The encoded information of retinal ganglion cells is sent via the optic nerve to downstream brain areas. At the optic chiasma, the optic nerves of the left and right eye meet and their optical bers are rearranged into the optic tracts of the left and right hemisphere with ipsi- and contralateral projections from each eye, e.g., the right eye makes ipsilateral and the left eye contralateral projections to the right hemisphere.
In order to understand how the motion-specic responses of retinal ganglion cells could be used for behavioral computations in downstream brain areas one has to observe the areas and nodes where retinal ganglion cells project. Although lower vertebrates as salamanders lack a cortex for more complex visual processing, there are striking similarities in the retinofugal projection areas of mammals and
am-Central projections of retinal ganglion cells phibians. Amphibians share central projection areas in hypothalamus, thalamus, pretectum and tectum with mice and rabbits (Herrick, 1925; Fritzsch, 1980).
These projection areas are related to dierent functions as object recognition, control of sleep-wake cycle and motor control depending on their downstream connections. Amphibians even have an accessory optic system (AOS) which is associated to the vestibulo-motor system. The AOS was rst discovered in rabbit and is important for correcting the retinal slip caused by eye and head movements (Simpson, 1984).
Figure 2.4 Central retinofugal projections of mouse (a) and salamander (b).
Retinal ganglion cells project to hypothalamus, thalamus, pretectum, tectum and acces-sory optic system (AOS). a In mouse, prominent nuclei of these areas are the suprachias-matic nucleus (SCN), the dorsal lateral geniculate nucleus (dLGN), the olivary pretectal nucleus (OPN), the superior colliculus (SC) and the medial terminal nucleus (MTN), respectively. Illustration adapted from Kim et al. (2008). b In salamanders, thalamic pro-jection sites are the lateral and medial neuropil Bellonci (NBl/m), the corpus geniculatum thalamicum (CGT), the uncinate eld (U) and the pretectal area (P). Other important projections go to the nucleus of the basal optic root (nBOR) of the AOS and the optic tectum (TO), adapted from Fritzsch (1980). NBl seems to be the analogue of dLGN while TO and BON correspond to SC and MTN, respectively.
A comparison between mammalian and amphibian systems is important since many genetic tools have been developed in mouse over the past years. For in-stance, cell type-specic genetic markers allow not only to classify dierent gan-glion cell types more accurately but are also useful to identify cell type-specic projection areas (Sanes and Masland, 2014).
In mouse and rabbit, three retinofugal projection areas are of particular impor-tance for visual motion processing, the dorsal lateral geniculate nucleus (dLGN) in thalamus, the superior colliculus (SC) in tectum (May, 2005) and the nuclei of the accessory optic system (AOS) with the medial terminal nucleus (MTN) as the most important one (Fig. 2.4a). The dLGN relays retinal inputs to the cortex for higher visual processing (V1/4) and maybe even conscious perception of motion (area MT in monkeys). The SC is an important node of the oculo-motor system for directing eye and head movements to a point of interest within
the visual scene. These movements are often triggered by the visual detection of moving objects. The object-motion-sensitive W3 cells in the mouse retina were found to project to SC and dLGN and might therefore be important for making voluntary eye movements towards moving objects. The AOS is involved in the optokinetic reex which corrects for image shifts caused by head and involuntary eye movements and which stabilizes the image that is projected onto our retinas (Simpson, 1984).
In the amphibian brain, the lateral neuropil Bellonci (NBl) and the optic tectum (TO) are the analogs of dLGN and SC, respectively (Fig. 2.4b, (Herrick, 1925;
Ebbesson, 1972; Fritzsch, 1980)). The amphibian AOS has only one terminal node, the nucleus of the basal optic root (nBOR) which seems to be the analog of the medial terminal nucleus (MTN) (Simpson, 1984).
Other retinal projection sites in mammals which are not related to motion pro-cessing are the suprachiasmatic nucleus (SCN) and the preoptic area (PO) in hy-pothalamus which are important for regulating the circadian rhythm (Klein et al., 1991) and the olivary pretectal nucleus (OPN) for the pupillary light reex. These areas receive input from intrinsically light sensitive retinal ganglion cells (Hattar et al., 2002, 2006).
Projections of DS cells ON, ON-OFF and OFF DS cells in mouse have been found to project to dierent downstream brain areas. Most ON-OFF and OFF DS cells project to the dLGN for higher visual processing and the SC which is important for involuntary motor responses to motion in visual scenes (Kim et al., 2010; Rivlin-Etzion et al., 2011). ON DS cells project to several nodes of the AOS (Oyster et al., 1980; Simpson, 1984; Dhande et al., 2013). The projections to the AOS provide visual input to the vestibulo-motor system which also controls eye movements to correct for head and body movements (Maekawa and Simpson, 1973). The detection of the motion direction from visual inputs is thought to work as a ne tuning of the rough inputs from the vestibular system. Most neurons of the three terminal nuclei of the AOS in rabbits (lateral, dorsal and medial ter-minal nucleus: LTN, DTN and MTN) and the nucleus of the optic tract (NOT) are directionally tuned. They show similar preferred directions as ON DS cells (Collewijn, 1975; Soodak and Simpson, 1988). DTN and NOT neurons prefer anterior motion while LTN and MTN neurons prefer up- or downward motion. In mouse, only DTN, MTN and NOT were found. ON DS cells with preference for anterior motion projected to DTN and NOT while ON DS cells preferring upward or downward motion projected to the dorsal or ventral MTN, respectively (Dhande et al., 2013). In rabbits, only the sustained ON DS cells seem to project to the
Population motion encoding MTN but not the transient ON DS cells (Yonehara et al., 2008; Kanjhan and Sivyer, 2010). A subtype of ON-OFF DS cells preferring anterior motion also pro-jected to the NOT apart from the usual projections to SC. Note here that upward, downward, posterior and anterior direction with respect to the animal refer to ventral, dorsal, nasal and temporal direction of the projected image on the retina, respectively. In frogs, extracellular recordings nBOR projecting axons revealed that DS retinal ganglion cells responding to image darkening are projecting to the AOS (Bastakov et al., 2015). Retrograde labeling showed that the nBOR-projecting retinal ganglion cells were OFF-type with large to medium sized receptive elds.
Some of them were displaced to the inner nuclear layer (Montgomery et al., 1981;
Cook and Podugolnikova, 2001).