Close but not quite

I wonder how often we are almost right but not quite. It seems to be a fairly common trap in biology.

Motor somatoIt has been thought for many years (140+ years) that the primary motor cortex (lying across the top of the head) mapped the muscles of the body and controlled their contractions. From this we got the comical homunculus with its huge lips and hands on a spindly little body. Each small area on this map was supposed to activate one muscle.

A recent paper by Graziano, Ethological Action Maps: A Paradigm Shift for the Motor Cortex (here), argues that this is not as it appears. What is being mapped are actions and not muscles. Here is the abstract:

The map of the body in the motor cortex is one of the most iconic images in neuroscience. The map, however, is not perfect. It contains overlaps, reversals, and fractures. The complex pattern suggests that a body plan is not the only organizing principle. Recently a second organizing principle was discovered: an action map. The motor cortex appears to contain functional zones, each of which emphasizes an ethologically relevant category of behavior. Some of these complex actions can be evoked by cortical stimulation. Although the findings were initially controversial, interest in the ethological action map has grown. Experiments on primates, mice, and rats have now confirmed and extended the earlier findings with a range of new methods.

Trends – For nearly 150 years, the motor cortex was described as a map of the body. Yet the body map is overlapping and fractured, suggesting that it is not the only organizing principle. In the past 15 years, a second fundamental organizing principle has been discovered: a map of complex, meaningful movements. Different zones in the motor cortex emphasize different actions from the natural movement repertoire of the animal. These complex actions combine multiple muscles and joints. The ‘action map’ organization has now been demonstrated in primates, prosimians, and rodents with various stimulation, lesion, and neuronal recording methods. The action map was initially controversial due to the use of electrical stimulation. The best argument that the action map is not an artifact of one technique is the growing confirming evidence from other techniques.”

Even settled science when it is neuroscience should be taken with a grain of salt. Any part of it could be something similar but not the same.

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