How perception adheres color to objects and surfaces: Studies using visual illusions and transcranial magnetic stimulation

The basic roadmap of the visual cortex starts with an early stage in which visual input is encoded primarily as edges. Information then flows through branching pathways into higher, more specialized regions of the brain. One such area is specialized in processing color, another area processes shape, etc. This encoding scheme poses some problems in explaining certain aspects of our daily experience of seeing. If the cortex processes the various visual features separately, how do we see all those elements integrated as unified objects? What mechanisms bind the features together? If the cortex encodes the visual scene in terms of its edges, then how do we see solid surfaces? What mechanisms fill-in the map of outlines?; This thesis investigates the problems of binding and filling-in using the techniques of visual illusion psychophysics and transcranial magnetic stimulation (TMS). TMS is known to cause the perception of a brief visual flash, or phosphene. Here, the appearance of the phosphene is found to depend on concurrent and previously viewed visual stimuli. In particular, TMS can cause an instant replay effect, which serves as an effective probe for the visual information contained within hidden internal brain states.; TMS-induced instant replay is used to probe the mechanisms of Cai's asynchronous binding illusion, in which a color change in a moving object is perceived to occur later in the motion stream. After subjects view this illusion, TMS can sometimes cause them to see a replay of the color change, but in the correct position. This indicates that the color change was properly encoded in some portion of the visual system, but that this representation normally remains unconscious due to further processing. It appears that TMS can selectively reactivate this representation, revealing its visual content without the distortion caused by other processes.; Further investigations of binding demonstrate a number of cases in which visual features are decomposed and/or misbound. TMS-induced instant replay can cause the color of one object to be bound to the position and orientation of another. It can also separately replay the color and orientation of a grating. Finally, in a non-TMS experiment, we create a stimulus that induces a steady-state misbinding of color and motion. The illusion is a vivid, long-lasting misbinding effect, and is ideal for neurophysiological investigation. These experiments confirm the separate encoding of visual features, the existence of an active binding mechanism, and they open the door to neurophysiological investigations of binding.; In an investigation of filling-in, a class of illusions derived from the artwork of Julian Stanczak is shown to defy the dominant model of color filling-in. It indicates that the extent of color filling is based on the high-level processing of globally defined perceptual surfaces rather than the low-level processing of locally defined retinotopic features. Based on a review of past electrophysiological studies and the phenomenology of the illusions presented here, it appears possible that the neural mechanisms of binding and filling-in might be intimately related, both of them highly integrated with the process of surface segregation.