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Phantom tactile sensations modulated by body position.

ABSTRACT: Bilateral activation of somatosensory areas after unilateral stimulation is assumed to be mediated by crosshemispheric connections. Despite evidence of bilateral activity in response to unilateral stimulation, neurologically intact humans do not experience bilateral percepts when stimulated on one side of the body. This may be due to active suppression of ipsilateral neural activity by inhibitory mechanisms whose functioning is poorly understood. We describe an individual with left fronto-parietal damage who experiences bilateral sensations in response to unilateral tactile stimulation-a rarely reported condition known as synchiria (previously described in visual, auditory, and somatosensory modalities). Presumably, the phantom sensations result from normal bilateral crosshemispheric activation, combined with a failure of inhibitory mechanisms to prevent bilateral perceptual experiences. Disruption of these mechanisms provides a valuable opportunity to examine their internal functioning. We find that the synchiria rate is affected by hand position relative to multiple reference frames. Specifically, synchiria decreases as the hands move from right (contralesional) to left (ipsilesional) space in trunk- and head-centered reference frames and disappears when the hands are crossed. These findings provide novel evidence that mechanisms that inhibit bilateral percepts operate in multiple reference frames.

SUBMITTER: Medina J

PROVIDER: S-EPMC2701404 | biostudies-literature | 2008 Dec

REPOSITORIES: biostudies-literature

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Phantom tactile sensations modulated by body position.

Medina Jared J Rapp Brenda B

Current biology : CB 20081208 24

Bilateral activation of somatosensory areas after unilateral stimulation is assumed to be mediated by crosshemispheric connections. Despite evidence of bilateral activity in response to unilateral stimulation, neurologically intact humans do not experience bilateral percepts when stimulated on one side of the body. This may be due to active suppression of ipsilateral neural activity by inhibitory mechanisms whose functioning is poorly understood. We describe an individual with left fronto-pariet ...[more]

PMID: 19062276

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Project description:While skin, joints and muscles receptors alone provide lower level information about individual variables (e.g., exerted limb force and limb displacement), the distance between limb endpoints (i.e., relative position) has to be extracted from high level integration of somatosensory and motor signals. In particular, estimation of fingertip relative position likely involves more complex sensorimotor transformations than those underlying hand or arm position sense: the brain has to estimate where each fingertip is relative to the hand and where fingertips are relative to each other. It has been demonstrated that during grasping, feedback of digit position drives rapid adjustments of fingers force control. However, it has been shown that estimation of fingertips' relative position can be biased by digit forces. These findings raise the question of how the brain combines concurrent tactile (i.e., cutaneous mechanoreceptors afferents induced by skin pressure and stretch) and non-tactile (i.e., both descending motor command and joint/muscle receptors signals associated to muscle contraction) digit force-related inputs for fingertip distance estimation. Here we addressed this question by quantifying the contribution of tactile and non-tactile force-related inputs for the estimation of fingertip relative position. We asked subjects to match fingertip vertical distance relying only on either tactile or non-tactile inputs from the thumb and index fingertip, and compared their performance with the condition where both types of inputs were combined. We found that (a) the bias in the estimation of fingertip distance persisted when tactile inputs and non-tactile force-related signals were presented in isolation; (b) tactile signals contributed the most to the estimation of fingertip distance; (c) linear summation of the matching errors relying only on either tactile or non-tactile inputs was comparable to the matching error when both inputs were simultaneously available. These findings reveal a greater role of tactile signals for sensing fingertip distance and suggest a linear integration mechanism with non-tactile inputs for the estimation of fingertip relative position.

Project description:Stochastic stimulation has been shown to improve movement, balance, the sense of touch, and may also improve position sense. This stimulation can be non-invasive and may be a simple technology to enhance proprioception. In this study, we investigated whether sub-threshold stochastic tactile stimulation of mechanoreceptors reduces age-related errors in wrist position estimation. Fifteen young (24.5±1.5y) and 23 elderly (71.7±7.3y) unimpaired, right-handed adults completed a wrist position gauge-matching experiment. In each trial, the participant's concealed wrist was moved to a target position between 10 and 30° of wrist flexion or extension by a robotic manipulandum. The participant then estimated the wrist's position on a virtual gauge. During half of the trials, sub-threshold stochastic tactile stimulation was applied to the wrist muscle tendon areas. Stochastic stimulation did not significantly influence wrist position sense. In the elderly group, estimation errors decreased non-significantly when stimulation was applied compared to the trials without stimulation [mean constant error reduction Δμ(θconof)=0.8° in flexion and Δμ(θconoe)=0.7° in extension direction, p = 0.95]. This effect was less pronounced in the young group [ Δμ(θcony)=0.2° in flexion and in extension direction, p = 0.99]. These improvements did not yield a relevant effect size (Cohen's d < 0.1). Estimation errors increased with target angle magnitude in both movement directions. In young participants, estimation errors were non-symmetric, with estimations in flexion [ μ(θconyf)=1.8° , σ(θconyf)=7.0° ] being significantly more accurate than in extension [ μ(θconye)=8.3° , σ(θconye)=9.3° , p < 0.01]. This asymmetry was not present in the elderly group, where estimations in flexion [ μ(θconof)=7.5° , σ(θconof)=9.8° ] were similar to extension [ μ(θconoe)=7.7° , σ(θconoe)=9.3° ]. Hence, young and elderly participants performed equally in extension direction, whereas wrist position sense in flexion direction deteriorated with age (p < 0.01). Though unimpaired elderly adults did not benefit from stochastic stimulation, it cannot be deduced that individuals with more severe impairments of their sensory system do not profit from this treatment. While the errors in estimating wrist position are symmetric in flexion and extension in elderly adults, young adults are more accurate when estimating wrist flexion, an effect that has not been described before.

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Phantom tactile sensations modulated by body position.

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Phantom tactile sensations modulated by body position.

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