A neural model of attention, perceptual grouping, and context-sensitive processing in the laminar circuits of visual cortex

Recent neurophysiological studies have shown that primary visual cortex, or V1, does more than passively process image features using the feedforward filters suggested in the Nobel-prize winning work of Hubel and Wiesel. It also uses horizontal interactions to group features preattentively into object representations, and feedback interactions to selectively attend to these groupings. All neocortical areas, including V1, are organized into layered circuits. This thesis presents a neural model showing how the layered circuits in cortical areas V1 and V2 elegantly join together these feedforward, horizontal, and feedback interactions. The model shows, in particular, how horizontal connections can complete perceptual groupings over positions that do not receive contrastive visual inputs, even while attentional feedback can only modulate or prime positions that do not receive such inputs. The model also demonstrates how attention can bias the groupings that do form. More generally, the model proposes how specific laminar circuits allow the responses of visual cortical neurons not only to be determined by the stimuli within their classical receptive fields, but also to be strongly influenced by stimuli in the extra-classical surround. This context-sensitive visual processing can greatly enhance the analysis of visual scenes, especially those containing targets that are low contrast, partially occluded, or crowded by distractors. Computer simulations of the model are presented, showing how recent neurophysiological and psychophysical data about context-sensitive processing can be explained. In particular, the simulations show how attention can selectively enhance object representations by propagating along boundary groupings of both real and illusory contours, and can also protect objects from competitive masking. It is also demonstrated how attention may have a stronger facilitatory effect on low contrast stimuli than on high contrast stimuli, how pop-out from textures can occur due to orientation contrast, and how contextual stimuli can enhance or suppress groupings in a contrast-sensitive manner. Finally, the specific functional roles that the model proposes for the cortical layers allow testable neurophysiological and psychophysical predictions to be made.