  It has been hypothesized that neural activities in the primary visual cortex
(V1) represent a saliency map of the visual field to exogenously guide
attention. This hypothesis has so far provided only qualitative predictions and
their confirmations. We report this hypothesis' first quantitative prediction,
derived without free parameters, and its confirmation by human behavioral data.
The hypothesis provides a direct link between V1 neural responses to a visual
location and the saliency of that location to guide attention exogenously. In a
visual input containing many bars, one of them saliently different from all the
other bars which are identical to each other, saliency at the singleton's
location can be measured by the shortness of the reaction time in a visual
search task to find the singleton. The hypothesis predicts quantitatively the
whole distribution of the reaction times to find a singleton unique in color,
orientation, and motion direction from the reaction times to find other types
of singletons. The predicted distribution matches the experimentally observed
distribution in all six human observers. A requirement for this successful
prediction is a data-motivated assumption that V1 lacks neurons tuned
simultaneously to color, orientation, and motion direction of visual inputs.
Since evidence suggests that extrastriate cortices do have such neurons, we
discuss the possibility that the extrastriate cortices play no role in guiding
exogenous attention so that they can be devoted to other functional roles like
visual decoding or endogenous attention.
