Verbal mediation of number knowledge: Evidence from semantic dementia and corticobasal degeneration

Patients with corticobasal degeneration (CBD) appear to have impaired number knowledge. We examined the nature of their number deficit while we tested the hypothesis that comprehension of larger numbers depends in part on verbal mediation. We evaluated magnitude judgments and performance on number conservation measures rooted in Piagetian theory in nonaphasic patients with CBD (n = 13) and patients with a fluent form of progressive aphasia known as semantic dementia (SD; n = 15). We manipulated the numbers of the arrays and the visual–spatial properties of the stimuli being compared during magnitude judgments and Piagetian conservation measures. CBD patients were consistently impaired judging the magnitudes of larger numbers (4–9), while they had minimal difficulty with smaller numbers (magnitudes ⩽3). By comparison, SD patients performed all measures of number knowledge at a ceiling level regardless of number magnitude. Neither patient group was significantly impacted by manipulations of the spatial properties of the stimuli. CBD patients’ impairment with larger numbers despite minimal aphasia, and SD patients’ intact performance despite an aphasia, challenge the proposal that understanding larger numbers depends on verbal mediation.