Two of the most interesting functions of the visual motion system are the analysis of optic flow for heading perception and the determination of 3D structure-from-motion (SFM). This proposal will examine how these functions are accomplished by visual cortex using single cell recording techniques in behaving monkeys. The first specific aim is to examine the unsolved problem of how flow generated by eye and head movements is subtracted from flow generated by observer translation in order to recover the direction of heading. Pilot experiments for the proposal indicate that eye pursuit signals shift the spatial tuning curves of flow sensitive neurons in the dorsal division of the medial superior temporal area (MSTd). This shift enables MSTd neurons to code heading direction irrespective of whether the eyes are still or moving. Experiments are also proposed to determine whether head movements also lead to spatial tuning shifts, and to examine whether the head movement signals are derived from efference copy, vestibular, and/or proprioceptive sources. It will also be determined whether MSTd neurons code heading in eye-, head-, body-, or world-centered coordinates. The second specific aim is to examine the neural mechanisms for 3D SFM perception. Monkeys will be trained to report the perceived direction of rotation of cylinders defined by disparity and motion cues and cylinders defined only by motion cues. These latter cylinders are perceived as 3D, but the perceived direction of rotation is bistable. Recordings will be made from the middle temporal area (MT) while animals perform this task. Pilot studies indicate that MT activity is correlated with the perceived rotation direction of the bistable stimulus, with cells giving different responses for the same physical stimulus depending on the percept. Experiments are also planned to examine a 3 stage model for SFM processing: the first stage measures motions and likely occurs in V1; the second stage segregates and depth orders surfaces and likely occurs in MT; the third stage uses speed gradients to calculate 3D shape and may occur in MT or MST. Recordings will be made in V1, MT and MST using variations of the bistable cylinder paradigm to determine where these three stages are located in the motion pathway. The experiments in this proposal will determine the neural mechanism for heading computation during eye and head movements and whether MSTd is directly involved. They will also determine if monkeys perceive SFM and the role of V1, MT, and MST in this important perceptual process.
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