Nick Holmes, University of Reading
The Role of Human Primary Somatosensory Cortex in Detecting and Discriminating Vibrotactile Stimuli: Psychophysics, MRI, & TMS Evidence
In humans, it is often claimed that a single pulse of transcranial magnetic stimulation (TMS) presented over the primary somatosensory cortex (S1) at around the same time as a brief tactile stimulus on the hand 'blocks' or 'extinguishes' perception of that stimulus. Luigi Tame and I could not replicate this effect. Instead, using single- and double-pulse TMS guided by individual functional magnetic resonance imaging (fMRI) activations of S1, we show that human S1 is involved in the comparison of two tactile stimuli, or else in maintaining a memory trace of the target stimulus across trials. We were unable to interfere with simple detection using single- or even paired-pulse TMS at high intensity. This result is very surprising given the strong claims made in the literature. A critical analysis of that literature reveals poor experimental design, and results that may have been affected by response biases. Instead, our finding that S1 is involved in the discrimination but not the detection of tactile stimuli complements what we know from macaque neurophysiology and lesion experiments.