Poster Presentation "The Development of Optic Flow Selectivity in MSTd Neurons Using Back-Propagation Networks"

In neurophysiological experiments examining the responses of MSTd neurons to visual motion components of optic flow stimuli in monkeys, Duffy & Wurtz (1991) reported neurons with double-component (plano-radial and plano-circular) and triple-component (plano-radial-circular) selectivities, while Graziano et al. (1994) reported neurons selective to spiral optic flows. Here we address the apparent disagreement of these reported findings under simulated experimental conditions. To examine the development of optic flow selectivities in the hidden units of a two-layer back propagation network, we constructed to computational models by using different input layer formats. The first simulation used a sparse optic flow. The input format consisted of eight circular MT receptive fields placed in the MSTd receptive field such that the MT receptive fields were radially symmetric without overlap. Each MT receptive field corresponded to eight directionally selective MT neurons which equally divided the directional vector space. The second simulation used a dense optic flow.The input format consisted of 67 overlapping MT receptive fields of normally distributed diameter placed pseudo randomly in the MSTd receptive field. Each MT receptive field corresponded to 16 directionally selective MT neurons which equally divided the vector space. The hidden units were classified as MSTd neurons whose receptive fields were coincident and encompassed the MT receptive fields from the input layer. The output layer of the network consisted of MSTd neurons whose receptive fields were consistent with the hidden layer. To minimize the effects of biasing, the output selectivities were designated as expansion, contraction, clockwise rotation, and counterclockwise rotation (consistent with MSTd selectivities found in both experiments). In both sets of simulations we found hidden units whose selectivities were consistent with the visual motion components of optic flow stimuli reported by Duffy & Wurtz and Graziano et al.
 

Duffy, C. J. & Wurtz, R. H. (1991a). "Sensitivity of MST neurons to optic flow stimuli. I. A continuum of response selectivity to large-field stimuli", J. Neurophysiol. 65, 1329-1345.

Duffy, C. J. & Wurtz, R. H. (1991b). "Sensitivity of MST neurons to optic flow stimuli. II. Mechanisms of response selectivity revealed by small-field stimuli", J. Neurophysiol. 65, 1346-1359.

Graziano, M. S. A., Anderson, R. A. & Snowden, R. J. (1994). "Tuning of MST neurons to spiral motions", J. Neuroscience 14, 54-67.