Experimental Manipulations of the Development of Ordered Projections in the Mammalian Brain
The ordered representation of the retina in the tectum of hamsters can be influenced by experimental manipulation of factors intrinsic to the developmental process, such as the direction and timing of arrival of retinal fibers. The orientation of the retinotopic map in the tectum with respect to the neural axes, the amount of representation of retinal subareas on the tectal surface, and the laminar specificity of retinal terminals in tectum are differentially affected by these developmental factors.
If the amount of tectal tissue is reduced at birth in hamster by a large amount, only a portion of visual field comes to be represented. The area of visual field represented is in every case lower nasal visual field, regardless of whether caudal, rostrolateral, central, or superficial tectal tissue is removed. This asymmetry in surface representation is also reflected in the laminar distribution of retinal fibers in the tectum. A source for the inhoniogeneity in visual field representation may be direction of optic tract arrival in tectum, which begins at the rostrolateral margin of the tectum, where lower nasal visual field is represented.
The polarity of the retinotopic map in tectum—its orientation with respect to the neural axes—may be dissociated from direction of fiber arrival, for if fibers are induced to enter the tectum medially, opposite to their normal entry point, a retinotopic map of normal order and polarity develops. Classes of mechanisms that account for both of these observations are discussed.
KeywordsCobalt Retina Neurol Hunt Sorting
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