Attentional effects on visual-tactile crossmodal enhancement at early stages of cortical processing

Abstract

Sensory processing can be facilitated through bimodal interactions between relevant visual-tactile sensory inputs in order to achieve goal-oriented behaviours. While the specific neural mechanisms contributing to this modulation remain unclear, the dorsolateral prefrontal cortex (DLPFC) may have a role in regulating the observed processing facilitation seen in the somatosensory cortex (S1), though the extent is not yet clear. We used electroencephalography (EEG) to observe the temporal contributions of visual priming to the enhancement of S1 responses. We hypothesized that inhibiting DLPFC cortical activity would result in a diminished facilitation of tactile processing in S1 (represented by the P50), observed by a visual-tactile stimuli onset with a 200-300 ms time delay. Somatosensory modulation was inferred through amplitude and latency shifts in tactile event-related potentials (ERPs) recorded while participants performed a sensory integration task that required scaled motor responses dependent on the amplitudes of tactile and visual stimuli. Tactile stimuli were discrete vibrations (25 Hz) presented to the left index finger, visual stimuli were presented as a central horizontal bar on a computer screen at varying heights, and graded motor responses were made by squeezing a pressure-sensitive rubber bulb. Healthy adults completed a training session to become familiar with the stimulus-response relationships for both visual and tactile stimuli prior to completing a task where pairs of discrete stimuli with random amplitudes were presented: Tactile-tactile (TT, 500 ms each, 30 ms ISI), visual-tactile with a 200-300 ms delay (vTd 200-300 ms), and visual-tactile with a 300-400 ms delay (vTd 300-400 ms). Stimuli pairs were administered in a block setting, where each block contained 60 trials, with 20 trials for each of the discrete stimuli presented in a randomized order. The study design consisted of 10 blocks, with a short transcranial magnetic stimulation (TMS) intervention at the halfway mark. Participants were randomly assigned to either an intervention group (n=16) or control group (n=16) where TMS modalities of theta burst stimulation (TBS); continuous TBS (cTBS) was given to the intervention group and intermittent TBS (iTBS) was given to the control group, both applied to the right DLPFC. Results revealed that P50 upregulation observed in condition vTd (200- 300 ms) is significantly lower following cTBS on the right DLPFC but still greater than unimodal TT stimulation. Following iTBS, bimodal facilitation was observed in condition vTd (300-400 ms) for P50 and N70. These findings improve our understanding of the role right DLPFC plays regarding crossmodal facilitation observed in visual-tactile processing.