What Has Four Legs . . .

Neurologists and psychologists learned long ago that brain injuries can be a powerful tool for investigating how human thought and memory are organized. A case study by two Johns Hopkins researchers, reported in Nature, is but the latest example. A 70-year-old retired librarian who suffered such an injury developed a remarkable symptom: she lost the ability to name animals, though % she could still name other living things, such as plants, and inanimate objects. Nor could she assign physical attributes to animals -- she could not, for example, answer the question "What color is an elephant?" Yet she could answer nonphysical questions, about whether a given animal lived on land or in the sea, or whether it was normally kept as a pet. And when shown a picture of, say, a lion with a horse's head, she could tell right away that the two didn't match.

So what does it mean? Evidently, the brain has distinct systems for classifying different types of objects, and it stores categories of information, such as physical versus nonphysical characteristics, in different ways. Moreover, the study shows that the brain has two classification systems, one language based and one sight based, and that one can be destroyed while the other stays intact.