The signing body: extensive sign language practice shapes the size of hands and face

Abstract

The representation of the metrics of the hands is distorted, but is susceptible to malleability due to expert dexterity (magicians) and long-term tool use (baseball players). However, it remains unclear whether modulation leads to a stable representation of the hand that is adopted in every circumstance, or whether the modulation is closely linked to the spatial context where the expertise occurs. To this aim, a group of 10 experienced Sign Language (SL) interpreters were recruited to study the selective influence of expertise and space localisation in the metric representation of hands. Experiment 1 explored differences in hands’ size representation between the SL interpreters and 10 age-matched controls in near-reaching (Condition 1) and far-reaching space (Condition 2), using the localisation task. SL interpreters presented reduced hand size in near-reaching condition, with characteristic underestimation of finger lengths, and reduced overestimation of hands and wrists widths in comparison with controls. This difference was lost in far-reaching space, confirming the effect of expertise on hand representations is closely linked to the spatial context where an action is performed. As SL interpreters are also experts in the use of their face with communication purposes, the effects of expertise in the metrics of the face were also studied (Experiment 2). SL interpreters were more accurate than controls, with overall reduction of width overestimation. Overall, expertise modifies the representation of relevant body parts in a specific and context-dependent manner. Hence, different representations of the same body part can coexist simultaneously.

Selective effects of a brain tumor on the metric representation of the hand: a pre- versus post-surgery comparison

“…It is important to note that the dimension where we found a change post-surgery was the width, but not the length. Indeed, variability in length perception seems less frequent, whereas width appears more susceptible to change (see Cocchini et al 2018 ; Coelho et al 2019 ; Longo 2022 ; Mora et al 2021 ). One reason for this finding is that width is intrinsically related to more representational flexibility to accommodate growth (De Vignemont et al 2005 ; Hashimoto and Iriki 2013 ).…”

“…We used a modified version of the hand localization task, as in previous research (Mora et al 2021 ). In this case, a horizontal transparent Perspex board (30 × 30 cm) was positioned on top of four metal posts (10 cm high).…”

“…The pictures were analyzed following the same method adopted in Mora et al ( 2021 ) with a bespoke-made image analysis program using Borland C ++ Builder (2007) that allowed conversion of pixel units into centimeters. The x and y coordinates for the real and perceived locations were obtained for each landmark (the origin was at the bottom right corner of each picture).…”

“…The x and y coordinates for the real and perceived locations were obtained for each landmark (the origin was at the bottom right corner of each picture). With the coordinate data, distances (in centimeters) between pairs of landmarks were calculated, as in previous studies (e.g., Mora et al 2021 ): (i) length of fingers were represented by the distance between each fingertip and adjacent interspace; (ii) the length of the hand’s dorsum was represented by the distance between the interspace between the ring and little fingers and the exterior side of the wrist; (iii) the width of the hand was represented by the distance from the interspace between the index and middle fingers, and the interspace between the ring and little fingers; and (iv) the width of the wrists was represented by the distance between the two sides of the wrists.…”

“…This task taps into the more implicit components of body representation (body model), in contrast to visual estimates that are more explicit and constructed by vision (Longo 2015a , 2017 , 2022 ; Longo and Haggard 2012b ). Indeed, previous research has identified differences in the body model of the hands using this task in participant experts in the use of hands (i.e., sign language interpreters; Mora et al 2021 ), a practice that is associated with anatomical structural changes in the motor representation of the hand (Allen et al 2013 ; Penhune et al 2003 ; Sastre-Janer 1998 ). We expected to observe specific modulation of his contralesional (right) hand representation that would modify following tumor resection.…”