✓ The topographical anatomy of the primary visual cortex in man was studied by macroscopic identification and measurement of the line of Gennari on coronal sections from 52 hemispheres collected at autopsy. Computer analysis of these data provided detailed, quantitative information concerning the amount, variability, and distribution of the striate area on the surface of the occipital lobe, and within the calcarine fissure and other sulci. This information is particularly applicable to the development of a functional visual prosthesis for the blind based on stimulation of the visual cortex. Taking conservative values for two-point discrimination (3 mm) on the cortex based on previous observations made during neurosurgical operations, and the surgically accessible areas available for stimulation reported in this study, it seems clear that a useful number of electrodes could be implanted in virtually every patient.
Histopathological changes of the cerebral cortex in response to small, penetrating metal and non-metal implants were analyzed by means of light and electron microscopy. The needle-shaped implants were left in place during all stages of histological preparation and embedded in plastic together with the cortex. Changes of the brain-implant boundary were classified as non-reactive, reactive, or toxic, according to the reactive cellular constituents. Among the non-reactive materials were several plastics and metals such as aluminum, gold, platinum, and tungsten. The boundary of these implants displayed little or no gliosis and normal neuropile with synapses within 5 micron of the implant's surface. The boundary of reactive materials such as tantalum or silicon dioxide was marked by multinucleate giant cells and a thin layer (10 micron)) of connective tissue. Toxic materials such as iron and copper were separated from the cortical neuropile by a capsule of cellular connective tissue and a zone of astrocytosis. Cobalt, a highly toxic material, produced more extensive changes in the zones of connective tissue and astrocytes. These results indicate that a variety of materials are well tolerated by the brain and could be used in the fabrication of neuroprosthetic devices.
The feasibility of developing a functional auditory prosthesis, or “artificial ear” to restore hearing to the totally deaf is discussed. The approach taken involves connecting electrodes to the auditory cortex of the brain. Previous work by other groups has concentrated on stimulation of the cochlea or auditory nerve. However, stimulation of auditory cortex bypasses the entire peripheral auditory system and this approach may thus be applicable to a wider group of people, including those with hearing loss caused by lesions of the auditory nerve, or even central pathways. Establishment of the feasibility of any such sensory prosthesis requires demonstration of meaningful information transfer in human subjects, demonstration that chronic implantation and stimulation can be achieved without damaging the brain, and the demonstration of engineering feasibility at reasonable cost. Each of these areas is discussed separately. Although progress to date gives reason for optimism, it must be stressed that large amounts of work remain to be done and the successful development of such a system may take many years. One principal limitation is the small number of patients suitable for experimentation. Other investigators are accordingly encouraged to take advantage of the rare clinical opportunities which permit progress in such areas.
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