TMS over primary somatosensory cortex affect tactile discrimination, but not detection thresholds at the fingers

Background

The present study, by using a novel approach combining the efficiency of the QUEST threshold estimation method with transcranial magnetic stimulation (TMS), aims to investigate whether tactile detection and discrimination thresholds at the fingers can be modulated by TMS on SI.

Method

Participants underwent a series of MRI scans (functional localisers) to produce somatotopic maps of the primary somatosensory cortex (SI). These maps were used to stimulate over SI with TMS during subsequent behavioural tasks. The threshold estimation method QUEST was used in a two-interval forced-choice design in order to establish threshold, respectively for detection and frequency discrimination, for a tactile stimulus of 50 ms, 100Hz sinusoidal vibration applied to the middle and/or index fingertips of one hand. This was done either when a single or a double pulse of TMS was applied over the SI (target site), or over the supramarginal gyrus (control site), or without applying TMS.

Results

The results showed that vibrotactile frequency discrimination thresholds at the fingers are increased by TMS over SI when two pulses are applied at 25 and 75ms after the onset of the vibrotactile stimulus. Moreover, preliminary data from two further experiments indicate that vibrotactile detection threshold is not modulated by TMS over SI.

Take home message

SI is required for discrimination, but may not be for detection, of tactile stimuli at the fingers.