Post on 14-Dec-2015
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?
Does Visual Attention Modulate Visual Evoked Potentials?
• 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
Does Visual Attention Modulate Visual Evoked Potentials?
• Result: several components of the visual ERP are modulated by attention– P1 and N1 are larger for attended relative to unattended stimuli
Does Visual Attention Modulate Visual Evoked Potentials?
• 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
Does Visual Attention Modulate Visual Evoked Potentials?
• 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
Does Visual Attention Modulate Visual Evoked Potentials?
• 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?
Does Visual Attention Modulate Visual Evoked Potentials?
• Cue – Target Paradigm:
Responses are faster and more accurate for validly cued stimuli
Notice that the target stimulus is identical across the attention conditions
Does Visual Attention Modulate Visual Evoked Potentials?
• 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
• 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
Intracranial Recordings of Attentional Selection
• Moran and Desimone (1985)
• “Classical” RF prediction: there should be no difference in responses in these two conditions
Intracranial Recordings of Attentional Selection
• Moran and Desimone (1985)• Result:
Intracranial Recordings of Attentional Selection
“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
Intracranial Recordings of Attentional Selection