Evidence for modality-specific meaning systems in the brain.

Patients with cerebral lesions offer a unique opportunity to investigate the organization of meaning systems in the brain. Clinical neurologists have long been aware that knowledge of particular classes or categories of information may be selectively impaired in some cases and selectively spared in others. For example knowledge of letters, colours, objects, or people may be lost as a consequence of damage to the left hemisphere of the brain. Recently there has been quantitative evidence for even more specific impairment and preservation of particular classes of knowledge. More recently the evidence for knowledge of living things as compared with inanimate objects is particularly striking. Such observations have suggested that our semantic knowledge base is categorical in its organization. In this preliminary report, we describe a patient whose semantic knowledge deficit was not only category specific, but also modality specific. Although his knowledge of the visual world was almost entirely normal, his knowledge of living things (but not objects!) was gravely impaired when assessed in the verbal domain. These findings call into question the widely accepted view that the brain has a single all-purpose meaning store.