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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
More filters
Journal Article•DOI•
Fingers crossed! An investigation of somatotopic representations using spatial directional judgements.

[...]

Alyanne M. de Haan1, Helen A. Anema2, Helen A. Anema1, H. Chris Dijkerman1•
Utrecht University1, University of Amsterdam2
24 Sep 2012-PLOS ONE
TL;DR: Performance did not improve with SOAs as long as 700 ms, suggesting that the localization of stimuli in a somatotopic reference and the integration of this representation with postural information are two separate processes that apply differently to the hands and fingers.
Abstract: Processing of tactile stimuli requires both localising the stimuli on the body surface and combining this information with a representation of the current posture. When tactile stimuli are applied to crossed hands, the system first assumes a prototypical (e.g. uncrossed) positioning of the limbs. Remapping to include the crossed posture occurs within about 300 ms. Since fingers have been suggested to be represented in a mainly somatotopic reference frame we were interested in how the processing of tactile stimuli applied to the fingers would be affected by an unusual posture of the fingers. We asked participants to report the direction of movement of two tactile stimuli, applied successively to the crossed or uncrossed index and middle fingers of one hand at different inter-stimulus intervals (15 to 700 ms). Participants almost consistently reported perceiving the stimulus direction as opposite to what it was in the fingers crossed condition, even with SOAs of 700 ms, suggesting that on average they did not incorporate the unusual relative finger positions. Therefore our results are in agreement with the idea that, by default, the processing of tactile stimuli assumes a prototypical positioning of body parts. However, in contrast to what is generally found with tactile perception with crossed hands, performance did not improve with SOAs as long as 700 ms. This suggests that the localization of stimuli in a somatotopic reference and the integration of this representation with postural information are two separate processes that apply differently to the hands and fingers.

14 citations

Journal Article•DOI•
Tactile Toe Agnosia and Percept of a "Missing Toe" in Healthy Humans.

[...]

Nela Cicmil1, Achim P. Meyer2, John F. Stein1•
University of Oxford1, Humboldt University of Berlin2
01 Mar 2016-Perception
TL;DR: These findings challenge current models of somatosensory localization, as they cannot be explained simply by a lack of distinct representations for toes compared with fingers, or by overt toe-finger correspondences.
Abstract: A disturbance of body representation is central to many neurological and psychiatric conditions, but the mechanisms by which body representations are constructed by the brain are not fully understood. We demonstrate a directional disturbance in tactile identification of the toes in healthy humans. Nineteen young adult participants underwent tactile stimulation of the digits with the eyes closed and verbally reported the identity of the stimulated digit. In the majority of individuals, responses to the second and third toes were significantly biased toward the laterally neighboring digit. The directional bias was greater for the nondominant foot and was affected by the identity of the immediately preceding stimulated toe. Unexpectedly, 9/19 participants reported the subjective experience of a "missing toe" or "missing space" during the protocol. These findings challenge current models of somatosensory localization, as they cannot be explained simply by a lack of distinct representations for toes compared with fingers, or by overt toe-finger correspondences. We present a novel theory of equal spatial representations of digit width combined with a "preceding neighbor" effect to explain the observed phenomena. The diagnostic implications for neurological disorders that involve "digit agnosia" are discussed.

13 citations

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

  • ...neighbouring digit, but more often in the direction of the central digits of the hand or foot (Cicmil et al., 2016; Manser-Smith et al., 2018)....

    [...]

  • ...These results provide a direct replication of the results of our 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....

    [...]

  • ...Moreover, we replicated the distinct patterns of tactile confusion found on the digits of the hand and the foot found in previous studies (Cicmil et al., 2016; Manser-Smith et al., 2018; Schweizer et al., 2001)....

    [...]

  • ...As the fingers have low pressure sensitivity thresholds in comparison to the toes, a near-threshold stimulus is needed to give a clear pattern of mislocalisations (Schweizer, Maier, Braun, & Birbaumer, 2000) and avoid ceiling effects such as encountered by Cicmil et al. (2016)....

    [...]

  • ...For example, digits of both the hands and feet are more frequently mislocalised to neighbouring than distant digits, however not equally to each neighbouring digit, but more often in the direction of the central digits of the hand or foot (Cicmil et al., 2016; Manser-Smith et al., 2018)....

    [...]

Related Papers (2)
Tactile confusions of the fingers and toes.

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09 Aug 2018-Journal of Experimental Psychology: Human Perception and Performance
Kelda Manser-Smith, Luigi Tamè, Matthew R. Longo
Structural representations of fingers rely on both anatomical and spatial reference frames.

[...]

01 Feb 2020-Journal of Experimental Psychology: Human Perception and Performance
Karina Dolgilevica, Matthew R. Longo, Luigi Tamè
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.