Inhibitory Input to the Direction Selective Ganglion Cells 1 Is Saturated at Low Contrast 2 Mikhail

18 Direction selective ganglion cells (DSGCs) respond selectively to motion towards a “preferred” 19 direction, but much less to motion towards the opposite “null” direction. Directional signals in the 20 DSGC depend on GABAergic inhibition, and are observed over a wide range of speeds, which 21 precludes motion detection based on a fixed temporal correlation. A voltage-clamp analysis, 22 using narrow bar stimuli similar in width to the receptive field center, demonstrated that 23 inhibition to DSGCs saturates rapidly above a threshold contrast. However, for wide bar stimuli 24 that activate both the center and surround, inhibition depends more linearly on contrast. 25 Excitation for both wide and narrow bars was also more linear. We propose that positive 26 feedback, likely within the starburst amacrine cell or its network, produces steep saturation of 27 inhibition at relatively low contrast, which renders GABA-release essentially contrast and speed 28 invariant, and thereby enhances the signal-to-noise ratio for direction selective signals in the spike 29 train over a wide range of stimulus conditions. This mechanism enhances directional signals at 30 the expense of lower sensitivity to other stimulus features such as contrast and speed. This 31 renders GABA-release essentially contrast and speed invariant, which enhances directional 32 signals for small objects, and thereby increases the signal-to-noise ratio for direction selective 33 signals in the spike train over a wide range of stimulus conditions. The steep saturation of 34 inhibition confers to a neuron immunity to noise in its spike train because when inhibition is 35 strong, no spikes are initiated. 36