TMS brain mapping in less than two minutes

Background
Transcranial magnetic stimulation (TMS) corticospinal excitability maps are a valuable tool to study plasticity in the corticospinal tract. Traditionally, data acquisition for a single map is time consuming, limiting the method's applicability when excitability changes quickly, such as during motor learning, and in clinical investigations where assessment time is a limiting factor.
Objective
To reduce the time needed to create a reliable map by 1) investigating the minimum interstimulus interval (ISI) at which stimuli may be delivered, and 2) investigating the minimum number of stimuli required to create a map.
Method
Frameless stereotaxy was used to monitor coil position as the coil was moved pseudorandomly within a 6 × 6 cm square. Maps were acquired using 1–4 s ISIs in 12 participants. The minimum number of stimuli was determined by randomly extracting data and comparing the resulting map to the original data set. To confirm validity, the pseudorandom walk method was compared against a traditional mapping method.
Results
Reliable maps could be created with 63 stimuli recorded with a 1 s ISI. Maps created acquiring data using the pseudorandom walk method were not significantly different from maps acquired following the traditional method.
Conclusions
To account for inter-participant variability, outliers, coil positioning errors and, most importantly, participant comfort during data acquisition, we recommend creating a map with 80 stimuli and a 1.5 s ISI. This makes it possible to acquire TMS maps in 2 min, making mapping a more feasible tool to study short- and long-term changes in cortical organization.