Single pulse TMS over the primary somatosensory cortex and its effect on vibrotactile detection thresholds at the fingers

Introduction

Many studies have investigated interactions in the processing of tactile stimuli presented at different fingers [1]. In a previous study [2] a Bayesian adaptive staircase procedure (QUEST) [3] and a two-interval forced-choice design was used in order to establish threshold for detecting a 200ms, 100Hz sinusoidal vibration applied to the left or right index fingertip (target). This was done either when the target was presented in isolation or concurrently with a distractor stimulus on another finger of the same or a different hand. We systematically varied the distractor finger (index, middle, ring or little finger of either hand), while also controlling the spatial location of the stimuli. The threshold for detecting the target was lower when it was presented in isolation, as compared to when a concurrent distractor was present. Moreover, detection thresholds varied as a function of the distractor finger stimulated both within (Fig. 1) and between (Fig. 2) the hands. However, the precise time-course of these interactions when stimuli arrive on opposite sides of the body remains uncertain. Specifically, it is not clear how, when, or by which anatomical pathway tactile stimulation of different fingers of the same and different hands interact. The aim of the present study is to address this issue by combining the QUEST threshold estimation method with MRI-guided single pulse TMS (spTMS). Here we will present some preliminary results.

Methods

Participants (N=2) underwent a series of MRI scans (localisers) to produce somatotopic maps of SI and SII cortices. The localiser included MRI-compatible piezoelectric vibrotactile stimulation of each digit on each hand in separate runs (10 conditions, 12s ON, 12s OFF, 10 blocks per digit). Data acquisition was optimised for the peri-rolandic, insular, and parietal cortices. During the scan, participants fixated a cross in the middle of a screen. The data were analysed in FSL by contrasting all stimulation blocks against rest. The peak activation focus in SI was used to position the spTMS during a modified version of the behavioural task described above.

Results

The results of this preliminary study show an increase of vibrotactile detection thresholds for the index finger when spTMS was applied to the contralateral SI with respect to the locus of stimulation, as compared to when spTMS was not present.

Conclusions

These preliminary data show that it is possible to modulate vibrotactile target detection thresholds at the fingers at an early level of somatosensory processing. This study lays the foundation for future research aiming to investigate low level interactions occurring between multiple tactile stimuli on the body surface. The differential effect of the distractor finger on target detection thresholds [2] is consistent with the segregation of different fingers in early somatosensory processing, from the periphery to S1 [4]. In this respect, it would be interesting to test whether TMS over somatosensory cortices would affect equally homologous and non-homologous finger pairs stimulated of the two sides of the body.

References

[1] Tamè L, Farnè A, & Pavani F (2011). Spatial coding of touch at the fingers: Insights from double simultaneous stimulation within and between hands. Neuroscience Letters, 487(1): 78-82.

[2] Tamè L, Johnstone T, Vig M, & Holmes NP (in preparation). Vibrotactile detection thresholds between fingers on the two hands reflect somatosensory receptive field organisation.

[3] Watson AB, & Pelli DG (1983). QUEST: A Bayesian adaptive psychometric method. Perception & Psychophysics, 33(2): 113-120.

[4] Nelson AJ, & Chen R (2008). Digit somatotopy within cortical areas of the postcentral gyrus in humans. Cerebral Cortex, 18: 2341-2351.