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Tamè L, Holmes NP (2016) Involvement of human primary somatosensory cortex in vibrotactile detection depends on task demand. NeuroImage, 138:184-196      
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Abstract
Detecting and discriminating between sensory stimuli are fundamental functions of the nervous system. Electrophysiological and lesion studies in the macaque suggest that primary somatosensory cortex (SI) is critically involved in discriminating between somatosensory stimuli, but is not required simply for detecting those stimuli. By contrast, studies in humans using transcranial magnetic stimulation (TMS) over sensorimotor cortex show near-complete disruption of detection when TMS is delivered with the somatosensory target. To address this discrepancy between human and monkey studies, we measured the sensitivity and decision criteria of participants detecting vibrotactile stimuli in a one-interval forced-choice (1IFC) design with individually-tailored fMRI-guided TMS applied over SI, over a control site not activated by vibrotactile stimuli (inferior parietal lobule, IPL), or away from the head (sham). TMS increased participants' likelihood of reporting 'no' target present regardless of site, but TMS over SI also prevented improvement in performance over time. We then measured tactile thresholds in a series of criterion-free 2IFC detection and discrimination tasks with relatively low working memory load. We found that thresholds for detecting vibrotactile stimuli were comparable with TMS over SI and over IPL, but TMS over SI significantly impaired vibrotactile frequency discrimination. Control experiments showed that stimulation over the median nerve at the wrist increased both detection and discrimination thresholds relative to stimulation over finger extensor muscles. We conclude that, in accordance with macaque studies, human SI is required for discriminating between tactile stimuli, and for maintaining stimulus representations over time, but may not be required for simple tactile detection