The Neural Control of Vision

B. Wiring Diagram of the Visual System

Figure 1 provides a schematic outline of the primate visual areas and their connectivity. The retinal ganglion cells from the nasal half of each retina send their axons to the contralateral half-brain whereas the ganglion cells from the temporal hemiretinae project ipsilaterally. As a result of this arrangement the left brain sees the right visual hemifield and the right brain the left visual hemifield; the neural signals produced by any one object in each eye end up in the same location in the brain.

The output cells of the retina are the retinal ganglion cells. There are more than a million of them in each eye and they connect to several brain structures that include the lateral geniculate nucleus of the thalamus, the superior colliculus, nucleus of the optic tract, and the terminal nuclei. The lateral geniculate nucleus (LGN) forms the gateway to visual cortex where in primates most of the computations for vision are carried out. The superior colliculus plays a central role in eye-movement control which is discussed in the second section of this web page.

In the retina, the LGN, and the visual cortex, each neuron sees only a small portion of the visual field. This small area is called the receptive field (RF) of the cell. The receptive fields of retinal ganglion cells, LGN cells and primary visual cortex cells are very small, often no more than a fraction of a degree in central vision. As we ascend to higher levels in the cortex the receptive fields become progressively larger. The visual field representation in most visual brain structures is laid out in a neat, topographic order.

The circle shown that runs through the nodal point of each eye and touches the point at which the two eyes fixate, is called the Vieth-Muller circle or the horopter. Light originating from any object that is positioned along this circle hits corresponding retinal points in the two eyes from which the ganglion cells connect, via subcortical structures, to corresponding brain regions. Stimuli that are either nearer or further away activate non-corresponding retinal points.