The bipolar cell terminal as a selective spatio-temporal filter

Abstract

The visual signal impinging on the photoreceptor array is passed through a series of spatio-temporal filters at the level of bipolar and ganglion cells before being passed to the brain as~ 20 parallel representations of the visual world. Key processing steps occur within the retina’s two synaptic layers, the outer and inner plexiform layers (OPL, IPL). In the OPL of the mouse retina, dendrites of~ 10-12 different types of bipolar cells integrate visual information forwarded from photoreceptors, under modulatory control of horizontal cells, setting up the initial building blocks from which complex “trigger features” are subsequently extracted by retinal ganglion cells in the IPL. Here, lateral inputs from amacrine cells directly impinge on ganglion cell and bipolar cell synapses further shaping the visual signal. To understand the visual signal harbored in BC terminals is therefore pivotal to our understanding of retinal circuit function. We characterized the ex-vivo light-response properties of mouse BC types by calcium imaging of synaptic terminals in the whole-mount retina and found that BCs can be reliably clustered based on their temporal properties and their propensity to generate spikes alone. The identified functional BC outputs are organized in a kinematopic map across different IPL strata, with fastest projecting centrally and slower cells projecting to either edge. Moreover, the fastest BC types generate all-or-nothing spikes, thereby enhancing time-precise processing of visual signals in the retina. But bipolar cells typically possess in the order of 20-30 individual axonal terminals, with potentially different intrinsic properties, at different electrotonic distance …