Semantic Dementia

Semantic dementia is a brain disorder characterized by progressive loss of world knowledge and conceptual understanding, which results from degeneration of the temporal lobes of the brain. The disorder typically affects people in the 6th to 8th decade of life. It is part of a spectrum of focal dementia syndromes affecting the frontotemporal lobes and is pathologically distinct from the more common degenerative dementia, Alzheimer's disease. This entry describes the clinical symptoms of semantic dementia and discusses factors that influence semantic loss, the capacity for relearning, and people's awareness of their semantic impairment.

The earliest symptoms are typically in the realm of language. People have difficulty remembering the names of things and understanding words. In conversation, they may make semantic errors, such as referring to a sheep as a dog, and use words overinclusively (e.g., water to refer to a wide range of liquids). Incorrect word usage reflects a loss of conceptual discrimination between related terms. On hearing some words (e.g., sheep), people with semantic dementia may ask what they mean, indicating that the problem is not simply one of word retrieval but a progressive loss in the person's knowledge of vocabulary. Ultimately, only a few stereotyped words or phrases remain. Nevertheless, people speak fluently and effortlessly within the confines of their increasingly restricted vocabulary. There is no effortful word search because words are no longer available to be sought.

The conceptual loss is not limited to words but encompasses the person's fund of knowledge relating to all sensory modalities. It affects the ability to recognize objects, faces, nonverbal environmental sounds, tactile, olfactory, and gustatory stimuli. Thus, people may no longer recognize fruits and vegetables in the supermarket, understand the significance of the sound of rain on the windowpane, or recognize the smell of coffee or taste of a lemon. These difficulties do not reflect a problem in sensory perception: People perceive and discriminate sensory stimuli entirely normally. The problem is in ascribing meaning to those percepts. They have lost their semantic associations. In contrast to the profound breakdown in semantic memory, day-to-day autobiographical memory is relatively well preserved, providing a striking contrast to the picture in classical amnesia.