DIRECTIONALLY SELECTIVE MOTION DETECTION IN THE SUSTAINING FIBERS OF THE CRAYFISH OPTIC-NERVE - LINEAR AND NONLINEAR MECHANISMS

1. Directional selectivity of crayfish sustaining fibers was examined with drifting sine wave gratings and with intracellular and extracellu lar recordings. Directionality was measured for variations in stimulus contrast, spatial frequency, and temporal frequency. 2. Sustaining fi bers exhibit directional selectivity in the magnitude of the compound postsynaptic potential (PSP), the impulse frequency modulation respons e, and the mean firing rate. The mean synaptic potential is insensitiv e to direction. The directionality of the mean impulse rate appears to arise by rectification in the voltage-to-impulse transduction. 3. The preferred directions of three identified sustaining fibers are simila r to those of head-down optomotor neurons to which these sustaining fi bers project. 4. The modulatory response, elicited by gratings driftin g in the preferred direction, increases linearly with contrast until s aturation (typically at a contrast of 0.5): where maximum directional selectivity obtains. 5. The magnitude of the directional response is a band-pass function of spatial and temporal frequency and exhibits rev ersal of directionality (i.e., aliasing) at high spatial and temporal frequencies. The results imply a spatial sampling interval of 4.5 degr ees and a temperature-dependent inhibitory delay of 40-90 ms. The PSP modulation response shares several features with that of neighboring t angential (Tan1) neurons. 6. A qualitative model is proposed for the t ransformation of a phase-sensitive, linear directional response to a p hase-insensitive and nonlinear time-averaged response, based on the fu nctional connections from Tan1 neurons to sustaining fibers to optomot or neurons. The model includes a threshold rectification, a synaptic b andpass filter, and differences in temporal phase among converging mod ulatory signals.