Visual object recognition in early Alzheimer's disease: deficits in semantic processing.

Objectives: The purpose of the present study was to divide visual object recognition into different stages and to reveal which of these stages are impaired in early Alzheimer's disease (AD).

Methods: Performance in object detection, familiarity detection, semantic name and word categorization, and identification with naming were studied by using two-choice reaction-time tasks. Ten patients with newly diagnosed AD and 14 healthy subjects were studied.

Results: Patients with early AD had impairments in several stages of the object recognition process. After controlling for the basic visuomotor slowness, they were as fast and as accurate as the controls in object detection, but had difficulties in all stages that required semantic processing.

Conclusions: Semantic memory impairments contribute to the deficits in visual object recognition in early AD. Thus, the semantic memory deficit may be manifested in several ways in the difficulties that AD patients experience in everyday life.