Parallel popout: further confirmation of the V1 Saliency Hypothesis (V1SH)

Finding a target among uniformly oriented non-targets is typically faster when this target is perpendicular, rather than parallel, to the non-targets. Here, by exploiting the properties of saliency computations in primary visual cortex (V1), I demonstrate a special case when exactly the opposite is true. Each item, target or non-target, comprises two disks of the same size; the centre of one disk is displaced 1.2 disk diameters from that of the other along a line defining the item’s orientation. A target has two black disks or two white disks; each non-target has one white disk and one black disk. The target is oriented 45 degree clockwise or counter-clockwise from horizontal; the non-targets are uniformly oriented either perpendicular or parallel to the target in a grey background. Unlike the target, each non-target activates a neuron in V1 more strongly when its orientation is perpendicular rather than parallel to the neuron's preferred orientation, since the white and the black disks best activate, respectively, the on- and off- subfields of the neural receptive field (Zhaoping, L., 2020, i- Perception, 11(4):1--5.). V1 neurons are suppressed more strongly by neighbouring neurons tuned to similar rather than dissimilar orientations. Thus, a target parallel (rather than perpendicular) to the non-targets evokes a higher V1 response and, according to V1SH, is more salient. Our behavioural confirmation of faster search in this condition supports V1SH's proposal that V1 is the neural basis for saliency of exogenous attentional selection (Zhaoping, L., 2002, Trends in Cognitive Sciences 6(1):9-16)