Vision of the body alters perceived finger numerosity

Patients with lesions of the left posterior parietal cortex commonly fail in identifying their fingers, a condition known as finger agnosia, yet are relatively unimpaired in skilled action. Several studies have shown that non-informative vision of the body enhances performance in numerous tactile tasks. However, it is unknown whether body structural representations are also affected by vision, given that finger agnosia is typically assessed while patients are blindfolded. Here, we investigated whether structural body representations are modulated by non-informative vision of the body. We tested healthy participants in a classic task used to assess finger agnosia, the “in-between” test, to determine whether tactile fingers identification vary when seeing the stimulated hand (non-informative vision). Across blocks, we used three different visual conditions: (1) viewing the stimulated hand, (2) viewing an object, and (3) viewing the other hand. Participants judged the numbers of unstimulated fingers “in between” the two touched fingers and responded vocally as quickly and accurately as possible. Critically, numerosity estimates were less accurate when they were seeing the stimulated hand compared to when an object or the unstimulated hand was visible. Our results demonstrate that vision of the body modulates body structural representations impairing tactile fingers identification.