Visual Cortical Mechanisms of Motion Perception and Navigation: Experiments and Models

The proposed research will explore human motion perception and navigation through a combination of neural modeling and experiments. A neural model is proposed in which optic flow is mapped to visual cortex via the cortical magnification factor, and this mapping serves as a basis for cell selectivity in the dorsal medial superior temporal cortex (MSTd). MSTd cells have large receptive fields, receive an eye movement efference copy, and respond to the spiral motion patterns that occur during combined observer rotation and translation. MSTd cells also exhibit position-invariant responses, which are useful for object motion processing. Model cells match MSTd neuron responses to optic flow stimuli placed in different parts of the visual field. The model demonstrates how MSTd cell responses can explain human judgments of self-motion direction (heading) and time-to-contact (TTC). Spiral-tuned model cells clarify when a subtractive efference copy is needed for accurate heading perception, and when retinal input is sufficient. Model cells also match human heading judgments across a range of heading angles, scene layouts, and rotation rates. TTC is processed by attentionally modulated cells selective for rate of expansion. Simulations match human TTC judgments when an observer moves towards a stationary object, and when an object moves towards a stationary observer. The experimental work addresses the mechanisms by which visual and extraretinal sources of information are combined. The foundation for this work is an optical illusion in which an expansion flow field of moving dots is overlapped by planar motion. In this case observers perceive an illusory displacement of the focus of expansion (FOE) in the direction of the planar motion (Duffy & Wurtz, 1993). The illusion may be a consequence of induced motion, wherein an induced component of motion relative to planar dots is added to the motions of expansion dots to produce the FOE shift. While such a process could be mediated by local “center-surround” receptive fields, the effect could also be due to a higher level process which detects and subtracts large-field planar motion from the flow field. In the experiments reported herein, larger FOE shifts were perceived for greater speeds and sizes of planar motion fields, although the speed effect saturated at high speeds. While the illusion appears to share a common mechanism with center-surround induced motion, the results also point to involvement of a more global mechanism that subtracts coherent planar motion from the flow field. Such a process might help to maintain visual stability during eye movements.