Monkey Manipulation

In the normal somatosensory pathway, when you touch something, an impulse is sent from:

the skin
to the dorsal root ganglion
to the dorsal columns of the spinal cord
to the nuclei of the dorsal columns in the medulla
to the medial lemniscus of the midbrain
to the ventral posterior nucleus of the thalamus
to the primary somatosensory cortex

To see an enlarged picture of the one above, click here. When you touch something painful, the pathway that the electrical impulse travels is a tad bit different.

If one were to say, amputate the index finger of an adult monkey and record signals reaching the associated part of the monkey's cortical map, the index finger of the homunculus, one would expect no neural activity at all. But when Merzenich and Kaas tried it, they discovered that neurons within the region fired anytime the two fingers adjacent to the amputated one were touched (Merzenich, 1986). From this they concluded that there are existing axon branches that become unmasked when normal input ceases.

Hubel and Wiesel's idea of a hardwired brain went unchallenged because although this unmasking sends new impulses to the previously empty region, there was no new neuronal growth since the impulse was sent only as far as a particular axon could reach.

To Go Back To Theories Main Page