There's Waldo! A Normalization Model of Visual Search Predicts Single Trial Human Fixations in an Object Search Task.
When searching for an object in a scene, how does the brain decide where to look next? Visual search theories suggest the existence of a global "priority map" that integrates bottom up visual information with top down, target specific signals. We propose a mechanistic model of visual search that is consistent with recent neurophysiological evidence, can localize targets in cluttered images, and predicts single trial behavior in a search task. This model posits that a high level retinotopic area selective for shape features receives global, target specific modulation and implements local normalization through divisive inhibition. The normalization step is critical to prevent highly salient bottom up features from monopolizing attention. The resulting activity pattern constitues a priority map that tracks the correlation between local input and target features. The maximum of this priority map is selected as the locus of attention. The visual input is then spatially enhanced around the selected location, allowing object selective visual areas to determine whether the target is present at this location. This model can localize objects both in array images and when objects are pasted in natural scenes. The model can also predict single trial human fixations, including those in error and target absent trials, in a search task involving complex objects.
Cerebral cortex