Neural activation state determines behavioral susceptibility to modified theta burst transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) allows one to investigate the effects of temporary interference of neural processing in neurologically intact subjects. In a previous study [J. Silvanto et al. (2007) Eur. J. Neurosci., 25, 1874-1881] we found that online TMS perceptually facilitates the attributes encoded by the least active neural populations. The objective of the present experiment was to extend this work to determine whether such state-dependent effects can be observed when offline high-frequency TMS is applied to suppress neural activity. The activity levels of direction-selective neural populations in the V1/V2 region were modulated by asking subjects to passively view either leftward or rightward motion during offline TMS. In a subsequent motion direction-discrimination task, their ability to discriminate motion direction was dependent on the type of motion they had passively viewed during offline TMS: detection of the congruent direction (i.e. direction viewed during offline TMS) was unaffected, whereas detection of the incongruent direction (i.e. opposite direction to the one viewed during offline TMS) was impaired. As the activity level of neurons tuned to the incongruent direction was presumably lower during the TMS than of those tuned to the congruent direction, this behavioral result demonstrates that the offline TMS preferentially suppressed attributes encoded by the least active neural populations. In contrast to direction discrimination, motion detection was not impaired in a direction-specific manner. This shows that the requirements of the psychophysical task, in conjunction with the relative activity states of neuronal populations when TMS is applied, can be used to selectively interfere with overlapping neuronal populations.