Fig. 2. Local circuitry for seeing and hearing.

In retinal (A, B) and cochlear (C, D) circuits, sensory stimuli are detected by primary sensory receptor cells (yellow and orange) and encoded by the activity of primary afferent neurons (blues and greens). In the retina, discrete types of photoreceptors are intermingled in a mosaic, and local interneuron microcircuits process sensory information before reaching a specific retinal ganglion cell (RGC) subtype (A, cross-section, blues and greens). RGCs extend fasciculated bundles of axons through the retina and into the optic nerve via the optic disc (B, wholemount). In the cochlea, each inner hair cell transmits information directly to multiple Type I spiral ganglion neuron (SGN) subtypes (C, cross-section), which extend their peripheral processes in tonotopically organized radial bundles (D, wholemount). Outer hair cells transmit information to Type II SGNs (C, green). Hair cell properties vary continuously along the tonotopic axis (D, inset).