The Retina

Signals from a large number of photoreceptors (rods and cones) converge on bipolar neurones which in turn synapse with retinal ganglion cells. The ganglion cell axons, which form the optic nerve, synapse at the lateral geniculate nucleus (LGN) and these in turn synapse at the visual cortex reaching it via the optic tract. A few axons are involved in the control of pupillary diameter and connect to the Edinger-Westphal nucleus and accessory optic tract and some other axons connect to the superior colliculus.

Bipolar cells can be classified into two types: ON bipolars, which depolarise to light, and OFF bipolars which hyperpolarise to light. Pathways for these two types of cells are anatomically distinct and segregated: the dendrites of ON-centre ganglion cells (excited by light) branch in the inner two thirds of the inner plexiform layer, whereas OFF-centre ganglion cells (inhibited by light) branch in the outer one third of the inner layer (Nelson et al., 1978). On and Off ganglion cells receive their inputs from the corresponding types of bipolar cells. These pathways remain distinct throughout the LGN where there is a predominance of ON-centre cells in layers 6 and 5 and a predominance of OFF-centre cells in layers 4 and 3. This ON/OFF distinction appears to carry on to the striate cortex (Schiller and Colby, 1983). It is thought the distinction between ON and OFF gives rise to orientation

and direction selectivities of cortical cells, however, Schiller et al (1986) suggested that it may be thus organised so as to provide fast transfer of information regarding light increments and decrements with equal sensitivity.

The concept of the 'receptive field' of individual neurones (Fischer, 1973) is important for understanding why spatial contrast influences VEP studies. Neurophysiologists have thought of the visual system in terms of neurons that are driven by stimulation within a small discrete area of the visual field. These receptive fields have been mapped by flashing or moving stimuli such as spots or bars. Signals from photoreceptors of a portion of stimulated (illuminated) retina converge onto two separate pools of the receptive field, these pools are known as centre and surround (Enroth-Cugell and Robson, 1966; Rodieck and Stone, 1965). The ganglion cell response recorded from its axon is a fluctuation (increase or decrease) from the mean background firing rate. When both centre and surround organisation of the ganglion cell receptive fields are illuminated, the modulated response decreases compared to if the illumination is o f the centre alone. As a result of this antagonistic organization, the size of centre relative to the surround establishes the spatial selectivity of individual neurones. Because o f the antagonistic organisation of separate pools for centre and surround portions of the receptive fields of bipolar neurones and ganglion cells, the retinal output does not simply represent the illumination of a patch of retina. Instead the response of bipolar and ganglion cells is determined by spatial contrast, that is a difference in illumination between adjacent retinal areas, rather than by the sum of their separate illuminations.

The distinction between centre and surround and hence their interaction, is sharpest for foveal ganglion cells. The more sharply defined is each mechanism, the more selective is each neuron for stimulus size. For example, bipolar cells of the retina are less selective for stimulus size and shape than cortical neurones. Retinal and LGN neurones are radially symmetrical- that is show little orientation selectivity. A vertical or horizontal slit-like

stimulus or grating will have a similar effect in the retinal and LGN but not so in the cortex (Hubei and Wiesel, 1968).

Ganglion cells differ in the size of their receptive fields. Generally, the closer a neurone to the anatomical fovea, the smaller its receptive field. However, each region of the retina is subserved by a range of ganglion cells with different receptive field sizes. The population of parafoveally located ganglion cells have larger receptive field centres than the foveal ones. It is estimated that the human foveal ganglion cell receptive field is smaller than 20' (Jones and Keck, 1978; Plant et al , 1983; Skrandies, 1984). Pattem element size thus relates strongly to the area of retina giving the strongest response.

The organisation described in the previous paragraphs applies to X ganglion cells (based on cat physiology). Another type of retinal ganglion cell is the Y cell. Y cells are somewhat different in that, opposing forces for centre and surround are never completely in balance. Y ganglion cells have a particular 'subunit' organisation that contributes signals that are not cancelled- i.e. centre and surround responses are not in perfect spatial opposition. (Hochstein and Shapley, 1976). Y cells are particularly sensitive to transient changes in illumination, and to spatial variation occurring in their receptive fields. Hochstein and Shapley (1976), and Victor and Shapley (1979), showed that an essential difference between X and Y cells is in the temporal relationship between stimulus and neuronal response. Y cells and target neurones in the LGN are more sensitive to low contrast stimuli than are X cells (Kaplan & Shapley, 1982). Enroth-Cugell and Robson (1966) noted that in cat, Y cell receptive field centres are generally larger than those of X cells and that X cells are more common in the area centralis of cat retina and thus associated with high resolution vision. Y cells, on the other hand, subserve motion detection and possibly low resolution pattem vision. In general, they have larger diameter axons with faster conduction compared with X cells (Stone and Freeman, 1971; Stone, 1983).

In monkeys, X cells are more common than Y and this is thought to reflect the higher spatial and chromatic acuity in the primate (Shapley and Perry, 1986). In monkey, the X ganglion cells synapse onto neurones in the parvocellular layers in the LGN and are thus called P-cells, and Y cells synapse onto the magnocellular layer and are called M-cells (see LGN section below).