Assignment #5 - Cortical plasticity and “phantom limbs”

Due Friday 3/22

We have discussed in this course how sensory neurons are tuned to different stimulus parameters, and how distributed representations underly our perception of the world. We have also discussed how neurons in the cortex are likely to change their tuning properties as the statistics of the input change - e.g., the tonotopic representation in primary auditory cortex will expand around a certain range of frequencies when an animal is given a discrimination task specifically involving those frequencies. The somatosensory cortex is no exception: neurons are tuned to different aspects of touch - e.g., sustained pressure or vibration - for various points along the body surface, forming a topographic representation of the body surface in the cortex. Importantly, though, this map is “fractured” in some places, meaning that neighboring neurons in the cortex sometimes (though rarely) represent vastly different portions of the body surface - for example, the hand and cheek. Based on these facts, explain how it is that we could observe phenomena such as “phantom limbs,” in which amputees report feeling a sensation on the missing body part when other regions of their body are touched.

You may find the following papers by Ramachandran helpful:

Archives of Neurology (2000), 57: 317-320
Brain (1998), 121: 1603-1630
Proc Roy Soc Lond, B (1998), 353: 1851-1859

Electronic versions are available on the class web page at

http://redwood.ucdavis.edu/bruno/psc129/