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Editors Selection IGR 8-4

Examination methods: Receptive fields

Comment by Ronald Harwerth on:

13654 Expansion of visual receptive fields in experimental glaucoma, King WM; Sarup V; Sauve Y et al., Visual Neuroscience, 2006; 23: 137-142

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During recent years the role of adult plasticity as a mechanism of recovery from CNS damage has become well established. Adult plasticity in somatosensory and motor systems has been studied extensively and, in most cases, the results have demonstrated considerable restoration of function. Much less is known about compensation for neural damage in the retina or visual cortex, and previous investigations have involved acute, focal lesions, that are of quite different from neural defects caused by glaucoma. Therefore, with respect to glaucoma, the investigation by King et al. (411) is an important study of the mechanisms of adult plasticity that occur following a protracted, non-localized loss of retinal neurons from elevated intraocular pressure. For these studies the authors induced experimental glaucoma in Wistar rats by cauterization of the episcleral veins, and after several months of sustained pressure elevation (25-30 mmHg, compared to normal pressures of 16-18 mmHg) the physiological effects were assessed by measurements of the sizes of visual receptive-fields in the superior colliculus. The principle finding was that collicular receptive fields driven by eyes with experimental glaucoma were significantly larger than receptive fields for control eyes. Further, the results showed that the relative amount of receptive field expansion was proportional to the degree and duration of intraocular pressure elevation. These findings suggest that the size of the somas and dendritic fields of surviving retinal ganglion cells have been enlarged as a result of glaucomatous damage to adjacent ganglion cells.

The size of the somas and dendritic fields of surviving retinal ganglion cells have been enlarged as a result of glaucomatous damage to adjacent ganglion cells

The results in the rodent model seem solid, but because of differences in the visual pathways of rodents and human patients, it is not clear that there is a direct application to clinical glaucoma. In the rat, there is almost 100% decussation of the optic nerve axons and, thus, unilateral ocular innervation of the superior colliculus. In this sense, the innervation of the superior colliculus is similar to the unilateral innervation of the somatosensory and motor systems where strong effects of adult plasticity have been shown. In contrast, similar studies in subjects with high degrees of binocular innervation to the visual cortex (cats and monkeys) have not found solid evidence of neural compensation from adult plasticity, unless the retinal lesions were bilateral or the fellow eye was enucleated. It, therefore, must be determined whether the changes in ganglion cell dendrites and soma that have been described in a rodent model of experimental glaucoma are expressed in clinical glaucoma.

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