The auditory space in the motor system

- TMS applied to the motor cortex (M1) at rest reveals ongoing covert motor programs.
- We delivered TMS to M1 while subjects heard sounds from different directions.
- The resulting TMS-evoked arm accelerations correlated with the direction of sound.
- Spatial tuning of TMS-evoked movements was evident at 50 ms from sound onset.
- A fast auditory spatial representation is present in the motor system of humans.

Sensory events in the space around us trigger specific motor patterns directed toward or away from the spatial location of the sensory source. Spatially-defined sensorimotor associations are well-known in the visual domain but less so for the auditory modality. In particular no spatially-directed audio-motor association has been described for the upper limb. We tested the instantaneous directional tuning of the corticospinal system by means of single-pulse transcranial magnetic stimulation (TMS) over the left motor cortex in 16 healthy volunteers while at rest. We recorded the lateral accelerations of the TMS-evoked movement by means of an accelerometer placed on the forearm. Acoustic stimuli (pure tone frequency = 1000 Hz, duration = 50 ms) coming from 25 different directions lying in the axial anterior half-plane at the height of the participant's ears were played on earphones. The entire set of sound directions covered a span of 160° (±80° where 0° is the frontal direction) at a fixed azimuth angle. Six different intervals between sound onset and TMS (0, 25, 50, 100, 150 and 200 ms) were tested for each sound direction. Significant correlations were found between sound origin and TMS-evoked arm accelerations only when TMS was delivered 50 ms prior to sound onset. We show the presence in the upper limb motor system of auditory spatial tuning. Sound information accesses the motor system at very short latency, potentially compatible with both a subcortical and a cortical origin of the response. The use of TMS-evoked accelerations allowed us to disclose a strict directional tuning in audio-motor associations.