In all mammals, much of the neocortex consists of orderly representations or maps of receptor surfaces
Figure 3 Some of the currently proposed areas of neocortex in rats. Like most other mammals, rats have a primary somatosensory area, S1, and a secondary somatosensory area, S2, a primary visual area, V1, and a secondary visual area, V2, a primary auditory area A1, and a primary motor area, M1. There is also evidence for a secondary or premotor area, M2, a dysgranular (Dys) somatosensory area with a less developed or "dysgranular" layer IV, and a parietal ventral (PV) somatosensory area. S1, S2, and PV each represent the receptors of the skin in an orderly manner. The locations of activating input relayed from major body parts are indicated for S1. Other areas of neocortex in rats have been proposed, but they are not indicated. Mammals, such as monkeys and humans, with larger brains and more neocortex have the same areas as those proposed here for rats, at least a few additional areas that they share with rats, and a number of additional areas that are not found in rats. Thus, species differ in numbers of cortical areas, as well as the proportional extent of neocortex. Piriform cortex and the olfactory bulb are indicated for reference.
(see Fig. 3). Visual areas of each cerebral hemisphere represent the contralateral half of the visual field via the nasal hemiretina of the contralateral eye and the temporal hemiretina (actually a smaller and species variable portion that is less than half) of the ipsilateral eye. Somatosensory areas represent the receptors of the contralateral body surface. Auditory areas represent tones from high to low frequencies as they activate successive locations along a row of cochlear hair cell receptors in the contralateral and ipsilateral ears. Other cortical areas relate to taste and vestibular input. Smell is relayed to piriform cortex from the olfactory bulb. The cortical representations are activated from the thalamus via orderly, topographic projections from the receptor sheets to subcortical nuclei that relay to the thalamus, or to the thalamus directly, and then to cortex. The maps closely, but not precisely, reflect the order of the receptors on the receptor sheet. The maps might have disruptions so that receptors next to each other are represented in nonadjacent parts of the map, and the maps might have modular repetitions related to receptor class or eye or ear of origin (see later discussion). In addition, the receptive fields, the portion of the receptive sheet that activates a neuron, become larger for neurons in each successive level of a series of connected nuclei and areas, so that the maps in initial subcortical structures are the most detailed, the maps in primary sensory areas are somewhat less detailed, and secondary and higher order sensory areas contain progressively less detailed maps. These changes in receptive field size and the detail of map topography are compatible with the changing functions of neural circuits in representations that are early or late in a processing sequence. The number of topographic maps of a sensory surface in cortex varies with the number of sensory areas, with the early areas in sequences being most topographic and later areas often reflecting little of the order of the receptor sheet, but possibly representing in a systematic manner some product or feature that has been derived from other sensory areas. Topographic maps place neurons that most interact within the area close together and provide a suitable substrate for the common spatiotemporal computations of neural circuits.
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