2B), light enters through the pupil and shadows are cast by retinal blood vessels. Once the baby monkey opens its eyes ( Fig. 2A), the right cortex, for instance, receives binocular input organized into nascent columns, except at the representation of the left optic disc, where input comes exclusively from the right eye. How do angioscotoma representations form?Īngioscotoma representations cannot form before birth, because the uterine environment is dark. In the periphery, a large pale zone corresponded to the representation of the far nasal retina of the left eye (monocular crescent). It stained darkly, because the left eye was enucleated in this animal. The left eye’s optic disc representation appeared as a large, homogeneous zone of CO activity in the right cortex. CO montage of this layer revealed a fuzzy pattern of fine ocular dominance columns ( Fig. Ocular dominance is expressed most strongly in layer 4C of striate cortex.
![eye for an eye cast eye for an eye cast](https://i.pinimg.com/736x/24/dd/63/24dd63f9c61ee65a6c0de019920eea5f.jpg)
Flatmount preparations enhance the likelihood of detecting subtle metabolic patterns, such as the optic disc representation, which lies buried in the calcarine sulcus ( 24). After perfusion, striate cortex was stripped from the white matter, flattened in a single piece, and sectioned tangentially to the surface.
![eye for an eye cast eye for an eye cast](http://www.cinemapassion.com/covers_temp/covers3/Oeil_pour_Oeil___Eye_for_an_eye__Canadienne_-14485717102012.jpg)
The ocular dominance columns are revealed by this approach because metabolic activity is reduced in the missing eye’s columns. To examine their pattern more closely, we enucleated one eye and stained striate cortex for the mitochondrial enzyme cytochrome oxidase (CO) ( 21- 23). They are present, in fact, but often appear poorly developed and irregular ( 20). The squirrel monkey was reported originally to lack ocular dominance columns ( 18, 19). We have found a representation of angioscotomas in striate cortex of the squirrel monkey ( Saimiri sciureus).
![eye for an eye cast eye for an eye cast](https://static.rogerebert.com/uploads/review/primary_image/reviews/eagle-eye-2008/hero_EB20080925REVIEWS809250304AR.jpg)
The hemoglobin in red blood cells absorbs light, casting a shadow from blood vessels in the inner retina onto photoreceptors located underneath. With refinement of perimetric techniques, subsequent investigators have plotted these so-called angioscotomas over wide portions of the visual field ( 16, 17). Helmholtz discovered while plotting his blind spot that he could discern the proximal stumps of large blood vessels emerging from the optic disc ( 15). We now show that visual deprivation occurs routinely in normal primates, affecting ocular dominance patterns. It has not been shown that under natural circumstances, competition between the two eyes has an important influence on the appearance of the ocular dominance columns.Ĭortical representation of retinal vessels The relevance of these experiments, however, can be questioned on the grounds that suture of the eyelids is an artificial manipulation, creating an extreme imbalance in the stimulation of each retina. Here, an imbalance in neuronal activity has a profound influence on the shape and size of the ocular dominance columns. Raising a newborn animal with monocular eyelid suture, to mimic congenital cataract, causes shrinkage of the deprived eye’s columns and expansion of the normal eye’s columns ( 11- 14). Although neuronal activity has a disputed role in column induction, it certainly can influence column morphology at a later stage in development. This finding suggests that column formation is ordained by intrinsic molecular cues, and not by patterns of neuronal activity.
![eye for an eye cast eye for an eye cast](https://financerewind.com/wp-content/uploads/2020/07/Qeye.jpg)
These small imbalances may be amplified by competitive synaptic interactions between geniculocortical afferents to generate a mature system of ocular dominance columns ( 6- 9).ĭoubt has been cast on this explanation for column development by recent experiments showing that ocular dominance columns appear in ferrets despite early bilateral enucleation ( 10). According to this idea, the correlated discharge of neighboring ganglion cells, perhaps generated by spontaneous waves of retinal activity ( 4, 5), induces local cortical imbalances in the density of geniculate afferents driven by each eye. Some experiments show an essential role for neuronal activity, because infusion of tetrodotoxin into both eyes seems to prevent the emergence of ocular dominance columns ( 3). The formation of these columns has been scrutinized in the hope of elucidating the processes that guide the development of the cerebral cortex ( 2). In many higher species, the geniculate afferents serving right and left eyes are segregated in the brain in layer 4 of primary (striate, V1) visual cortex into a mosaic of interdigitated inputs called ocular dominance columns ( 1).