A case of autotopagnosia in Alzheimer's disease: Mechanistic insights into body-part localization.

Autotopagnosia is a rare neuropsychological syndrome characterized by disordered localization of body parts. Reports remain scarce and the cognitive mechanisms underlying this syndrome are not fully understood. We describe the case of a 65-year-old Japanese man whose clinical and neuropsychological profiles and neuroimaging findings supported a diagnosis of probable Alzheimer's disease. Notably, he had episodic memory deficits and visuospatial impairment without aphasia, along with Gerstmann syndrome and marked autotopagnosia. Neuropsychological examinations assessed body-part knowledge and processing, combining pictorial representations, spoken instructions, and tactile input with pointing and naming responses; single-case comparisons were benchmarked against age-matched, cognitively healthy adults (n = 20). The patient could name body parts and retain general knowledge and visual recognition of them, yet consistently failed to localize those parts on whole-body stimuli: his own body, another person's body, and a full-body diagram. Language-only probes of inter-part spatial relations were impaired, whereas non-spatial definitions were preserved. This cross-modal profile is not attributable to a single sensory modality or language/motor factor, nor is it readily explained by accounts invoking failures to map visual/somatosensory inputs to body-part location or the primary disruption of an egocentric, multisensory body schema. Instead, it is most consistent with a modality-independent disruption of body-part localization knowledge, i.e., a high-level body-centered spatial map linking body-part labels to their canonical positions on the human body. Our findings suggest that autotopagnosia can be parsimoniously unified through this mechanism. These observations underscore the need for standardized, modality-diverse examinations to clarify the mechanisms underlying this rare clinical phenomenon.