Abstract
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 sensation and skilled action. Such dissociations have traditionally been interpreted as evidence that structural body representations (BSR), such as the body structural description, are distinct from sensorimotor representations, such as the body schema. We investigated whether performance on tasks commonly used to assess finger agnosia is modulated by changes in hand posture. We used the ‘in between’ test in which participants estimate the number of unstimulated fingers between two touched fingers or a localization task in which participants judge which two fingers were stimulated. Across blocks, the fingers were placed in three levels of splay. Judged finger numerosity was analysed, in Exp. 1 by direct report and in Exp. 2 as the actual number of fingers between the fingers named. In both experiments, judgments were greater when non-adjacent stimulated fingers were positioned far apart compared to when they were close together or touching, whereas judgements were unaltered when adjacent fingers were stimulated. This demonstrates that BSRs are not fixed, but are modulated by the real-time physical distances between body parts.
Highlights
Provided behavioural evidence in favour of the existence of body structural representations involving an allocentric representation of finger order, independent of hand posture[5]
We examined in healthy humans whether structural body representations are modulated by changes in body posture using two classical tests of finger agnosia[3]
The results of the present work show that the representations used to identify tackily the fingers, can sometimes act as stable representations that code the relative position between the fingers regardless of postural changes
Summary
Provided behavioural evidence in favour of the existence of body structural representations involving an allocentric representation of finger order, independent of hand posture[5]. Knowledge of the spatial relations between body parts, and between the fingers, may play a role in the production of motor responses (e.g., finely tuned movements), these representations could potentially be less fixed than is commonly believed. With this idea in mind, we investigated whether structural body representations are modulated by changes in body posture. Some authors have suggested that it derives primarily from visual inputs that define body part boundaries and proximity relationships[6] Note that, unlike these previous works, in the present study we focused on the contribution of touch and postural information in generating structural body representations, while visual information was not manipulated
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