The neural circuit mechanisms for retinal response to motion reversal

Introduction

The ability of anticipating an object’s location is a crucial property for retinal circuitries. However, motion discontinuities, such as motion reversal, is unpredictable. Experimental results have shown that the retina responds to motion reversal with a synchronized burst of activities with fixed latency, regardless of its position relative to the ganglion cell receptive field. In this paper, the authors elucidate the mechanisms underlying this phenomenon.

Neurons respond to motion reversals of the opposite polarity

Neurons tend to respond to the reversal of stimuli with opposite polarity. For instance, the ON cells are more likely to respond to the reversal of dark edges or bars. The reason for this for reversal of edges is simpler: as a dark edge reverts, the resulting stimulation across the bipolar cell receptive field (stimulus convolved with the spatial filter) will increase, thereby activating the ON pathway. The same is true for a bright edge and the OFF pathway. For reversal of a bar-stimulus, the situation becomes more complicated, since the bar consists of a leading and trailing edge. Therefore, the pathway that will be triggered will be dependent on the location of the reversal. If the leading edge has just passed the center of the receptive field, then the ON pathway will be activated due to the same reason as aforementioned. However, if the location is far away from the center, then the effect of the trailing edge dominates, and the OFF pathway is activated instead.

Amacrine cells have little effect on generating response to motion reversal

Using whole-cell patch clamps, the authors demonstrated that excitatory currents into the retinal ganglion cells are the main contribution for the response to motion reversal. This is also confirmed by them blocking inhibition, and showing that the response remains intact.

Bipolar gain control as the main mechanism

The authors showed that the ACM (adaptive cascade model) captures this effect. The ACM incorporates both the ON and OFF pathway, and includes the gain control effects for both the bipolar cells and the ganglion cells. It is shown that the bipolar cell gain control is the main contributor behind this effect. In particular, the response of motion reversal always occurs at a fixed latency, which is partially due to the gain of the bipolar cells requiring some time to recover after stimulation by the original, smooth moving stimulus. Adjusting the time constant for bipolar cell gain control confirms this.

Author: Belle Liu

Original paper: Chen, Eric Y., et al. "The neural circuit mechanisms underlying the retinal response to motion reversal." Journal of Neuroscience 34.47 (2014): 15557-15575.