The ability to detect moving objects in the visual scene is fundamental to our daily survival. It originates within the sensory organ, the retina, where a significant proportion of retinal ganglion cell (RGC) responses is dedicated to detecting motion in different directions (Figure1).
The first evidence for such retinal computation was established in the rabbit retina by Barlow and colleagues in the 1960s. By moving small spots of light across parts of the receptive field of a direction-selective RGC (DSGC), they showed that a moving stimulus can elicit strong spiking of action potentials in one direction (henceforth referred to as the preferred direction) but not in the opposite, or null, direction (Figure 2) (1).
In fact, there are different types of DSGCs and they can be segregated based on two criteria. First, they either respond to both light onset and offset (ON-OFF), or just to the former (ON). Second, they prefer different directions of motion. These functional characteristics give rise to four types of ON-OFF DSGCs and three types of ON DSGCs. The ON-OFF DSGCs (ooDSGCs) each detect motion in one of four cardinal axes while the ON DSGCs detect movement in the dorsal, ventral and nasal directions (2) (Figure 3).
Since then, advances in mouse genetics has enabled the study of analogous DSGC populations in the mouse retina, and also led to the discovery of an OFF DSGC type that detects upward motion in the visual scene (3). These new tools complement existing technologies like electron microscopy and dye-filling techniques by allowing specific types of RGCs to be genetically labeled and manipulated. This facilitates the search for morphological correlates of their physiological function. In the following segments, I shall describe the morphology and physiological properties that underlie the direction-selective responses of each DSGC type.
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