Some people have trouble remembering members of a specific class of objects. Particularly semantic class members are forgotten, for example names, numbers, or animal names. However, objects themselves may not be recognized, such as small tools, or human faces, so the deficit can be perceptual rather than purely or necessarily linguistic. About 20 classes seem to be affected.
The deficit for each set of forgotten category members has been linked to damage in a specific area of the brain, due mainly to injuries such as falls, infections or strokes. There is often a logical link between other functions of that brain area and the class of lost memories; for example, damage near the motor area of the brain may be associated with forgetting action verbs, like sweeping.