lesion studies

Meaning: A lesion is damaged or destroyed tissue. Researchers compare performance after damage with performance before it, or with an unaffected control group. In humans the damage is usually natural (stroke, tumor, trauma, surgery); in animals it can be made deliberately and precisely. The reasoning is that if a function is lost or impaired after damage to a region, that region probably contributes to the function. Classic cases are Phineas Gage (frontal lobe damage, changes in personality and impulse control), patients with Broca's or Wernicke's area damage (different language deficits), and H.M. (hippocampal removal, severe anterograde amnesia with procedural memory largely intact).

On the MCAT: A passage describes damage to a structure and asks you to predict the deficit, or asks you to identify the method. Know how lesion studies compare with fMRI, PET, and EEG, which record activity and show correlation, while lesions suggest a region is needed. A double dissociation (damage to region A impairs function X but not Y, damage to region B does the reverse) is stronger evidence that the functions rely on separate regions.

Common confusion: A deficit after a lesion does not prove the function is stored only in that spot. The region may be one part of a network, damage can also affect nearby tissue or fibers passing through, and the brain may partly compensate through plasticity. Human lesions are also uncontrolled, so conclusions are tentative.