Homeostatic Metaplasticity in the Human Somatosensory Cortex

Abstract

Long-term potentiation (LTP) and long-term depression (LTD) are regulated by homeostatic control mechanisms to maintain synaptic strength in a physiological range. Although homeostatic metaplasticity has been demonstrated in the human motor cortex, little is known to which extent it operates in other cortical areas and how it links to behavior. Here we tested homeostatic interactions between two stimulation protocols -- paired associative stimulation (PAS) followed by peripheral high-frequency stimulation (pHFS) -- on excitability in the human somatosensory cortex and tactile spatial discrimination threshold. PAS employed repeated pairs of electrical stimulation of the right median nerve followed by focal transcranial magnetic stimulation of the left somatosensory cortex at an interstimulus interval of the individual N20 latency minus 15 msec or N20 minus 2.5 msec to induce LTD- or LTP-like plasticity, respectively [Wolters, A., Schmidt, A., Schramm, A., Zeller, D., Naumann, M., Kunesch, E., et al. Timing-dependent plasticity in human primary somatosensory cortex. Journal of Physiology, 565, 1039-1052, 2005]. pHFS always consisted of 20-Hz trains of electrical stimulation of the right median nerve. Excitability in the somatosensory cortex was assessed by median nerve somatosensory evoked cortical potential amplitudes. Tactile spatial discrimination was tested by the grating orientation task. PAS had no significant effect on excitability in the somatosensory cortex or on tactile discrimination. However, the direction of effects induced by subsequent pHFS varied with the preconditioning PAS protocol: After PAS(N20-15), excitability tended to increase and tactile spatial discrimination threshold decreased. After PAS(N20-2.5), excitability decreased and discrimination threshold tended to increase. These interactions demonstrate that homeostatic metaplasticity operates in the human somatosensory cortex, controlling both cortical excitability and somatosensory skill.

Modifying somatosensory processing with non-invasive brain stimulation

“…For example, PAS N20 - 2.5 to S1 paired with pHFS facilitated P25 amplitudes and improved tactile spatial acuity as measured with the GOT task (Werhahn, Mortensen, Van Boven, Zeuner, & Cohen, 2002) while PAS N20 - 2.5 paired with pHFS inhibited P25 components and decreased tactile acuity. Unlike previous studies, PAS alone had no effect perhaps explained by slight differences in stimulation protocols (150% RMT, 225 pairs at a faster rate of 0.25Hz) (Bliem, Muller-Dahlhaus, Dinse, & Ziemann, 2008). …”

Consensus Paper: Probing Homeostatic Plasticity of Human Cortex With Non-invasive Transcranial Brain Stimulation

“…(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) evidence for homeostatic plasticity in primary somatosensory cortex [85,86]. Both studies used an NTBS prime preceding a highfrequency tactile stimulation (HFS) protocol of the contralateral hand in order to demonstrate a homeostatic response in the somatosensory cortex.…”

Normalization of sensorimotor integration by repetitive transcranial magnetic stimulation in cervical dystonia

“…For instance, animal experiments demonstrated that depending on the degree of neuronal depolarization the same tetanic stimulation protocol induced either LTP or LTD [26]. In healthy subjects, it has been shown that metaplasticity operates not only in M1 but also in S1 and that it influences corticospinal excitability as well as somatosensory skills [27] 2 SAI before and after 1 Hz rTMS over S1 and M1 is demonstrated for CD patients. Also, SAI of controls before and after S1 rTMS is shown.…”