Abstract
From a computational theory of V1, we formulate an optimization problem to investigate neural properties in the primary visual cortex (V1) from human reaction times (RTs) in visual search. The theory is the V1 saliency hypothesis that the bottom-up saliency of any visual location is represented by the highest V1 response to it relative to the background responses. The neural properties probed are those associated with the less known V1 neurons tuned simultaneously or conjunctively in two feature dimensions. The visual search is to find a target bar unique in color (C), orientation (O), motion direction (M), or redundantly in combinations of these features (e.g., CO, MO, or CM) among uniform background bars. A feature singleton target is salient because its evoked V1 response largely escapes the iso-feature suppression on responses to the background bars. The responses of the conjunctively tuned cells are manifested in the shortening of the RT for a redundant feature target (e.g., a CO target) from that predicted by a race between the RTs for the two corresponding single feature targets (e.g., C and O targets). Our investigation enables the following testable predictions. Contextual suppression on the response of a CO-tuned or MO-tuned conjunctive cell is weaker when the contextual inputs differ from the direct inputs in both feature dimensions, rather than just one. Additionally, CO-tuned cells and MO-tuned cells are often more active than the single feature tuned cells in response to the redundant feature targets, and this occurs more frequently for the MO-tuned cells such that the MO-tuned cells are no less likely than either the M-tuned or O-tuned neurons to be the most responsive neuron to dictate saliency for an MO target.
Highlights
Background on visual attention, saliency, and their neural substratesSpatial visual selection, often called spatial attentional selection, enables vision to select a visual location for detailed processing using limited cognitive resources [1]
Behavioral data confirmed an unexpected prediction that an eye of origin singleton that is hardly distinctive from other visual inputs can attract attention and gaze qualitatively just like a salient and highly distinctive orientation singleton does – observations [13,15] show that the eye of origin singleton can be more salient than an orientation singleton
The goal and the plan for the current study Whereas the previous studies used known facts about V1 physiology to test, and confirm, the V1 saliency hypothesis, the current study aims to probe the unknown or less known V1 properties assuming that the V1 saliency hypothesis holds (Fig. 2)
Summary
Background on visual attention, saliency, and their neural substratesSpatial visual selection, often called spatial attentional selection, enables vision to select a visual location for detailed processing using limited cognitive resources [1]. Behavioral data confirmed an unexpected prediction that an eye of origin singleton (e.g., an item uniquely shown to the left eye among other items shown to the right eye) that is hardly distinctive from other visual inputs can attract attention and gaze qualitatively just like a salient and highly distinctive orientation singleton does – observations [13,15] show that the eye of origin singleton can be more salient than an orientation singleton This finding provides a hallmark of the saliency map in V1, because the eye of origin feature is not explicitly represented in any visual cortical area except V1. Functional magnetic resonance imaging and event related potential measurements confirmed that, when top-down confounds are avoided, a salient location evokes brain activations in V1 but not in the parietal and frontal regions [14], which are thought to be involved in saliency by traditional views [1]
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