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Journal Article•DOI•

A common representation of fingers and toes

Kelda Manser-Smith1, Luigi Tamè1, Matthew R. Longo1•
Birkbeck, University of London1
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.
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 (2 min read)

Jump to: [1. Introduction] – [2.1. Participants] – [2.3. Task] – [2.4. Analysis] – [3.1. Directional bias for localisation of the toes] – [3.2. Directional bias for localisation of the fingers] – [3.3. Shared individual differences between the fingers and the toes] and [4. Discussion]

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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Figures (2)
Citations
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Journal Article•DOI•
Evolution of the Human Foot

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Richard Hope
18 Apr 1942-BMJ

58 citations

Journal Article•DOI•
Four meta-analyses across 164 studies on atypical footedness prevalence and its relation to handedness.

[...]

Julian Packheiser1, Judith Schmitz2, Gessa Berretz1, David P. Carey3, Silvia Paracchini2, Marietta Papadatou-Pastou4, Sebastian Ocklenburg1, Sebastian Ocklenburg5 •
Ruhr University Bochum1, University of St Andrews2, Bangor University3, National and Kapodistrian University of Athens4, University of Duisburg-Essen5
02 Sep 2020-Scientific Reports
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.

33 citations

Journal Article•DOI•
Tactile distance anisotropy on the feet.

[...]

Kelda Manser-Smith1, Luigi Tamè2, Luigi Tamè1, Matthew R. Longo1•
Birkbeck, University of London1, University of Kent2
08 Jul 2021-Attention Perception & Psychophysics
TL;DR: In this paper, the authors 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.
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.

6 citations

Journal Article•DOI•
Fingers hold spatial information that toes do not.

[...]

Kelda Manser-Smith1, Daniele Romano2, Daniele Romano1, Luigi Tamè1, Luigi Tamè3, Matthew R. Longo1 •
Birkbeck, University of London1, University of Milan2, University of Kent3
01 Jan 2021-Quarterly Journal of Experimental Psychology
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.

3 citations

Journal Article•DOI•
Body structural representation in schizotypy.

[...]

Francesca Fotia1, Loes van Dam2, Loes van Dam1, John Sykes, Ettore Ambrosini3, Marcello Costantini4, Francesca Ferri4 •
University of Essex1, Technische Universität Darmstadt2, University of Padua3, University of Chieti-Pescara4
01 Jan 2022-Schizophrenia Research
TL;DR: This article found that individuals with high schizotypal traits in the general population may be characterized by a progressive sense of detachment from one's lived body, which may represent a potential marker for schizophrenia proneness.

2 citations

References
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Journal Article•DOI•
Evolution of hallucial grasping in the primates

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Frederick S. Szalay1, Marian Dagosto2•
Hunter College1, Johns Hopkins University2
01 Feb 1988-Journal of Human Evolution
TL;DR: The “prehallux” hypothesis for the explanation of the sesamoid in the entocuneiform-hallucial articulation of anthropoids cannot be corroborated by either topographical, developmental, or functional evidence.

142 citations

Journal Article•DOI•
Spatial remapping of touch: confusion of perceived stimulus order across hand and foot.

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Tobias Schicke1, Brigitte Röder1•
University of Hamburg1
01 Aug 2006-Proceedings of the National Academy of Sciences of the United States of America
TL;DR: A remapping of tactile location of all body parts into a nonanatomically anchored and common reference frame rather than a specific remapping for eye-hand coordination only is suggested.
Abstract: The "body schema," a spatial representation of the body in its environment, has been suggested to be an emergent property of a widespread network of effector-specific frontal and parietal areas, many of which integrate sensory input from the different modalities. On a behavioral level, such multimodality has been shown with temporal order judgment tasks, in which participants decide which of the two hands received a tactile stimulus first. The accuracy of these judgments is influenced by body posture, indicating that tactile stimuli are not simply represented in an anatomical reference frame, but are transformed into external spatial coordinates. So far, these studies have only investigated the hands. It is therefore unclear whether a default remapping of touch into external space is a special feature of visual-manual control or whether all body parts are represented in a common nonanatomically anchored reference frame. In the present study, blindfolded participants made temporal order judgments of stimuli presented to both hands, both feet, or one hand and one foot. The stimulated limbs were held in either a parallel or a crossed posture. Judgments were equally impaired by limb crossing in all limb combinations (hands only, feet only, hand and foot). These results suggest a remapping of tactile location of all body parts into a nonanatomically anchored and, importantly, common reference frame rather than a specific remapping for eye-hand coordination only.

141 citations

Journal Article•DOI•
The coevolution of human hands and feet.

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Campbell Rolian1, Daniel E. Lieberman2, Benedikt Hallgrímsson1•
University of Calgary1, Harvard University2
01 Jun 2010-Evolution
TL;DR: The data support the hypothesis that human hands and feet coevolved, and suggest that the evolution of long robust big toes and short lateral toes for bipedalism led to changes in hominin fingers that may have facilitated the emergence of stone tool technology.
Abstract: Human hands and feet have longer, more robust first digits, and shorter lateral digits compared to African apes. These similarities are often assumed to be independently evolved adaptations for manipulative activities and bipedalism, respectively. However, hands and feet are serially homologous structures that share virtually identical developmental blueprints, raising the possibility that digital proportions coevolved in human hands and feet because of underlying developmental linkages that increase phenotypic covariation between them. Here we show that phenotypic covariation between serially homologous fingers and toes in Homo and Pan is not only higher than expected, it also causes these digits to evolve along highly parallel trajectories under episodes of simulated directional selection, even when selection pressures push their means in divergent directions. Further, our estimates of the selection pressures required to produce human-like fingers and toes from an African ape-like ancestor indicate that selection on the toes was substantially stronger, and likely led to parallel phenotypic changes in the hands. Our data support the hypothesis that human hands and feet coevolved, and suggest that the evolution of long robust big toes and short lateral toes for bipedalism led to changes in hominin fingers that may have facilitated the emergence of stone tool technology.

118 citations

Journal Article•DOI•
Finger representations in human primary somatosensory cortex as revealed by high-resolution functional MRI of tactile stimulation.

[...]

Renate Schweizer1, Dirk Voit1, Jens Frahm1•
Max Planck Society1
01 Aug 2008-NeuroImage
TL;DR: Fine-scale functional organization of the finger areas in the human primary somatosensory cortex was investigated by high-resolution BOLD MRI at 3 T using a multi-echo FLASH sequence with a voxel size of 2 mm(3) for the identification of individual variations in finger somatotopy.

116 citations

Journal Article•DOI•
Tactile spatial acuity at the human fingertip and lip : Bilateral symmetry and interdigit variability

[...]

Krish Sathian1, Andro Zangaladze1•
Emory University1
01 May 1996-Neurology
TL;DR: The spatial threshold for discrimination of grating orientation is used to assess tactile spatial acuity at the lower lip and each fingertip, bilaterally, in seven normal human subjects and it is shown that acuity does not differ significantly between right and left sides.
Abstract: We used the spatial threshold for discrimination of grating orientation to assess tactile spatial acuity at the lower lip and each fingertip, bilaterally, in seven normal human subjects. We confirmed previous findings that: (1) this test is highly reliable, and (2) acuity at the lip is substantially higher than at the fingertips. In addition, we showed that: (1) acuity does not differ significantly between right and left sides, and (2) among fingertips, acuity is lowest at the fifth digit and comparable on the other digits.

113 citations

"A common representation of fingers ..." refers background in this paper

  • ...Moreover, although tactile acuity differs significantly across the fingers (Duncan & Boynton, 2007; Sathian & Zangaladze, 1996) and toes (Manser-Smith et al., 2018), for ease of testing and consistency with our previous study, one strength of von Frey hair was used across all five fingers/toes....

    [...]

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Tactile confusions of the fingers and toes.

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09 Aug 2018-Journal of Experimental Psychology: Human Perception and Performance
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Structural representations of fingers rely on both anatomical and spatial reference frames.

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01 Feb 2020-Journal of Experimental Psychology: Human Perception and Performance
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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.