Clock test deficits are associated with semantic memory impairment in Alzheimer disease

Based on previous findings, we hypothesized that Clock Test deficits in patients with Alzheimer disease (AD) are associated primarily with the impairment of semantic memory regarding the appearance and functionality of a clock. To scrutinize this hypothesis, we developed a Clock Questionnaire that examined the semantic knowledge of the concept of a ''Clock'' and correlated scores from the questionnaire with performance in Clock Drawing, Clock Setting, and Clock Reading by healthy control participants and patients with mild cognitive impairment, early AD, and progressed AD. The Rey-Osterrieth-Complex-Figure is known to measure both visuospatial abilities and executive functions, and was chosen as a control variable. We found that deteriorated semantic memory best predicted Clock Test performance. In progressed AD, degraded knowledge regarding the appearance of a clock can explain the reduced ability to draw a clock face, while in early AD, impaired access to semantic knowledge about the minute hand might explain observed difficulties in drawing, setting, and reading the minute hand.