The neural bases of attention

1. At what levels of the nervous system does attention exert its effects?

Answer: Both ‘early’ and ‘late’. 2. What neural systems control the allocation

of attention? Answer: Prefrontal and parietal cortex.

Early and late selection for attention: Event related potential (ERP) evidence

For spatial attention (location) the P1 event (~100 msec) is enhanced for attended relative to unattended location.

Gradient of the effect suggests this occurs in V2/V3 (extrastriate occipital) but not in V1.

Nonspatial visual attention (rather than location) shows increases in eventrelated potential components (ERPs) at 250-300 msec consistent with color, form & motion only being explicitly represented in areas V4; IT; and MT .

Is enhancement an increase of the attended stimulus event or a decreaseof unattended events? Need a neutral stimulus control to compare them to.

Found both things for location attention.

1. Decrease: in P1 (80-120 ms) showed inhibition of unattended, butnot facilitation for attended stimuli.

2. Increase: In N1 (140-180 ms) showed facilitation of attended, but not inhibition of unattended, stimuli.

1. Actually a decrease: in P1 (80-120 ms) relative to neutral so inhibition of unattended, not facilitation for attended stimuli.

2. Increase: In N1 (140-180 ms) showed facilitation of attended, but not inhibition of unattended, stimuli.

Early and late selectionPET evidence

Task makes a difference. Earliest studies didn’t find effects, but didn’t have good designs.

Hillyard compared ERP and PET (bloodflow) in the same task and found occipital (but not V1) enhancement with location attention for both. So supports ERP story.

Corbetta used stimuli that differed in color, form and motion, and subjects only attended to changes in one of those three. Where do you thinkblood flow increases occurred with attention?

Blood flow increases occurred in V4 for color, IT for form and MT for motion with attention.

Early and late selectionfMRI evidence

With spatial location attention effect is seen in extrastriate cortex again (retinotopic).

But also in primary visual cortex (compared T and +++ to find upside down T; harder task). No modulation of ERP in V1 when both ERP and fMRI compared. (But there is a timing explanation. See it then decide it needs attention.)

Speed discriminations versus passive viewing also showed MT attention effect and V1 effect.

For V1 difficult discriminations or many distractors are associated with an attention modulation of stimuli.

Effects may be increases in responding to attended, as well as decreases in responding to unattended, stimuli.

Voluntary Attention Modulates fMRI Activityin Human MT–MST

Attention here is nonspatial orobject based attention.

Single unit recording evidence

Desimone recorded cells in V1, V4 and IT when monkeys were performing same/different task.

When both stimuli were in thereceptive field, the cell respondedless to the stimulus when it was unattended(V4 and IT).This was not seen in V1, but field toosmall for both stimuli.

More recent cell data

In IT and V4 attention outside the receptive field can also reduce response to stimuli in receptive field.

Attentional state may be accompanied by an increase inbaseline firing (V2 and V4).

Object attention effects in ITregardless of location.This figure illustrates effects whentriangle is cue and attendedstimulus. If triangle isunattended then firing wouldbe inhibited after initial activation.