Perceptual grouping of visual form and motion: Modeling spiking cortical circuits and moving reference frames

This thesis models how the brain achieves perceptual grouping in both form and motion. Perceptual grouping of form is modeled using spiking neurons. Perceptual grouping is a challenge for spiking cells because its properties of collinear facilitation and analog sensitivity occur due to binary spikes with irregular timing across many interacting cells. hi particular, illusory contours can take hundreds of milliseconds to resolve by integrating multiple spikes over time. The form grouping model reconciles fast feedforward processing with slower feedback processing in a laminar cortical network of spiking cells whose emergent properties simulate neurophysiological data These laminar dynamics shed new light on how the brain resolves local informational ambiguities through the use of properly designed nonlinear feedback spiking networks which run as fast as they can, given the amount of uncertainty in the data that they process.;The motion grouping model analyzes how spatially disjoint and ambiguous local motion signals in multiple directions generate coherent and unambiguous representations of object motion. Various motion percepts, starting with those of Duncker and Johansson, obey a rule of vector decomposition, where global motion appears to be subtracted from the true motion path of localized stimulus components, so that objects and their parts are seen as moving relative to a common reference frame. The motion grouping model predicts how vector decomposition results from multiple-scale and multiple-depth interactions within and between the form and motion processing streams in V1-V2 and V1-MST, which include form grouping, form-to-motion capture, figure-ground separation, and object motion capture mechanisms. In particular, these mechanisms solve the aperture problem, group spatially disjoint moving objects via illusory contours, capture object motion direction signals on real and illusory contours, and use inter-depth directional inhibition to cause a vector decomposition. Due to inter-depth directional inhibition, motion directions of a moving frame at a nearer depth suppress these directions at a farther depth, and thereby cause a peak shift in the perceived directions of object parts moving with respect to the frame. This peak shift helps to explain the vector decomposition that is perceived between the frame and its moving parts.