H2 Anatomy and physiology of the visual pathway

Despite almost a half century of experimentation, the best site for implementing a visual neuroprosthesis has yet to be resolved. As will be seen in subsequent sections, numerous sites have been investigated, each with its relative merits and shortcomings. To provide a better understanding of each neuro-prosthetic approach and to give grounds for comparing the competing approaches, an awareness of the underlying neural system is necessary. As details of the visual pathway are readily available in any elementary anatomy and physiology text, the discussion here is limited to a short review highlighting the potential sites for a neuroprosthesis. In particular, we have chosen to highlight the neural organization of these sites and how this organization relates to a neuroprosthetic application. The majority of the information provided in this section comes from either well-known neuroscience texts12 or the excellent Web site http://www.webvision.med.utah.edu.

The visual pathway, highlighting the points where vision neuroprosthe-ses have been or could be implemented is shown schematically in Figure 11.1. Light entering the eye falls upon the retina, located at the back surface of the eye. Here, photoreceptor neurons convert the electromagnetic energy of the light into electrochemical signals. This is the first stage of a series of

Figure 11.1 Schematic view of the visual pathway, highlighting points where a visual neuroprosthesis has been or could be investigated. This illustration also shows the concept of visuotopy with the letters A and B. The visuotopic organization of the pathway results in nearby objects in visual space being represented in nearby neurons throughout the visual pathway.

Figure 11.1 Schematic view of the visual pathway, highlighting points where a visual neuroprosthesis has been or could be investigated. This illustration also shows the concept of visuotopy with the letters A and B. The visuotopic organization of the pathway results in nearby objects in visual space being represented in nearby neurons throughout the visual pathway.

retinal neurons that process features of the visual scene. The axons of the last of the series, the retinal ganglion cells, are collected together into the optic nerve (ON). These axonal fibers are reorganized at the optic chiasm (OC) and then project to a number of subcortical structures through the optic tract (OT). The majority of these axons form synapses in the lateral geniculate nucleus (LGN) of the thalamus, where another series of neurons further process the visual scene. In turn, the axons of the LGN neurons project through the optic radiation (OR) to the cerebral cortex. Here, a hierarchical scheme of visual processing occurs over much of the posterior region of the occipital cerebral cortex. The regions subserving this processing are collectively called visual cortex (VC).

A number of themes are common to the neural organization at all sites along the visual pathway. The first of these is that, for any neuron along the pathway, its receptive field describes the type of the visual stimuli that causes the neuron to respond. The receptive field characterization of a neuron details the nature of the visual stimulus (location in visual space, shape, size, intensity, color, etc.) that optimally drives the neuron and how the neuron's activity changes for other than the optimal stimulus. For example, a particular photoreceptor neuron may reserve its greatest response for a small blue spot of light at a particular location relative to central vision. Most probably, external electrical stimulation of the same neuron could evoke in a similar percept. A related and very critical theme for a neuroprosthesis is that the map from visual space to neural space is visuotopic. That is, the neurons at any site along the visual pathway are arranged so that their receptive field locations form an organized and approximately linear map of visual space. This implies that nearby objects in visual space evoke activity in nearby neurons. Hence, the outline of a rectangle presented in visual space will result in activity in a similarly shaped arrangement of neurons. The arrangement of neurons may be differentially stretched in each axis, rotated, and evenly slightly warped but still would appear as the outline of a rectangle. Presumably, external electrical stimulation of the same ensemble of neurons will result in the perception of the outline of a rectangle. A last theme is that the visual pathway represents a massively parallel method of signal processing. Associated with this parallelization are a number of distinct processing pathways found in primates, including humans. The principal two pathways, the M pathway (for magno, or large) and the P pathway (for parvo, or small), begin at the retina and are segregated throughout much of the visual pathway. It is thought that the M and P pathways represent two broad features of an object in visual space: where the object is located and what the object is, respectively. At this point, it is unclear whether a neuropros-thesis should (or could) preferentially evoke activity in the M pathway, the P pathway, or both pathways in order to provide the best percept. Nevertheless, due to the segregation of the two pathways, there could be a preference for stimulating one pathway over the other at various points along the visual pathway.

0 0

Post a comment