Effects of motor imagery on finger force responses to transcranial magnetic stimulation

The purpose of this study was to investigate whether characteristics of finger interaction seen in voluntary finger force production tasks could also be observed during motor imagery. Transcranial magnetic stimulation (TMS) was applied over the contralateral M1 hand area. Three conditions were tested in eight young healthy volunteers: At rest, during motor imagery of maximal force production by the index finger (ImIn), and during motor imagery of maximal force production by all four fingers simultaneously (ImAll). We obtained measures of motor threshold (MT), motor-evoked potentials (MEP) from the contralateral flexor digitorium superficialis, and TMS-induced forces from individual fingers. Increased MEP and decreased MT during motor imagery tasks suggested enhanced excitability of structures involved in the generation of TMS-induced responses. TMS-induced forces were larger during motor imagery tasks than at rest. This effect was present, albeit significantly smaller, in the middle, ring, and little fingers during ImIn as compared to ImAll. This finding has been interpreted as a correlate of the phenomenon of unintended finger force production (enslaving). The motor imagery effect on finger forces evoked by TMS was significantly larger during ImIn (4% MVC) than during ImAll (2.8% MVC) tasks, corresponding to the phenomenon of force deficit. These results provide direct evidence of the neural origin of the main phenomena of finger interaction. Furthermore, the similarities between characteristics of finger interaction during motor imagery and during voluntary movement suggest the involvement of similar neural structures (including M1).