A common representation of fingers and toes

TL;DR: This work obtained confusion matrices showing the pattern of mislocalisation on the hairy skin surfaces of both the fingers and toes, which suggest that there is a common representation of the hands and toes.

About: This article is published in Acta Psychologica.The article was published on 2019-08-01 and is currently open access. It has received 8 citations till now.

Summary

1. Introduction

• These similarities in tactile mislocalisation of the digits indicate that there may be commonalities in mental representations of the hands and feet, despite their divergent physical and functional properties.
• Moreover, the authors used a form a representational similarity analysis (RSA; Kriegeskorte, Mur, & Bandettini, 2008) to investigate whether individual differences between participants are shared across the glabrous and hairy skin surfaces of each limb.
• Together these results suggest that mislocalisations arise at the level of complete digits, not of individual skin surfaces, consistent with their arising from higher-level body representations.

2.1. Participants

• In their previous study (Manser-Smith et al., 2018), the individual differences found between the two surfaces of the fingers and toes using their decoding approach showed Cohen’s d’s of 1.76 and 1.04, respectively.
• As the authors reduced the number of trials completed by each participant in the present experiment (due to time constraints during testing), and they expected a weaker effect than in their previous study because they were comparing two different body parts, they conducted a power analysis using an effect size of half the smaller value found in their previous study.
• The authors based their calculations on a one-tailed t-test, as they have a clear directional prediction that classification accuracy should be greater than chance levels, rather than lower than chance.

2.3. Task

• The testing procedure closely resembled that used in their previous study (Manser-Smith et al., 2018).
• This posture was kept consistent regardless of whether the hand or foot was being tested, and they were instructed to remain as still as possible throughout each experimental block.
• Touch was only applied to the hairy skin, and not the glabrous skin.
• Vision was prevented throughout the experiment using a blindfold.
• The order of digit stimulation was pseudo-randomised within each block of trials, so that there was an approximately equal number of each type of preceding trial.

2.4. Analysis

• The analyses carried out closely resembled those of their previous study, and were exactly as described in the pre-registration document.
• The authors regressed the 20 off-diagonal cells (i.e., the localisation errors) of each participant’s confusion matrix (Ci) on the grand average confusion matrix for the other 39 participants (CGA), as in Equation 2.
• These residuals were calculated separately for the confusion matrices on the fingers and the toes, resulting in two sets of residuals per participant.
• Classification accuracy was calculated for each participant as the percentage of those 78 between-participant correlations which were smaller than the within-participant cross-correlation.
• The authors preregistered analysis plan specified a one-sample t-test to assess whether classification accuracy was significantly greater than chance (i.e. 50%).

3.1. Directional bias for localisation of the toes

• Figure 2 (left panel) shows the confusion matrix for tactile toe localisation on the hairy skin of the toes.
• As in their previous study, the majority of mislocalisations were made onto neighbouring toes.
• These results provide a direct replication of the results of their previous study and of Cicmil et al. (2016), corroborating the presence of directional biases for tactile toe localisation in response to stimulation of the hairy skin of the toes.

3.2. Directional bias for localisation of the fingers

• Figure 2 (right panel) shows the confusion matrix for tactile finger localisation on the hairy skin of the fingers.
• Once again, these results provide a direct replication of the results of their previous study, that there are consistent directional biases for tactile finger localisation in response to stimulation of the hairy skin of the fingers.

3.3. Shared individual differences between the fingers and the toes

• The key question was whether person-to-person differences in the pattern of mislocations is shared between the fingers and toes.
• A Bayesian onesample t-test provided moderate evidence in support of the alternative hypothesis against the null hypothesis, BF10 = 3.56.
• This result provides evidence for shared individual differences between the fingers and toes in how people mislocalise touch on the digits.

4. Discussion

• These results provide evidence for a common representation of fingers and toes.
• A shared high-level mental representation of the hands and feet may have developed to be beneficial to their primate ancestors, and would still beneficial to primates that retain similar structure and functional use of the hands and feet, to facilitate co-ordinated use.
• In their previous study the authors found strong idiosyncratic differences in the tactile localisation task comparing performance on the two skin surfaces of the hand or foot, providing strong evidence that there is a shared mental representation of the two skin surfaces of the hands and feet (such as a volumetric 3-D model of the body part).

Figures

Figure 2. Confusion matrices showing the proportion of stimuli judged as located on each of the five digits as a function of which digit was actually stimulated. Digits were identified by numbers one (the big toe/thumb) through five (the little toe/finger). Data from the foot is shown on the left panel, and data from the hand is shown in the right panel. The proportion of correct responses for each digit is shown along the diagonal from the top-left to the bottom-right. The off-diagonal cells represent mislocalisations between digits.

Figure 1. Experimental set-up. Participants sat in a chair with their left foot resting on a foot rest, and their left hand resting on a table. This posture gave the experimenter easy access to both the fingers and toes. Vision was prevented using a blindfold.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "A common representation of fingers and toes" ?

Manser-Smith et al. this paper found that the human hands and feet are serially homologous structures that have co-evolved, resulting in numerous similarities between the two body parts. 

Q2. What future works have the authors mentioned in the paper "A common representation of fingers and toes" ?

To attempt to disentangle how the body representation itself and the body ’ s position in external space contribute to localisation biases, future experiments may focus on manipulating posture of the fingers and toes relative to one another, or relative to the gaze-direction, for example. From the results of this experiment and others the authors have suggested that patterns of tactile confusions may arise from high-level body representations, which likely originate in the posterior parietal cortex. Cortical somatotopy suggests manual dexterity is primitive and evolved independently of bipedalism.