Abstract
Sensing movements across the skin surface is a complex task for the tactile sensory system, relying on sophisticated cortical processing. Functional MRI has shown that judgements of the direction of tactile stimuli moving across the skin are processed in distributed cortical areas in healthy humans. To further study which brain areas are important for tactile direction discrimination, we performed a lesion study, examining a group of patients with first-time stroke. We measured tactile direction discrimination in 44 patients, bilaterally on the dorsum of the hands and feet, within 2 weeks (acute), and again in 28 patients 3 months after stroke. The 3-month follow-up also included a structural MRI scan for lesion delineation. Fifty-nine healthy participants were examined for normative direction discrimination values. We found abnormal tactile direction discrimination in 29/44 patients in the acute phase, and in 21/28 3 months after stroke. Lesions that included the opercular parietal area 1 of the secondary somatosensory cortex, the dorsolateral prefrontal cortex or the insular cortex were always associated with abnormal tactile direction discrimination, consistent with previous functional MRI results. Abnormal tactile direction discrimination was also present with lesions including white matter and subcortical regions. We have thus delineated cortical, subcortical and white matter areas important for tactile direction discrimination function. The findings also suggest that tactile dysfunction is common following stroke.
Highlights
Humans have a sophisticated sensitivity to the features of movements across the skin, which relies on complex cortical processing (Pei et al, 2010; McIntyre et al, 2016)
Using functional magnetic resonance imaging with healthy humans, we have shown that direction discrimination of tactile motion applied to the leg is processed in a wide set of cortical regions, and that the specific areas engaged depend on the cues present in the stimulus
Comparing the Tactile direction discrimination (TDD) scores in the acute phase compared to 3 months after the stroke, we found no significant difference when testing on the feet [mean difference in relative TDD score 1⁄4 À0.01, t(55) 1⁄4 0.16, P 1⁄4 0.971, paired samples t-test]
Summary
Humans have a sophisticated sensitivity to the features of movements across the skin, which relies on complex cortical processing (Pei et al, 2010; McIntyre et al, 2016). Tactile direction judgements are typically more sensitive in the presence of skin stretch (Olausson et al, 1998; Gleeson et al, 2010), likely due to activity in the slowly adapting type 2 peripheral afferents, which are highly sensitive to the direction of skin stretch (Olausson et al, 1998, 2000) Low friction stimuli, such as a rolling wheel, moving air jet or a tactile array, cause little to no skin stretch, providing only the successive positions cue for movement direction (Norrsell and Olausson, 1994; McIntyre et al, 2016). Such stimuli engage all classes of peripheral low-threshold mechanoreceptor afferents, both myelinated and unmyelinated, and the directional information is most likely conveyed via central integration of the successively activated primary afferents with neighbouring receptive fields (Gardner and Palmer, 1989; Srinivasan et al, 1990; Pei et al, 2010; McIntyre et al, 2016; Saal et al, 2017)
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