Motion Processing in the Retina

Abstract

The retina is the first stage in the visual pathway, and is responsible for encoding light patterns from the outside environment into a neural code to be sent to the brain. Rather than functioning as a simple camera, signaling the relative light intensity of every "pixel" in space, the retina performs a variety of different computations, such as adaptation to changing contrast and luminance levels or detection of complex motion patterns. To further our understanding of retinal computation, we investigated how the retina encodes two types of behaviorally relevant motion discontinuities: motion onset and motion reversal. First, we found that a subset of retinal ganglion cells, fast OFF cells, produce a stronger response to motion onset than to smooth motion and that this response only requires the OFF pathway. We also developed a phenomenological model, the Adaptive Cascade Model, which can accurately reproduce the retinal responses to motion onset, smooth motion, and object appearance over a wide range of stimulus conditions.

In regards to motion reversal, previously described work (Schwartz et al., 2007b) found that a significant fraction of retinal ganglion cells respond to motion reversal with a synchronized burst of firing of fixed latency in regards to reversal. Continuing to characterize this phenomenon, we found that reversal responsive cells arise from three main cell types, fast OFF, fast ON, and medium OFF cells and that reversal responsive cells were generally only reversal responsive to one type of reversal. Input currents to ganglion cells during smooth motion and motion reversal were found to be predominately excitatory and could also exhibit a fixed latency response to reversals. Finally, we applied the Adaptive Cascade Model to predict the motion reversal response. We found that the Adaptive Cascade Model could indeed capture the reversal response, setting the groundwork for the possible development of a model that can account for the retinal responses to all types of object motion.