Finger posture modulates structural body representations

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.

Tactile confusions of the fingers and toes.

“…For example, recent research has shown that finger posture alters our structural body representation to account for changing position (Tam•, Dransfield, et al, 2017). Moreover, localising touch in external space uses a distorted body representation common to the underlying position sense, as opposed to tactile localisation .…”

“…This is evident in body representation disturbances such as finger agnosia, a symptom of Gerstmann syndrome in which individuals experience the selective loss of ability to recognise, identify, Tactile Confusions of Finger and Toes distinguish or indicate individual fingers, on either the patientÕs own or anotherÕs fingers (Anema et al, 2008;Gerstmann, 1939;Kinsbourne & Warrington, 1962;Mayer et al, 1999). Even in healthy adults, identifying touch to an individual finger is not as straightforward as one might expect (Rusconi et al, 2014;Rusconi, Gonzaga, Adriani, Braun, & Haggard, 2009;Tam•, Dransfield, Quettier, & Longo, 2017).…”

Structural representations of fingers rely on both anatomical and spatial reference frames.

“…Procedure. Procedures were similar to our previous study (Tamè, Dransfield, et al, 2017). A detailed description of procedures is in supplementary material.…”

“…This is intuitive, since body posture changes constantly but the structural configuration of the body remains stable. Recent evidence from our laboratory has shown that BSRs are not as fixed as previously believed, but are modulated by the relative positions of body parts -i.e., fingers (Tamè, Dransfield, Quettier, & Longo, 2017), suggesting that "on-line" and "off-line" representations of the body are not completely distinct, but may dynamical interact.…”

“…A classic measure of finger agnosia is the so called 'in-between' test (IBT) in which participants judge the number of unstimulated fingers in-between two simultaneously touched fingers (Kinsbourne & Warrington, 1962). We recently used this task to show that the judged number of fingers in-between is generally underestimated, though increases when the fingers are splayed compared to when they are pressed together (Tamè, Dransfield, et al, 2017). This demonstrates that BSRs are influenced by finger posture, or the physical distance between fingers in external space, or possibly both.…”