Topographic map (neuroanatomy)

A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems.

The visual system refers to the part of the central nervous system that allows an organism to see. It interprets information from visible light to build a representation of the world. The ganglion cells of the retina project in an orderly fashion to the lateral geniculate nucleus of the thalamus and from there to the primary visual cortex(V1); adjacent spots on the retina are represented by adjacent neurons in the lateral geniculate nucleus and the primary visual cortex. This projection pattern has been termed topography. There are many types of topographic maps in the visual cortices, including retinotopic maps, occular dominance maps and orientation maps. Retinotopic maps are the easiest to understand in terms of topography. Retinotopic maps are those in which the image on the retina is maintained in the cortices (V1 and the LGN). In other words, if a specific region of the cortices was damaged that individual would then have a blind spot in the real world, they would not be able to see the bit of the world that corresponded to the retina damage. Orientation maps are also topographic. In these maps there are cells which have a preference to a certain orientation, the maximum firing rate of the cell will be achieved at that preference. As the orientation is moved away from the firing rate will drop. An orientation map is topographic because neighboring neural tissues have similar orientation preferences.

The retinotopic layout of the visual topographic map refers to the fact that afferent connections from the retina that hold their particular order through sub-cortical structures, to V1, and throughout other cortical visual regions. The primary visual cortex (V1, Brodmann's area 17) is the first cortical area to receive visual input. The stria of Gennari – a set of heavily myelinated, horizontally projecting axons within the termination zone of lateral geniculate nucleus (LGN) input to V1 – provides an anatomical marker particular to V1.

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