NEUROLOGY'S dictum—localize the lesion—is being applied with vigor to neuropsychiatric disorders. A century ago, Hughlings Jackson admonished us to avoid the pitfall of equating the deficits after lesions with the functions of the area destroyed. Similarly, Walsh1 rejected the theory of cerebellar "braking" to explain dysmetria after cerebellar lesions. The cerebellum was considered to inhibit overshooting and undershooting. He provided a lovely metaphor for misunderstanding brain function: that of an automobile transmission with a gear tooth knocked off, causing a "clunk" when the drive shaft turns slowly and a vibration at faster speeds. One might conclude that the gear tooth prevents clunks and vibrations, supported by their resolution after the tooth is replaced. The gear teeth, however, transmit power from the drive to the shaft.
Devinsky O. A Mind That Tics. Arch Gen Psychiatry. 2000;57(8):753. doi:10.1001/archpsyc.57.8.753
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