Book T of C
Chap T of C
If each part of the nervous system stores memories of its own role in neural activity, this may help explain a famous series of experiments that baffled psychologists in the 1930s and 1940s. Karl Lashley, one of the world's foremost brain researchers, tried to locate the area in the brain where engrams or memory traces were stored. He sliced or removed sections of rat brains after teaching the rats to run mazes. None of the brain injuries abolished the "maze-running habit," although Lashley tried removing tissue in almost every area that allowed the rat to remain alive. Lashley concluded that memories had to be spread all over the brain, throughout the tissue.
What led Karl Lashley to his conclusion? In what way did Lashley "pick the worst possible laboratory task to study"?
In retrospect, Lashley probably picked the worst possible laboratory task to study, if he was trying to find a specific location that stored a memory. Maze running involves many parts of the brain. At minimum it involves vision (remembering the sight of correct pathways), spatial sense (remembering the direction to turn), olfaction (smelling the cheese and moving toward the more powerful odor), and kinesthesis (the feeling of arms and legs running a certain direction). If one type of clue is eliminated, there are many others remaining, allowing the rat to guide itself to the end of the maze. So Lashley was half right: memory is widely distributed. He was also half wrong, because he assumed memory was unitary and there was one type of memory trace stored all over the brain.
What bias was created by Lashley's research? How was the idea "put to rest" by brain scanning technologies?
Lashley's research was very influential and led to a persistent anti-localization bias among psychologists. For decades, psychologists were inclined to believe that complex mental functions would be spread all over the brain in equal measure, with one area of brain tissue being able to substitute for another after brain injury (how else to explain Lashley's results?).
Lashley's conclusions implied that a skill would never depend upon one tiny area of neurons. However, as it turned out, almost the exact opposite is true. Specialized circuits exist in every part of the brain. But so many locations are involved in a complex act like running a maze that eliminating one part of the brain is not enough to disrupt the entire act.
Posner (1993) noted the "popularly held belief in psychology that the cognitive functions of the brain are widely distributed among different brain areas." However, he concluded, "imaging studies reveal a startling degree of region-specific activity." In other words, most brain tissue is highly specialized. A typical cognitive act does indeed activate many places in the brain, but each area is doing something different from the others: something for which it is specialized.
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Copyright © 2007 Russ Dewey