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Abstract:
Responses of macaque V1 neurons to oriented stimuli can be modulated by stimuli outside the classical receptive field (CRF). Generally, responses are inhibited by extra-CRF stimuli with a parallel orientation and inhibited less by orthogonally oriented stimuli. We have shown previously [Soc Neurosci Abstr, 27, 12.5, 2001] that for contextual stimuli of opposite contrast-sign the strength of inhibition does not depend on orientation. We asked if this interaction between orientation and contrast-sign generalizes to surface segmentation cues other than contrast-sign, such as disparate color.
We recorded the responses of orientation selective V1 cells in alert behaving macaques to an optimally oriented red line (target) on a gray background. The target was presented either in isolation or in the presence of a 6.5 deg2 mask surrounding the CRF. The mask consisted of red or green lines oriented either parallel or orthogonal to the target. The lines were all equally bright and brighter than the background.
About 2/3 of the cells studied were inhibited more strongly by masks parallel to the target than by orthogonal masks. This orientation specific masking effect occurred irrespective of the color of the mask. Experiments with a green target line yielded similar results.
Disparate colors do not modulate the effect of disparate orientation on the masking power of contextual stimuli, like disparate contrast-sign does. Hence, interaction between different surface segmentation cues does not appear to be a general effect. Instead, partial separation between ON and OFF pathways in area V1 can explain our previous result with contrast-sign as well as the failure of this effect to generalize to color.
