Metaplasticity in human primary somatosensory cortex: effects on physiology and tactile perception

Abstract

Theta-burst stimulation (TBS) over human primary motor cortex evokes plasticity and metaplasticity, the latter contributing to the homeostatic balance of excitation and inhibition. Our knowledge of TBS-induced effects on primary somatosensory cortex (SI) is limited, and it is unknown whether TBS induces metaplasticity within human SI. Sixteen right-handed participants (6 females, mean age 23 yr) received two TBS protocols [continuous TBS (cTBS) and intermittent TBS (iTBS)] delivered in six different combinations over SI in separate sessions. TBS protocols were delivered at 30 Hz and were as follows: a single cTBS protocol, a single iTBS protocol, cTBS followed by cTBS, iTBS followed by iTBS, cTBS followed by iTBS, and iTBS followed by cTBS. Measures included the amplitudes of the first and second somatosensory evoked potentials (SEPs) via median nerve stimulation, their paired-pulse ratio (PPR), and temporal order judgment (TOJ). Dependent measures were obtained before TBS and at 5, 25, 50, and 90 min following stimulation. Results indicate similar effects following cTBS and iTBS; increased amplitudes of the second SEP and PPR without amplitude changes to SEP 1, and impairments in TOJ. Metaplasticity was observed such that TOJ impairments following a single cTBS protocol were abolished following consecutive cTBS protocols. Additionally, consecutive iTBS protocols altered the time course of effects when compared with a single iTBS protocol. In conclusion, 30-Hz cTBS and iTBS protocols delivered in isolation induce effects consistent with a TBS-induced reduction in intracortical inhibition within SI. Furthermore, cTBS- and iTBS-induced metaplasticity appear to follow homeostatic and nonhomeostatic rules, respectively.

Test-Retest Reliability of Homeostatic Plasticity in the Human Primary Motor Cortex

“…Finally, our findings are limited to homeostatic plasticity in the healthy M1 using a double tDCS protocol. Further research is needed to determine the test-retest reliability of homeostatic plasticity induced using other noninvasive brain stimulation methodologies in M1, as well as homeostatic plasticity induced in other brain regions relevant to different pathologies [ 55 – 58 ].…”

Impact of different intensities of intermittent theta burst stimulation on the cortical properties during TMS‐EEG and working memory performance

“…Across different sessions, participants received iTBS at different intensities (50%, 75%, or 100% rMT). 75% rMT was chosen as smaller EEG responses to TMS have been observed in the prefrontal cortex compared to the motor cortex (Kahkonen et al, ) and to be within the range of 70–80% motor threshold as previously been described (Gentner et al, ; Goldsworthy, Pitcher, & Ridding, ; Jones et al, ; Nettekoven et al, ; Pedapati et al, ; Tsang et al, ). In addition, the intensity was set relative to rMT rather than aMT to avoid potential metaplastic influences related to prior muscle activation (Cash, Mastaglia, & Thickbroom, ).…”

The effects of individualised intermittent theta burst stimulation in the prefrontal cortex: A TMS‐EEG study

“…The variability of the plastic effects were also reduced in this patient group, emphasising the importance and the impact of the intrinsic physiological properties on TBS-induced aftereffects. Furthermore, iTBS may evoke an opposite effect to that of an expected direction (i.e., decrease in MEP amplitude) in patients with spinal cord injury (Fassett et al, 2017), and the resulting outcome may also depend on the cortical area stimulated (Jones et al, 2016). It remains to be determined whether Ind iTBS will elicit desired effects in neurological disorders, and different methods of individualisation may be required for different brain regions.…”