Effects of Attention

Studies in several areas of occipital and inferotemporal cortices in monkeys have indicated that spatially direct attention can modulate the activity of individual neurons. These studies indicate that, when two stimuli are presented in a neuron's receptive field, the neuron's response is dictated by the attended stimulus. This result has been interpreted as a reduction in the suppressive effect of the second stimulus when attention is directed to the first stimulus in the receptive field. This modulatory effect was observed both in the stimulus-evoked activity and in the spontaneous activity when no stimulus was present.

The role of attention in the activity of area V2 in human visual cortex has been investigated using fMRI with paradigms that are largely identical to those used in single-unit investigations in monkeys used to study stimulus-evoked activity and background modulation. To study the effect of attention on evoked activity, fMRI were investigated in areas V1, V2, VP, V4, and TEO during conditions of sequential or simultaneous stimulus presentation both with and without spatially directed attention. The suppressive effect of simultaneous stimulation with four different stimuli increased from area V1 to area TEO. In addition, the magnitude of the response in both the simultaneous and sequential conditions was increased when attention was directed to the visual stimuli in areas V2, V4, and TEO, but not in area V1. To study the effect of attention on spontaneous activity, the baseline activity prior to stimulus presentation was measured with and without spatially directed attention. A baseline shift index (BSI) was used to quantify this difference in activity during the expectation period prior to stimulus presentation. A statistically significant BSI was observed for all cortical areas studied including areas V1 and V2. Thus, area V2 showed an increase in both stimulus-evoked activity and background activity under conditions of spatially directed attention. Interestingly, area V1 showed a significant effect of attention on background activity but not on stimulus-evoked activity. The increase in background activity during spatially directed attention is similar to the results observed in single-unit recording in areas V2 and V4 of awake macaque monkeys.

Thus, functional imaging techniques have been used to study the role of area V2 in a range of perceptual and cognitive tasks. These functional properties of human V2 are summarized in Table IV. This table represents the beginning of detailed investigations about the function of area V2. Future work will ask more specific

Table IV

Perceptual and Cognitive Functions of Human V2a Task Functional activity

Attention +

Color +

Disparity +

Illusory contour weak

Global object —

Motion +

Illusory motion —

Higher order motion —

Texture segmentation — (weak)

Imagery ?

"Summary of the functional properties of human V2 assessed using functional imaging techniques. + indicates function localized to V2; — indicates function not localized to V2. Weak indicates inconsistent or nonsignificant result; ? indicates conflicting results in literature.

questions about the specific roles that V2 plays in these perceptual tasks. Important questions remain concerning the spatial ranges in which V2 performs motion, form, and color analyses and whether these analyses can be attributed to individual functional compartments in V2.

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