Spikes and ribbon synapses in early vision

• The retina of vertebrates and invertebrates encodes visual signals with both spikes and graded signals.
• In vertebrates, ‘digitization’ of the visual signal begins in the axonal system of bipolar cells.
• Spikes and graded signals are both transmitted through ribbon synapses.

Image processing begins in the retina, where neurons respond with graded voltage changes that must be converted into spikes. This conversion from ‘analog’ to ‘digital’ coding is a fundamental transformation carried out by the visual system, but the mechanisms are still not well understood. Recent work demonstrates that, in vertebrates, graded-to-spiking conversion of the visual signal begins in the axonal system of bipolar cells (BCs), which transmit visual information through ribbon-type synapses specialized for responding to graded voltage signals. Here, we explore the evidence for and against the idea that ribbon synapses also transmit digital information. We then discuss the potential costs and benefits of digitization at different stages of visual pathways in vertebrates and invertebrates.