Inter-hemispheric interaction of touches at the fingers: a combined psychophysics and TMS approach

Many studies have investigated interactions in the processing of tactile stimuli presented at different fingers. However, the precise time-scale of these interactions when stimuli come on opposite sides of the body remains uncertain. Specifically, it is not clear how tactile stimulation of different fingers of the same and different hands can interact with each other. The aim of the present study was to address this issue using a novel approach combining the QUEST threshold estimation method with single pulse TMS (spTMS). First, QUEST was used in a two-interval forced-choice design in order to establish threshold for detecting a 200ms, 100Hz sinusoidal vibration applied to the index fingertip (target finger threshold). This was done either when the target was presented in isolation and 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) and kept constant the target finger (left or right index), while also controlling the spatial location of the stimuli. Second, the same participants underwent a series of MRI scans (localisers) to produce somatotopic maps of SI and SII cortices. These maps were used to stimulate over SI with spTMS during a subsequent behavioural task, with the aim of modulating the behavioural interactions between the different fingers. The results showed that 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. These preliminary data support a number of recent findings from behavioural, neuroimaging, and neuropsychological studies. The differential effect of the distractor finger on target detection thresholds is consistent with the segregation of different fingers in early somatosensory processing, from the periphery to S1.