Electrophysiology of Visual Attention

1. Electrophysiology of Visual Attention

2. 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?

3. 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

4. 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

5. 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

6. 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

7. 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?

8. 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

9. 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

10. Consequences of Attentional Selection Single unit recordings

11. 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?

12. 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?

13. Intracranial Recordings of Attentional Selection • 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

14. Intracranial Recordings of Attentional Selection • Moran and Desimone (1985) • “Classical” RF prediction: there should be no difference in responses in these two conditions

15. Intracranial Recordings of Attentional Selection • Moran and Desimone (1985) • Result: Response to “Sample” Response to “Sample” Response to Target Response to Target “effective” stimulus at unattended location “effective” stimulus at attended location

16. Intracranial Recordings of Attentional Selection • 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