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Journal ArticleDOI

A common representation of fingers and toes

01 Aug 2019-Acta Psychologica (Elsevier)-Vol. 199, pp 102900

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.

AbstractThere are many similarities and differences between the human hands and feet. On a psychological level, there is some evidence from clinical disorders and studies of tactile localisation in healthy adults for deep functional connections between the hands and feet. One form these connections may take is in common high-level mental representations of the hands and feet. Previous studies have shown that there are systematic, but distinct patterns of confusion found between both the fingers and toes. Further, there are clear individual differences between people in the exact patterns of mislocalisations. Here, we investigated whether these idiosyncratic differences in tactile localisation are shared between the fingers and toes, which may indicate a shared high-level representation. We obtained confusion matrices showing the pattern of mislocalisation on the hairy skin surfaces of both the fingers and toes. Using a decoding approach, we show that idiosyncratic differences in individuals' pattern of confusions are shared across the fingers and toes, despite different overall patterns of confusions. These results suggest that there is a common representation of the fingers and toes.

Summary (2 min read)

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).

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TL;DR: It was showed that the prevalence of atypical footedness ranges between 12.10% using the most conservative criterion of left-footedness to 23.7% including all left- and mixed-footers as a single non-right category, and that footing is a valuable phenotype for the study of lateral motor biases, its underlying genetics and neurodevelopment.
Abstract: Human lateral preferences, such as handedness and footedness, have interested researchers for decades due to their pronounced asymmetries at the population level. While there are good estimates on the prevalence of handedness in the population, there is no large-scale estimation on the prevalence of footedness. Furthermore, the relationship between footedness and handedness still remains elusive. Here, we conducted meta-analyses with four different classification systems for footedness on 145,135 individuals across 164 studies including new data from the ALSPAC cohort. The study aimed to determine a reliable point estimate of footedness, to study the association between footedness and handedness, and to investigate moderating factors influencing footedness. We showed that the prevalence of atypical footedness ranges between 12.10% using the most conservative criterion of left-footedness to 23.7% including all left- and mixed-footers as a single non-right category. As many as 60.1% of left-handers were left-footed whereas only 3.2% of right-handers were left-footed. Males were 4.1% more often non-right-footed compared to females. Individuals with psychiatric and neurodevelopmental disorders exhibited a higher prevalence of non-right-footedness. Furthermore, the presence of mixed-footedness was higher in children compared to adults and left-footedness was increased in athletes compared to the general population. Finally, we showed that footedness is only marginally influenced by cultural and social factors, which play a crucial role in the determination of handedness. Overall, this study provides new and useful reference data for laterality research. Furthermore, the data suggest that footedness is a valuable phenotype for the study of lateral motor biases, its underlying genetics and neurodevelopment.

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Journal ArticleDOI
TL;DR: Spatial information held by the fingers is stronger and more reliable than for the toes, so is not a general characteristic of limbs, but possibly related to hand use.
Abstract: Fingers have preferential associations with relative spatial locations. Tactile localisation is faster when the fingers are in these locations, such as when the index finger is in a relatively higher spatial position, and the thumb in a relatively lower position. However, it is unclear whether these associations are related to hands specifically, or are a more general characteristic of limbs. The present study therefore investigated whether toes have similar spatial associations. If these associations reflect the statistics of natural limb usage, very different patterns of association would be expected for the fingers and toes, given their different functional roles in daily behaviour. We measured reaction time (RT) and error rates of responses to tactile stimuli applied to the middle finger/toe or thumb/big toe, when they were positioned in a relative upper or lower location. We replicated the finding that fingers have preferential associations that facilitates localisation – RT and error rate were lower when the index finger was in the top position, and the thumb in the bottom position. We found that toes do not hold the same spatial information, though it remains unclear whether toes hold different spatial information or none at all. These results demonstrate spatial information held by the fingers is stronger and more reliable than for the toes, so is not a general characteristic of limbs, but possibly related to hand use.

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Journal ArticleDOI
Abstract: A deficient sense of self, typically observed in schizophrenia spectrum disorders, is often accompanied by abnormalities in bodily perception and awareness. These abnormalities are seemingly among the most powerful predictive factors for the onset of schizophrenic illnesses. According to the hypothesis of the psychosis continuum, high schizotypal traits in the general population may be characterized by a progressive sense of detachment from one's lived body. Building upon previous research that found an abnormal Body Structural Representation (BSR) in individuals with schizophrenia, this study aims to extend these findings to schizotypy. To investigate this, we utilized the Finger Localization Task (FLT), in which participants must identify the finger touched by the experimenter, and the In Between Task (IBT), in which two fingers are touched and participants must specify the number of fingers in between the two stimulated fingers. We found that individuals with high schizotypy were significantly less accurate than individuals with low schizotypy in determining the spatial configuration of their own fingers relative to each other. Most significantly, performances on both tasks were negatively correlated with the score on the Dissociative Experiences Scale (DES). These findings support the hypothesis that the progressive loss of one's sense of self is associated with abnormal bodily experiences and dissociative symptomatology which may represent a potential marker for schizophrenia proneness.

Journal ArticleDOI
Abstract: Perception of distance between two touches varies with orientation on the hand, with distances aligned with hand width perceived as larger than those aligned with hand length. Similar anisotropies are found on other body parts (e.g., the face), suggesting they may reflect a general feature of tactile organization, but appear absent on other body parts (e.g., the belly). Here, we investigated tactile-distance anisotropy on the foot, a body part structurally and embryologically similar to the hand, but with very different patterns of functional usage in humans. In three experiments, we compared the perceived distance between pairs of touches aligned with the medio-lateral and proximal-distal foot axes. On the hairy skin of the foot dorsum, anisotropy was consistently found, with distances aligned with the medio-lateral foot axis perceived as larger than those in the proximo-distal axis. In contrast, on the glabrous skin of the sole, inconsistent results were found across experiments, with no overall evidence for anisotropy. This shows a pattern of anisotropy on the foot broadly similar to that on the hand, adding to the list of body parts showing tactile-distance anisotropy, and providing further evidence that such biases are a general aspect of tactile spatial organization across the body. Significance: The perception of tactile distance has been widely used to understand the spatial structure of touch. On the hand, anisotropy of tactile distance perception is well established, with distances oriented across hand width perceived larger than those oriented along hand length. We investigated tactile-distance anisotropy on the feet, a body part structurally, genetically, and developmentally homologous to the hands, but with strikingly different patterns of functional usage. We report highly similar patterns of anisotropy on the hairy skin of the hand dorsum and foot dorsum. This suggests that anisotropy arises from the general organization of touch across the body.

References
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TL;DR: An inventory of 20 items with a set of instructions and response- and computational-conventions is proposed and the results obtained from a young adult population numbering some 1100 individuals are reported.
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Frequently Asked Questions (2)
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. 

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.