The size of the middle temporal area in primates

The increase in the size of the lateral geniculate body and the primary visual cortex from prosimians to apes and man reflects at an anatomical level the importance of the visual system in primates. In the prestriate cortex visual modalities are processed separately in specialized pathways and areas. This arrangement facilitates the allometric analysis of cortical areas subserving defined visual modalities. Its heavy myelination makes the middle temporal area, a visual cortical field specialized for the detection of moving stimuli, an easily detectable and reliably delineable area in histological sections. The size and position of the middle temporal area can therefore be compared between species, in order to collect quantitative data about the development of a defined visual submodality during primate evolution. The volume of the middle temporal area was measured in 27 primate species. Allometric comparisons show that the middle temporal area is larger in simians than in most prosimians. In Callitrichidae, both the middle temporal area and the striate cortex are well developed. In cebids and cercopithecids, however, the sizes of the middle temporal area and primary visual cortex show divergent trends. Whereas the striate cortex is still enlarging, the size of the middle temporal area is reduced as compared to callitrichids. Previous studies have revealed a close correlation between area striata and neocortex sizes, as well as area striata and lateral geniculate sizes. Such a close correlation does not exist for the middle temporal area versus neocortex or area striata. Therefore, the size of a visual structure serving a special submodality (e.g., the middle temporal area for the detection of moving stimuli) may develop in a species relatively independently from the lateral geniculate and primary visual cortex sizes.