Background: The Necker cube is usually used for evaluating the visuoconstructional ability of patients with mild cognitive impairment (MCI) and dementia. However, the Necker cube is often considered a drawing with a visual illusionary perspective. The purpose of this study was to investigate whether Necker cube copying could detect participants with MCI due to dementia. Methods: We retrospectively analyzed the database of the 1998 prevalence study that was part of the Tajiri Project (n = 599). Pencil drawings of the Necker cube on A4-sized white paper by non-demented people (Clinical Dementia Rating (CDR) 0 and 0.5, n = 256) were classified into two patterns: non-three-dimension (3-D) and 3-D. Two neuropsychologists assessed Necker cube copying according to the criteria of the classification. After the classification, the database of the 2003 incidence study was used according to the subjects’ conversion to dementia. Results: In the prevalence study, among those who made a non-3-D drawing of the Necker cube, there were significantly fewer people in the CDR 0 group than in the CDR 0.5 and CDR 1+ groups; similarly, there were significantly fewer people in the CDR 0.5 group than in the CDR 1 + group (χ2 = 32.6, P < 0.001; post-hoc tests using χ2 tests, CDR 0 > CDR 0.5 > CDR 1+, P < 0.001). In the incidence study, among those who made a non-3-D drawing of the Necker cube, there were significantly fewer people in the non-converter group than in the converter group (χ2 = 19.9, P < 0.001). However, there was no significant difference between the non-converter group (n = 21) and the converter group (n = 21) when age, sex, educational levels, and Mini-Mental State Examination scores were controlled (χ2 = 0.0, P = 1.000). Conclusions: Our results suggested that Necker cube copying may evaluate visual illusion as well as visuoconstructional ability. The Necker cube may not be an appropriate test to detect participants with MCI due to dementia.
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