Rod + cone signal interference impairs mesopic motion discriminability in a model circuit.

Under mesopic conditions, such as dawn or dusk, signals from both rod and cone photoreceptors contribute to perception. These parallel inputs are combined within the retina before being sent to subsequent visual areas. The integration of these kinetically distinct parallel signals poses unique challenges for human vision. Though previous behavioral studies have found that dim lighting conditions specifically impair motion perception in human subjects, the origin of this dependence is unclear. In the present study, we created a model circuit that predicts ganglion cell responses to moving stimuli by incorporating electrophysiologically derived circuit components into a Hassenstein-Reichardt correlator, a classical motion-detection model. The model circuit demonstrates that interactions between rod- and cone-derived signals negatively impact the encoding of the direction of a moving object under mesopic conditions. Furthermore, we found that motion discriminability improved if the model circuit was only sensitive to the cone-activating components of the stimuli. We conclude that rod+cone signal interference occurring at the lowest level of vision has an impact on motion direction discrimination, a higher-level task with relevance for behavior.