Tactile versus Motor Imagery: Differences in Corticospinal Excitability Assessed with single-pulse TMS

Abstract Tactile Imagery (TI) remains a fairly understudied phenomenon despite an increased attention to this topic in recent years. Here we investigated the effects of TI on corticospinal excitability by measuring motor evoked potentials (MEPs) induced by single-pulse transcranial magnetic stimulation (TMS). The effects of TI were compared with those of tactile stimulation (TS) and kinesthetic motor imagery (kMI). Twenty-two participants performed three tasks in randomly assigned order: imagine finger tapping (kMI); experience vibratory sensations in the middle finger (TS); and mentally reproduce the sensation of vibration (TI). MEPs increased during both kMI and TI, with a stronger increase for kMI. No statistically significant change in MEP was observed during TS. The demonstrated differential effects of kMI, TI and TS on corticospinal excitability have practical implications for the development of imagery-based and TS-based brain-computer interfaces (BCIs), particularly the ones intended to improve neurorehabilitation by evoking plastic changes in sensorimotor circuitry. Significance Statement While it is known that tactile imagery (TI) engages the primary somatosensory cortex similarly to physical tactile perceptions, it is not well understood how TI affects neural processing in the primary motor cortex (M1), the area that controls voluntary movements while receiving somatosensory feedback. This study employed transcranial magnetic stimulation (TMS) to examine the responsiveness of M1 to different types of somatosensory imagery in response to TMS. TI facilitated the responses in the forearm and hand muscles but to a significantly lesser extent compared to kinesthetic motor imagery (kMI). This demonstration of the distinct effects of TI and kMI on corticospinal excitability highlights the importance of selecting an imagery strategy when using imagery to modulate cortical representation of the body. These findings have practical implications for the development of imagery-based brain-computer interfaces (BCIs) intended for rehabilitation of sensorimotor impairments.