Body posture affects tactile discrimination and identification of fingers and hands

Abstract

It is an unresolved question whether the posture of single fingers relative to each other is represented in the brain within an external frame of reference. In two experiments, we investigated postural influences on the processing of tactile stimuli at fingers and hands. Healthy subjects received two simultaneous tactile stimuli at the fingertips while the fingers of both hands were either interleaved or not. In speeded response tasks, they were asked to discriminate (experiment 1) or to identify (experiment 2) the touched body parts, either regarding hand laterality or finger type. The results demonstrate that both finger discrimination and finger identification are influenced by body posture. We conclude that the assumption of a solely somatotopic representation of fingers is not tenable and that an external reference system must be available for the detection of single fingers. The results are discussed in terms of a mental segmentation of external space, based on body posture and task requirements.

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Notes

  1. 1.

    Equality judgements require discrimination rather than identification of body parts. To account for these deviant requirements, we will distinguish between identification and discrimination tasks. According to this label distinction, identification tasks were used in Haggard et al. (2006, except experiment. 2), while we will implement discrimination tasks in experiment 1.

  2. 2.

    This cut-off value is higher than that used in experiment 4 of Haggard et al. (2006) (2000 ms). It accounts for the fact that discrimination tasks require more time than identification tasks (Haggard et al. 2006). The cut-off value of 3000 ms lies within 2.7 standard deviations above the mean of reaction times. Nevertheless, applying different cut-off values (2000 ms and 5000 ms) to the data produced the same results.

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Acknowledgments

This study was supported by the European Commission within the 6th FP through the SOMAPS research consortium (Project No 043432, www.somaps.eu). Jörg Trojan is indebted to the Baden-Württemberg foundation for the financial support of this research project by the elite programme for postdocs.

Declaration

The study was approved by the local ethical committee and conducted according to the ethical standards laid down in the 6th Revision of the Declaration of Helsinki (Version Seoul 2008). Furthermore, the authors declare that they have no conflict of interest.

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Correspondence to Martin Riemer.

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Riemer, M., Trojan, J., Kleinböhl, D. et al. Body posture affects tactile discrimination and identification of fingers and hands. Exp Brain Res 206, 47–57 (2010). https://doi.org/10.1007/s00221-010-2396-y

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Keywords

  • Finger
  • Hand
  • External frame of reference
  • Response fields
  • Somatosensory