Quadri-pulse stimulation induces stimulation frequency dependent cortical hemoglobin concentration changes within the ipsilateral motor cortical network.

Background: Imaging studies investigating repetitive transcranial magnetic stimulation (rTMS) mediated hemodynamic consequences revealed inconsistent results, mainly due to differences in rTMS parameters and technical difficulties with simultaneous recordings during rTMS.

Objective/hypothesis: Quadri-pulse rTMS (QPS) induces bidirectional long-term plasticity of the human primary motor cortex (M1). To evaluate its on-line effects, near infrared spectroscopy (NIRS) recordings were performed during QPS. We hypothesized that on-line effects during QPS are different from long-term aftereffects.

Methods: Using a novel TMS - on-line multi-channel NIRS setup we recorded hemoglobin concentration [Hb] changes at the stimulated M1 and adjacent sensory-motor areas during QPS protocols inducing oppositely directed aftereffects (QPS-5: interstimulus interval (ISI) 5 ms, potentiation; QPS-50: ISI 50 ms, depression). In two experiments we studied NIRS changes during either single or repeated QPS bursts.

Results: The repetitive QPS-5 bursts significantly decreased oxyhemoglobin concentration ([oxy-Hb]) in the ipsilateral M1. A single QPS-5 burst decreased [oxy-Hb] in the M1 and premotor cortex. QPS-50 induced no significant NIRS changes at any sites.

Conclusions: QPS can significantly alter cortical hemodynamics depending on the stimulation frequency. While bidirectional long-term aftereffects of QPS reflect synaptic efficacy changes, unidirectional on-line effects during QPS may represent pure electrophysiological property changes within the cell membrane or synapse. Since neuronal postexcitatory inhibitory postsynaptic potentials typically peak within the first 10-20 ms, only pulses delivered at higher frequencies may lead to summation of the inhibitory effects, resulting in [oxy-Hb] decrease only after QPS-5. Our new TMS-NIRS setup may be valuable to investigate TMS induced neurovascular coupling mechanisms in humans.