Linear gain effect of theta-burst and 15 Hz rTMS on intra-cortically measured neuronal responses to oriented grating visual stimuli

Transcranial magnetic stimulation (TMS) involves the application of time-pulsed magnetic fields to cortical tissue through a coil positioned near the head. TMS is widely used for studying the mechanisms underlying perception and behaviour and is considered a potential therapeutic technique for various conditions. However, the application of TMS has been hindered by the lack of understanding of its mechanism of action. Here we studied the effects of three repetitive TMS (rTMS) paradigms on intra-cortical neuronal responses to oriented gratings in cat area 18. Each of stimulation protocols, including continuous theta-burst, intermittent theta-burst, and 15 Hz rTMS, consisted of 600 pulses. Application of continuous theta-burst and 15 Hz rTMS suppressed the action potential response to oriented grating stimuli for 1.5-6 minutes. In contrast, application of intermittent theta-burst stimulation was associated with enhancement of the action potential response [~]15 minutes following TMS. Neuronal assemblies that were more responsive before the application of rTMS, were affected more than neurons that were less responsive before rTMS. In spite of these changes, the preferred orientation, the orientation tuning of the multi-unit activity and the spatial pattern of the responses recorded from assemblies neurons remained unaffected. Our findings demonstrate that rTMS does not modify the functional selectivities of ensembles of neurons; rather, it has a linear gain effect on their responses.