Somatosensory Cortex of Macaque Monkeys is Designed for Opposable Thumb

The evolution of opposable thumb has enabled fine grasping ability and precision grip, which led to the capacity for fine manipulation of objects and refined tool use. Since tactile inputs to an opposable thumb are often spatially and temporally out of synch with inputs from the fingers, we hypothesized that inputs from the opposable thumb would be processed in an independent module in the primary somatosensory cortex (area 3b). Here we show that in area 3b of macaque monkeys, most neurons in the thumb representation do not respond to tactile stimulation of other digits and receive few intrinsic cortical inputs from other digits. However, neurons in the representations of other digits respond to touch on any of the four digits and are significantly more interconnected in the cortex. The thumb inputs are thus processed in an independent module, whereas there is significantly more inter-digital information exchange between the other digits. This cortical organization reflects behavioral use of the hand with an opposable thumb.