The role of primary somatosensory cortex in tactile detection and discrimination: fMRI-guided TMS investigations

Introduction

The contribution of the primary somatosensory cortex (SI) to the perception of tactile stimuli at the fingers is still not fully understood. In particular, while previous reports have agreed on the crucial role of SI in tactile discrimination, its role in simple detection remains controversial. The aim of the present study is to address this issue by combining the QUEST threshold estimation method in a two-interval forced choice design (2IFC), with functional and anatomical magnetic resonance imaging (MRI)-guided single and --double pulse transcranial magnetic stimulation (TMS).

Methods

Participants (N=7 or 12 per experiment) underwent a series of MRI scans (localisers) to produce somatotopic maps of SI. The maps were used to stimulate over SI with TMS during subsequent experiments. In six experiments, QUEST was used to establish threshold for detection or frequency discrimination for tactile stimuli (50ms, 200Hz sinusoidal vibration) applied to the middle and/or index fingertips of one hand. In Experiments 1-3 (n=12), TMS was applied as a single (75ms after stimulus onset) or a double pulse (25 and 75ms after onset) over the contralateral SI (target site), or over contralateral supramarginal gyrus (control site), or with sham TMS (coil held away from the head, no brain stimulation). In Experiments 4-6 (n=7), double pulse TMS (0 and 50ms after onset) was applied over the median nerve at the wrist (target site), or over the extensor digitorum communis of the forearm (EDC, control site), or with sham TMS (no peripheral stimulation). Finally, in Experiment 7 double pulses of TMS were applied over SI, SMG, and away from the head in a single interval “yes-no” detection task, with tactile stimuli presented at the threshold level previously determined by QUEST.

Results

The results showed that 2IFC frequency discrimination thresholds at the fingers were significantly increased by double-pulse TMS over SI (Figure 1a). By contrast, 2IFC detection thresholds at the fingers were not increased by either single (Figure 1c) or double (Figure 1b) pulse TMS over SI relative to SMG (Exp. 2-3). Importantly, both frequency discrimination and simple detection thresholds at the fingers were increased by both single and double-pulse TMS applied over the median nerve at the wrist, relative to the muscle control site (Exp. 4-6, see Figure 1d-f). Finally, double-pulse TMS over SI in the yes-no task decreased the participants’ sensitivity relative to TMS over SMG (Figure 2a), while TMS over both SI and SMG increased the response criterion, making participants more conservative with TMS, regardless of stimulation site (Figure 2b).

Conclusions

The present findings suggest that SI is required for frequency discrimination of vibrotactile stimuli at the fingers. By contrast, SI is not necessary for forced-choice detection in a criterion-free 2IFC task, whereas it is required for detection of tactile stimuli in a single interval “yes-no” task. This discrepancy suggests a different involvement (and importance) of SI in tactile processing as a function of task demands, and with respect to tactile working memory [3,4]. This dissociation reconciles previous literature on monkey [3] and humans [4,5,6].

References

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[2] Tamè L, Charlton J, & Holmes NP (in preparation). A cost effective and fast psychophysics procedure to determine tactile thresholds at the fingers.

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