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selective regions of temporal cortex in macaque monkeys and humans resemble domains (Tsao et al., 2003, 2008a; Pinsk et al., 2005; Rajimehr et al., 2009). Likewise, the large visual area termed V4 or DL has been divided into large regions or domains of neurons that are either color selective or orientation selective (Tanigawa et al., 2010), although these large regions might also be considered separate cortical areas (Cusick and Kaas, 1988; Stepniewska and Kaas, 1996).

How Do Columns and Modules Emerge in Development?

A number of factors likely contribute to the functional organization of cortex, but at the modular level, activity-dependent selection of coactive afferents together with cellular signals that are position dependent probably are two of the most important variables (Erzurumlu and Kind, 2001; Sur and Leamey, 2001; Kaas and Catania, 2002). There is considerable evidence to support this conclusion, but some of the most impressive evidence comes from studies that created three-eyed frogs (Constantine-Paton and Law, 1978; Katz and Constantine-Paton, 1988). In frogs, each optic tectum normally receives inputs from only the contralateral eye, but when a third eye is added experimentally to one side of the head during embryonic development, both eyes on that side compete for territory in the same contralateral optic tectum. The projections from each of these eyes respond to molecular signals that tend to produce the same retinotopic pattern in the optic tectum, but local groups of tectal neurons favor inputs from one eye or the other. The result is that the afferents from the two eyes form alternating bands or stripes that resemble the ocular dominance bands in cats and anthropoid primates. The borders between these bands in the optic tectum and visual cortex correspond to locations where abrupt differences in activity patterns occur, and they do not develop or they degrade when activity is blocked (Cline et al., 1987). Obviously, the ability to form ocular dominance bands did not evolve via natural selection in the optic tectum of frogs for some future function. Instead, the developmental factors that produced these columns were present for other reasons that are not clear but apparently are widely important in nervous system development (Katz and Constantine-Paton, 1988). The capacity for module formation seems to be inherent in all cortical tissue, as well as in other tissue such as the optic tectum or superior colliculus, where inputs of different activation patterns compete for location with an overall global map. Thus, ocular dominance bands and other configurations, as well as orientation modules and other types of columns, including those based on discontinuities of the receptor sheet, have emerged independently in several lines of mammalian evolution. For some of these types of modules, asking what they do (Horton and Adams, 2005) may be the wrong



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