Errors of somatosensory localisation in a patient with right-hemisphere stroke

Intact tactile detection yet biased tactile localization in a hand-centered frame of reference: Evidence from a dissociation

We examined the performance of an individual with subcortical damage, but an intact somatosensory thalamocortical pathway, to examine the functional architecture of tactile detection and tactile localization processes. Consistent with the intact somatosensory thalamocortical pathway, tactile detection on the contralesional hand was well within the normal range. Despite intact detection, the individual demonstrated substantial localization biases. Across all localization experiments, he consistently localized tactile stimuli to the left side in space relative to the long axis of his hand. This was observed when the contralesional hand was palm up, palm down, rotated 90° relative to the trunk, and when making verbal responses. Furthermore, control experiments demonstrated that this response pattern was unlikely a motor response error. These findings indicate that tactile localization on the body is influenced by proprioceptive information specifically in a hand-centered frame of reference. Furthermore, this also provides evidence that aspects of tactile localization are mediated by pathways outside of the primary somatosensory thalamocortical pathway.

Biases in tactile localization by pointing: compression for weak stimuli and centering for distributions of stimuli

Weak electrocutaneous stimuli applied to the forearm are erroneously localized toward its middle (Steenbergen P, Buitenweg JR, Trojan J, Veltink PH. Exp Brain Res 232: 597-607, 2014). We asked whether mechanical touch stimuli exhibit a similar bias and whether the bias is toward the middle of the forearm or toward the middle of the recent stimulus distribution. In experiments 1 and 2, participants ( n = 12 and n = 10) localized by pointing von Frey filaments applied to four locations on the dorsal forearm. Individually adjusted weak and strong stimuli ( experiment 1) or two levels of strong stimuli ( experiment 2) were presented in single sessions in random order. Weaker stimuli were localized with greater variability than the strong, with compression toward the middle of the forearm. Responses to the two levels of strong stimuli did not differ. In experiment 3, participants ( n = 16) were presented two spatially offset stimulus distributions (8 cm center-to-center), each offset from the forearm middle, on 2 different days. Out of four target locations comprising each distribution, two were shared. Responses to weak stimuli were compressed compared with responses to strong stimuli. Importantly, biases for the common targets had opposite directions, each being toward the middle of the distribution within which targets were presented. Responses to strong stimuli also exhibited a distribution-dependent bias, a 2-cm overall shift across the forearm midpoint. We conclude that touch localization is subject to intensity-dependent biases determined by the recent history of stimulation and possibly also by the available or perceived response space. NEW & NOTEWORTHY Recent findings show that weak electrical stimuli applied to the forearm are mislocalized toward the forearm middle, relative to strong stimuli. We found that weak mechanical stimuli are similarly mislocalized. The bias changed if, as a group, stimuli were not centered on the forearm middle: weak stimuli gravitated toward the center of prior stimulation. Localization of strong stimuli was also biased, consistent with the tendency to center responses within the available response space.

Somatotopic mismatch of hand representation following stroke: is recovery possible?

Well-organized somatotopic representation of the hand is required to interpret input from cutaneous mechanoreceptors. Previous reports have identified patients with various distortions of somatotopic representation after stroke. Importantly, those patients were investigated years after the stroke, indicating that afferent signal regained access to the cortical circuits; however, further plastic changes, which would re-establish somatotopic order and ability to correctly localize tactile stimuli, did not follow. Thus, it was not known whether somatotopic organization could be restored in such patients and whether there is a potential for new rehabilitation strategies. This is the first case report demonstrating normalization of somatotopic representation.

A new method for assessing self-touch enhancement of the foot in stroke patients with mobility problems

Patient NG is the first reported case of lower-limb 'self-touch enhancement' following stroke. Mobility problems prevented NG from reaching to touch her foot, thus we used a self-touch rubber-hand paradigm to mimic the conditions of self-administered touch. With vision precluded, NG administered stimulation to a prosthetic limb while the Examiner administered synchronous stimulation to NG's affected left foot. NG detected all stimulation administered with our self-touch paradigm, whereas in the control condition (with NG not involved in administering stimulation), NG had failed to detect one-third of Examiner-administered stimulation. When mobility problems are a barrier to investigating self-touch enhancement, the self-touch paradigm can be used to demonstrate residual tactile sensation following stroke.

The role of self-touch in somatosensory and body representation disorders after stroke

Somatosensory impairments occur in about half of the cases of stroke. These impairments range from primary deficits in tactile detection and the perception of features, to higher order impairments in haptic object recognition and bodily experience. In this paper, we review the influence of active- and self-touch on somatosensory impairments after stroke. Studies have shown that self-touch improves tactile detection in patients with primary tactile deficits. A small number of studies concerned with the effect of self-touch on bodily experience in healthy individuals have demonstrated that self-touch influences the structural representation of one's own body. In order to better understand the effect of self-touch on body representations, we present an informal study of a stroke patient with somatoparaphrenia and misoplegia. The role of self-touch on body ownership was investigated by asking the patient to stroke the impaired left hand and foreign hands. The patient reported ownership and a change in affect over all presented hands through self-touch. The time it took to accomplish ownership varied, based on the resemblance of the foreign hand to the patient's own hand. Our findings suggest that self-touch can modulate impairments in body ownership and affect, perhaps by helping to reinstate the representation of the body.