The murine absolute visual threshold: behavior & retinal pathways

Abstract

Connexin 36 (Cx36) gap junctions are important for governing the sensitivity of the dark-adapted retina. Despite its importance for physiological sensitivity, the degree to which retinal Cx36 governs the psychophysical absolute threshold is not known. The purpose of this work is to study to what extent inner- and outer-retinal Cx36 governs the absolute visual threshold. In Chapter 2 we developed a one-alternative forced choice (1AFC) task for measuring murine absolute visual thresholds to full-field flash stimuli. We found that our 1AFC task, in conjunction with the theory of signal detection, gave response bias-independent absolute visual threshold estimated. Using this assay, we found that decision criteria are related to response times. In Chapter 3 we used the 1AFC task and the power of transgenic mice to assess the relative contributions of inner and outer retinal Cx36 to the absolute visual threshold. We concluded that inner, not outer, retinal Cx36 is most responsible for governing the absolute visual threshold. In parallel, by testing mice with disrupted rod vision, we determined that rod OFF pathways, and not cones, set the absolute visual threshold in the absence of Cx36. Finally, we studied the impact of Cx36 on temporal summation at absolute threshold, by obtaining thresholds for a range of flash durations. Threshold-vs-duration data was then fit with a model of temporal summation that allowed us to determine whether Cx36 influences the temporal filtering properties of scotopic vision. Our model fits suggest that photoreceptor Cx36 may play a role in temporal processing at absolute visual threshold. Overall, this work sheds new light on the behavioral dynamics and neural underpinnings of rod mediated vision.