The natural history of "pure" late-stage semantic dementia

Relatively little is known about the neuropsychological profile of late-stage semantic dementia. This article provides a detailed assessmentof patient MK who, despite her very severe semantic impairments, remained cooperative to testing and, unusually, did not show additionalbehavioral/personality changes. Although MK’s initial presentation was typical of semantic dementia (SD), her performance began todeviate from the normal pattern. She developed impairments of single word repetition and regular word reading, and began to producephonological errors in picture naming and spontaneous speech. These deficits might suggest that late-stage SD includes an independentdisorder of phonology. An alternative possibility, however, is that phonological processing cannot proceed normally in the face of profoundsemantic degradation. A series of experiments supported the latter explanation of MK’s deficits. In picture naming, MK showed little effectof progressive phonological cueing, did not reveal an increased sensitivity to word length or phonological complexity and continued toshow a high degree of item-specific consistency in both accuracy and errors: she tended to produce the same erroneous phonemes for eachitem. She remained sensitive to the effects of phonological similarity in immediate serial recall. Letter substitution errors in regular wordreading were more common for lower frequency letters (e.g., Q, Z). These letters also produced more item errors in immediate serial recall,suggesting that a frequency-graded loss of letter knowledge, rather than separate orthographic and phonological deficits, accounted for thedeficits in both of these tasks. These findings are discussed in terms of theories that posit strong interactivity between phonology andsemantics.