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Abstract:
We investigated visual evoked potentials (VEPs) during
presentation of a texture defined bar oriented 45° clockwise or
counterclockwise, segregating from a textured surround. Local
orientation contrast at the edges between figure and surround
remained constant whereas bar elements were either collinear or
orthogonal to bar orientation. In one task (viewing bar with
attention), observers discriminated the orientation of the texture
defined bar. In the other (viewing bar without attention), a digit
identification task engaged visual attention at the centre of the
textured display. VEPs were recorded during onset-offset
stimulation when the bar appeared and disappeared cyclically from a
uniform textured background. The difference component, obtained by
subtraction of offset-from onset-VEP, showed two negative peaks
with latencies around 140-160 and 190-210 ms. In the orientation
discrimination task, the second component only had significantly
larger amplitude and reduced latency in the collinear condition,
suggesting that discriminability of bar orientation improves with
collinear configurations. In the digit identification task, the
first component only had a significantly larger amplitude in the
non-collinear condition. This second effect is similar to that
previously obtained in orientation discrimination of textured edges
(Caputo & Casco, 1999), suggesting that without attention
figure-ground segmentation occurs, but is based on local
orientation contrast at the textured edge rather than perceptual
grouping.
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