes remain fixed at centre (covert attention)• Detect onset of a specific target• Record the visual ERP in response to stimuli


• Result: several components of the visual ERP are modulated by attention– P1 and N1 are larger for attended relative to unattended stimuli


• P1 effect is the difference between P1 peaks on attended and unattended trials

• Has contralateral topography (isopotential voltage maps and scalp current density maps)

• consistent with sources in extrastriate cortex


• Timing of P1 effect (70 to 90) ms suggests that sustained attention affects the feed-forward sweep but not at primary cortex

• Can we conclude that V1 is unaffected by sustained attention?

• P1 effect can be caused only by attending to locations– Selection by colour, orientation,

conjunctions or identity does not modulate early visual processes


• Transient visual spatial attention also enhances perceptual abilities for attended relative to unattended stimuli

• The theory is that transient visual attention causes a P1 and N1 effect like sustained visual attention

• How could you test that?


• Cue – Target Paradigm:

Responses are faster and more accurate for validly cued stimuli

Notice that the target stimulus is identical across the attention conditions


• Both P1 and N1 effects are observed• Effects are most pronounced over contralateral

cortex• Note this is different from the auditory transient

attention case

Consequences of Attentional Selection

Single unit recordings

Consequences of Attentional Selection

• Selection of one location or object or auditory stream has consequences for sensory responses evoked by that stimulus– ERP responses in auditory and visual cortex

• Are there effects of attentional selection observable at the cellular level?– This is going to require intracranial recordings– What animal would you choose?

Intracranial Recordings of Attentional Selection

• Moran and Desimone (1985)

• Approach this as a “Vision Scientist” - Recall that:– Cells in ventral stream pathway are selective for color, orientation,

complex shapes– “classical” notion of RF is that a cell should fire actively whenever its

preferred stimulus is present in its RF– V4 RFs are a few degrees of visual angle – much larger than the

resolution of attention

• What happens when attention selects an object in a cell’s RF if that cell isn’t “tuned” to the features of the object?

• Moran and Desimone (1985)– Response properties of cells are identified a priori

– Each cell is characterized by what is an “effective” and “ineffective” stimulus

– Monkeys were trained to attend to one of several locations within a V4 RF

– Monkey is given a target in a delayed match-to-sample task– Respond when target stimulus occurs at cued location


• Moran and Desimone (1985)

• “Classical” RF prediction: there should be no difference in responses in these two conditions


• Moran and Desimone (1985)• Result:


“effective” stimulus at attended location

Response to Target

“effective” stimulus at unattended location

Response to “Sample”

Response to Target

Response to “Sample”

• Moran and Desimone (1985)• Result:

– Neuron responds vigorously only if its effective stimulus is attended

– Interesting caveat: this only applies when there is an ineffective stimulus (to which the monkey attends) present in the V4 RF

• When the ineffective stimulus is outside of the cell’s RF, it’s responses are largely unmodulated


electrophysiology of visual attention

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