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Abstract:
Abstract: The responses of many neurons in V1 are modulated
by orientation contrast between the receptive field (RF) and its
surround, suggesting that these cells participate in figure-ground
segmentation. We studied whether the responses of V1 and V2 neurons
are also modulated when their RFs lie within figures that are
defined by illusory contours rather than orientation contrast and
whether this modulation is affected by attention. Single units were
recorded in V1 (27 units) and V2 (35 units) of a fixating Rhesus
monkey while it performed an attentional task. In the Orthogonal
Grating condition, a circular figural grating (diameter = 2.5 x RF)
was presented over the RF and surrounded by an
orthogonally-oriented grating. In the Offset Grating condition, an
identical figural grating was presented over the RF and surrounded
by an iso-oriented grating shifted 90 degrees in relative phase,
creating an illusory contour that defined the figure. The baseline
response was measured by presenting a homogenous grating over the
RF. The Orthogonal Grating condition yielded facilitation
frequently in both V1 and V2, with average responses increasing 37%
and 75%, respectively. However, in the Offset Grating condition,
facilitation was infrequent and weak (11% on average) in V1 but
common and robust (37% on average) in V2. In both V1 and V2,
attention had little effect on the facilitation. These data provide
evidence for hierarchical aspects of figure-ground
segmentation.
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