Abstract
Paired associative stimulation (PAS), a form of non-invasive cortical stimulation pairing transcranial magnetic stimulation (TMS) with a peripheral sensory stimulus, has been shown to induce neuroplastic effects in the human motor, somatosensory and auditory cortex. The current study investigated the effects of acoustic PAS on late auditory evoked potentials (LAEP) and the influence of tone duration and placebo stimulation. In two experiments, 18 participants underwent a PAS with a 4 kHz paired tone of 400 ms duration using 200 pairs of stimuli (TMS-pulse over the left auditory cortex 45 ms after tone-onset) presented at 0.1 Hz. In Experiment 1 this protocol was contrasted with a protocol using a short paired tone of 23 ms duration (PAS-23 ms vs. PAS-400 ms). In Experiment 2 this PAS protocol was contrasted with sham stimulation (PAS-400 ms-sham vs. PAS-400 ms). Before and after PAS, LAEP were recorded for tones of 4 kHz (same carrier frequency as the paired tone) and 1 kHz as control tone. In Experiment 1, there was a significant difference between LAEP amplitudes of the 4 kHz tone after PAS-23 ms and PAS-400 ms with higher LAEP amplitudes after PAS-23 ms. Before both conditions, no difference could be detected. In Experiment 2 we observed a significant overall decrease in LAEP amplitudes pre to post PAS. Unspecific decreases of LAEP following PAS with a long paired tone (PAS-400 ms) might be related to habituation effects due to repeated presentation of sound stimuli which are not evident for PAS with a short paired tone (PAS-23 ms). Interpreting this result using the concept of temporal integration time allows us to discuss it in the context of spike-timing dependent plasticity.
Highlights
Transcranial magnetic stimulation (TMS) can inhibit or facilitate neuronal activity via non-invasive electromagnetic stimulation
Paired associative stimulation (PAS) is one specific rTMS protocol which combines direct stimulation of the brain via very low-frequency rTMS (e.g. 0.1 Hz) with a corresponding peripheral sensory stimulation (Wolters et al 2005)
The non-significant interaction effects frequency by time (F = 3.562; df = 1,17; p = 0.076; η2p = 0.173) and tone duration by time (F = 3.685; df = 1,17; p = 0.072; η2p = 0.178) with medium effect sizes can be neglected in the light of the threefold interaction
Summary
Transcranial magnetic stimulation (TMS) can inhibit or facilitate neuronal activity via non-invasive electromagnetic stimulation. Electric currents are induced in superficial brain areas via rapidly changing magnetic fields generated by a coil of wires acting as an electromagnet (Barker et al 1985; Di Lazzaro et al 2004; Merton and Morton 1980; Siebner and Ziemann 2007). Depending on the frequency of the applied pulses, rTMS has inhibitory (up to 1 Hz) or facilitatory (over 1 Hz) effects on brain activity. According to the model of STDP the synaptic strength between two neurons is enhanced if postsynaptic activity is preceded by presynaptic activity. In the model of STDP the pairing needs to occur within a critical time period [“tens of milliseconds or less” (Markram et al 1997)] in order to induce changes on synaptic strength.
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