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

TL;DR: Investigating finger identification using the "in-between" test demonstrates that BSRs rely on both anatomical and external reference frames, and finger identification was modulated by anatomical and spatial proximity in external space between touches.

Abstract: Finger agnosia refers to a neurological condition in which patients with left posterior parietal lesions fail to identify their fingers, despite having relatively preserved abilities in sensation and skilled action. This dissociation suggests that the structural body representations (BSRs) may be distinct from sensorimotor representations. However, recent research has reported that postural changes modulate representation of hand structure, revealing dynamic interactions between structural and sensorimotor body representations. However, it is unknown how and to what extent anatomical and spatial proximity contribute to shape the hand structural representation. We investigate this question using the "in-between" test in which participants estimate how many unstimulated fingers are in between 2 touched fingers of the left hand placed palm down. The first phalange of the participants' fingers was touched on the left or right side. Judged finger numerosity was greater when fingers were stimulated on far sides (i.e., opposite sides of the 2 fingers) compared to when they were stimulated on close (i.e., sides facing each other's) or middistance (i.e., sides facing in the same direction) sides. Therefore, finger identification was modulated by anatomical and spatial proximity in external space between touches. This demonstrates that BSRs rely on both anatomical and external reference frames. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

Summary

Introduction

• The authors knowledge about the spatial relationships between body parts (e.g., limbs and fingers) is thought to be mediated by a representation known as the body structural description.
• Here the authors explored the nature of BSRs and the spatial reference frames used for finger structural representation using an adapted version of the in-between test in which they stimulated the sides of fingers .
• Participants’ task was to judge how many unstimulated fingers were touched in-between the two touched fingers.

Method

• Participants reported normal touch and normal or corrected to normal vision.
• Procedures were similar to their previous study (Tamè, Dransfield, et al., 2017).
• Between trials there was a variable interstimulus interval, ranging from 1200 to 2200 ms, after participant’s response.
• The participant’s task was to estimate how many unstimulated fingers there were in-between the two touched fingers, responding as quickly and accurately as possible.
• The raw data are publicly available at https://osf.io/k6q5h.

Results

• As Figure 1B illustrates, participants generally underestimated the number of unstimulated fingers.
• The authors participants’ judgements closely corresponded to the actual changes in the spatial distance between stimuli, when there were one or two fingers in-between.
• This suggests that BSRs in such a context, rely also on external spatial coordinates.

Discussion

• The authors used the in-between test to examine the reference frames used by structural body representations when coding touch on the fingers.
• Judgements were higher when the far sides were stimulated compared to when middistance or close sides were stimulated.
• These results corroborate the notion that both the anatomical and the external spatial coordinates significantly contribute to finger representation.
• Neighbouring finger stimulation resulted in overestimation of finger numerosity, but only when far finger sides were stimulated.
• Note that for the three fingers space condition this can be over weighted given that there could be no more than three spaces.

Interplay of reference frames

• The authors findings suggest that anatomical and external spatial reference frames are integrated and used to locate touch on the fingers and that these coordinates are used in finger structural representation based on touch.
• Evidence discussed earlier supports this idea by demonstrating that the authors can employ multiple reference frames to represent touch on the body or body parts in space (e.g., Badde & Heed, 2016; Haggard et al., 2006; Tamè, Farnè, & Pavani, 2011b).
• It has been proposed that spatial touch perception is achieved through the integration of multiple location codes that are weighted on the basis of the availability and reliability of all the spatial information (Badde & Heed, 2016).
• In conclusion, their findings clearly demonstrate that a combination of the anatomical and the external spatial coordinates of touch are used in finger structural representation based on touch on the fingers of the same hand.
• The present study supports the view that body structural representations are more flexible rather than fixed as previously thought.

Figures

Figure 1. The “in-between” task (A) for the conditions in which close finger sides were stimulated (green circles), mid-distance finger sides (white circles) and far finger sides (red circles) were stimulated. (B) Judged finger numerosity for the different distance in finger space as a function of fingers’ side. Error bars indicate 95% within participants Confidence Intervals (95%CI) (Loftus & Masson, 1994).

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Citation for published version
Dolgilevica, Karina and Longo, Matthew R. and Tamè, Luigi (2019) Structural representations
of fingers rely on both anatomical and spatial reference frames. Journal of Experimental Psychology:
Human Perception and Performance . ISSN 0096-1523. (In press)
DOI
Link to record in KAR
https://kar.kent.ac.uk/77647/
Document Version
Author's Accepted Manuscript

1
Structural representations of fingers rely on both anatomical and
spatial reference frames
Karina Dolgilevica
1
, Matthew R. Longo
1
, and Luigi Tamè
1,2
1
Department of Psychological Sciences, Birkbeck, University of London, London, United Kingdom
2
School of Psychology, University of Kent, Canterbury, United Kingdom
Address for correspondence:
Luigi Tamè
School of Psychology
University of Kent
CT2 7NP, Canterbury, United Kingdom
E-Mail: l.tame@kent.ac.uk
Pages: 19; Figures: 1; Abstract: 200/200; Words: 2,468/2,500
Keywords: TACTILE, FINGER AGNOSIA, BODY STRUCTURAL REPRESENTATIONS, SIDE, SPACE

2
Abstract
Finger agnosia refers to a neurological condition in which patients with left posterior parietal
lesions fail to identify their fingers, despite having relatively preserved abilities in sensation
and skilled action. This dissociation suggests that the structural body representations (BSRs)
may be distinct from sensorimotor representations. However, recent research has reported
that postural changes modulate representation of hand structure, revealing dynamic
interactions between structural and sensorimotor body representations. However, it is
unknown how and to what extent anatomical and spatial proximity contribute to shape the
hand structural representation. We investigate this question using the “in-between” test in
which participants estimate how many unstimulated fingers are in-between two touched
fingers of the left hand placed palm down. The first phalange of the participants’ fingers was
touched on the left or right side. Judged finger numerosity was greater when fingers were
stimulated on far sides (i.e., opposite sides of the two fingers) compared to when they were
stimulated on close (i.e., sides facing each other’s) or mid-distance (i.e., sides facing in the
same direction) sides. Therefore, fingers identification was modulated both by anatomical
and spatial proximity in external space between touches. This demonstrates that BSRs rely on
both anatomical and external reference frames.

3
Significance Statement
Traditionally body representation research focuses on dissociation between the structural
body representations (BSRs) and the sensorimotor representations (e.g., body schema). Our
research supports the idea of a dynamic relationship between different body representations
as well as a more flexible representation of body’s structure in the bodily experience, which
are not fixed but varies as a function of the spatial relationships between the fingers. In the
context of finger BSRs based on touch, for instance, the adoption of an anatomical reference
frame is associated with neighbouring fingers, while an external reference frame with non-
neighbouring fingers. Despite this reference frame bias, we show that both the anatomical
and the external spatial coordinates of touch are considered when representing the fingers’
structure based on touch.

4
Introduction
Our knowledge about the spatial relationships between body parts (e.g., limbs and
fingers) is thought to be mediated by a representation known as the body structural
description. Neuropsychological cases of autotopagnosia, in which patients show a specific
deficit in pointing to their own or another’s body parts (Semenza & Goodglass, 1985) and
finger agnosia, a selective deficit in differentiating between one’s own or another’s fingers
(Kinsbourne & Warrington, 1962) provide evidence for the existence of such representations.
Notably, these patients often demonstrate relatively unimpaired skilled actions (Buxbaum &
Coslett, 2001).
The fact that patients with autotopagnosia are impaired at identifying body parts,
despite unaltered sensation and skilled action has traditionally been interpreted as evidence
that structural body representations (BSRs) are mediated by mechanisms different from
sensorimotor representations, such as the body schema (Anema et al., 2008; de Vignemont,
2010; Longo, Azañón, & Haggard, 2010; Schwoebel & Coslett, 2005; Tamè, Azañón, & Longo,
2019). 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.,