[Number of neurons and synapses in the visual cortex of different species]

The number of neurons under 1 mm2 of visual cortex (area 17) is about 200 000 in monkey and man, and it varies between 45 000 and 70 000 in non-primates which have been studied. The number per hemisphere increases with the surface of area 17, passing from less than 1 million in mouse to about 538 million in man. The number of synapses under 1 mm2 of visual cortex has been estimated by different authors at between 480 million (mouse) and 1270 million (rat) : the number per hemisphere increases with brain size from 32 billion in rat to 3 084 billion (x10(9)) in man. The number of synapses per neurons tends to be higher in species with fewer neurons per mm3. Our laminar study in monkey shows this correlation at the level of each lamina : those having the largest number of neurons per mm3 have the least number of synapses per neuron.