On the Integration of Tactile, Proprioceptive and Visual Signals by the Brain

The integration of different sensory inputs in the brain is crucial not only for taking appropriate motor actions but also for body perception and awareness of the bodily self. Integration occurs in higher areas in the brain usually in areas belonging to the parietal lobe

1) Integrating Vision and Proprioception in Area 5 ( Graziano et. al (2000))

Brodmann Area 5 (or Area 5) is part of the parietal cortex in humans, and in monkeys, is a subdivision of the parietal lobe, occupying primarily the superior parietal lobule. Graziano et. al (2000) studied single neuron responses from Area 5 of monkeys. In their experiments, the arm contralateral to the recording site was outstretched. The real arm of the monkey was covered, and instead a realistic fake hand was kept in view.  The experimental design was 2 x 2

2 x 2 experimental design. Cross indicates fixation spot. The gray arm is the

 fake arm. The real arm is covered in the experiment (Graziano et. al (2000))

Single neuron recordings indicated that the firing rate of individual neurons depends not only on the position of the real arm, but also on the position of the fake arm. The neuron is significantly affected by the  position of the real arm, firing more when he real arm is to the left. Additionally, the firing rate further increases when the fake arm is also to the left. 

Single neuron recording from Area 5.  The firing rate is maximum when 

the real and the fake arms are in congruent position (Graziano et. al (2000))

2) Integrating Vision and Touch in Ventral Intraparietal Area (Duhamel et. al (1998))

The Ventral Intraparietal Area (or VIP) is a discrete area in the depths of the intraparietal sulcus. Duhamel et. al carried out single neuron recordings in the VIP. They found the neurons to possess a bimodal receptive field. Not only did they respond to a combination of visual and tactile stimuli, but the bimodal receptive fields were arranged in an orderly manner.

Bimodal receptive fields of VIP neurons (Duhamel et. al (1998))

Small central visual receptive fields  were associated with small tactile receptive fields on the muzzle, whereas large peripheral receptive fields were associated with  large tactile receptive fields on the side of the head or body. The neurons also demonstrated direction selectivity, in the sense that a visual or tactile stimulus moving in one direction was preferred over the other, and this preferred direction of visual and tactile stimuli coincided in the majority of cells. 

Direction selectivity of VIP neurons (Duhamel et. al (1998))

References

Graziano, M. (2000). Coding the Location of the Arm by Sight Science, 290 (5497), 1782-1786 DOI: 10.1126/science.290.5497.1782

Duhamel JR, Colby CL, & Goldberg ME (1998). Ventral intraparietal area of the macaque: congruent visual and somatic response properties. Journal of neurophysiology, 79 (1), 126-36 PMID: 9425183