Is the retina critical?

Recent works suggest that sensory systems have their internal connectivity fine tuned to be near a critical point of a phase transition in order to maximize their dynamic range. In this work, we use a minimal conductance-based model of the rod pathways in the vertebrate retina to study the effects of rod-rod coupling by gap junctions on the dynamic range of a ganglion cell. The model is composed of multiple arrays of cells, containing single compartment models of rods and cones, rod and cone bipolar cells, AII amacrine cells and ganglion cells. Signal transmission between cells of consecutive layers is made by saturating chemical synapses according to realistic convergence and divergence factors. Gap junctions are used to couple rods and cones in the receptors layer, AII amacrine cells, and cone bipolar cells. We used the model to investigate the effects of different degrees of rod-rod coupling on the dynamic range of the ganglion cell. Our simulation results show that the dynamic range is maximized for a rod-rod connectivity degree below the critical value for bond percolation. The presence of gap junctions between AII amacrine cells is less relevant to dynamic range amplification when compared to the influence of receptor coupling.