Motion processing using asymmetric shunting lateral inhibitory networks

Asymmetrically connected inhibitory shunting networks are believed to occur in many areas of the brain, including the visual system; they can explain many peripheral visual phenomena. This paper illustrates their usefulness in motion detection, and compares details of their performance with experimental results. Two stimuli are considered: drifting gratings and jumping bars. An elementary motion detector (EMD) is defined, consisting of a pair of opposite-directed asymmetric inhibitory networks. A frequency-space expansion (equivalent to a Volterra-Wiener expansion) is used to examine the EMD's response to a drifting grating. The EMD can be tuned to independent spatial and temporal frequencies: by summing the outputs of a number of EMDs, a velocity-tuned network is obtained. This velocity-tuned network is very well suited to certain field effect transistor (FET) implementations. Since the network is nonlinear, the response to a drifting grating does not fully characterize its behaviour. The response to a jumping bar stimulus is also examined. Two animals are considered: *Cats: the work of Emerson et al on the response of directionally selective (DS) complex cells in cat visual cortex to a white noise signal. *Flies: the work of Franceschini et al on the response of the fly H1 cell to stimulation of two neighbouring receptor cells, and that of Horridge and Marcelja on the fly H1 response to various jumping bar and edge stimuli. Agreement with all three of these experiments cannot be expected, because the results for the two animals are not consistent. There are two main differences: *Time development of the velocity-tuned signal: for the fly H1 cell this generally consists of two sharp (ON and OFF) responses to the second presentation of the bar signal, while for the cat DS cell a more slowly varying signal is observed. *Response to two presentations of the bar signal in the same position (i.e., a 'jumping bar' that jumps zero distance): for the fly H1 cell there is generally no response, while the cat DS cell responds significantly. The velocity-tuned network agrees well with the results for the cat DS cells.