In the past decade, several studies have examined the effects of transcranial direct current stimulation (tDCS) on long-term episodic memory formation and retrieval. These studies yielded conflicting results, likely due to differences in stimulation parameters, experimental design and outcome measures.
In this work we aimed to assess the robustness of tDCS effects on long-term episodic memory using a meta-analytical approach.
We conducted four meta-analyses to analyse the effects of anodal and cathodal tDCS on memory accuracy and response times. We also used a moderator analysis to examine whether the size of tDCS effects varied as a function of specific stimulation parameters and experimental conditions.
Although all selected studies reported a significant effect of tDCS in at least one condition in the published paper, the results of the four meta-analyses showed only statistically non-significant close-to-zero effects. A moderator analysis suggested that for anodal tDCS, the duration of the stimulation and the task used to probe memory moderated the effectiveness of tDCS. For cathodal tDCS, site of stimulation was a significant moderator, although this result was based on only a few observations.
To warrant theoretical advancement and practical implications, more rigorous research is needed to fully understand whether tDCS reliably modulates episodic memory, and the specific circumstances under which this modulation does, and does not, occur.
BACKGROUND: Repetitive transcranial magnetic stimulation (rTMS) of the motor cortex activates corticospinal neurons mainly through the depolarization of cortico-cortical axons belonging to interneurons of superficial layers. OBJECTIVE: We used single-fiber electromyography (SFEMG) to estimate the "central jitter" of activation latency of interneural pools from one pulse of TMS to another. METHODS: We evaluated 10 healthy subjects and one patient with multiple sclerosis. By recording SFEMG evoked activity from the left first dorsal interosseous (FDI), we first used a standard repetitive electrical 3 Hz stimulation of the ulnar nerve at the wrist to calculate the mean consecutive difference from at least 10 different potentials. The same procedure was applied during 3 Hz repetitive TMS of the contralateral motor cortex. The corticospinal monosynaptic connection of the FDI and the selectivity of SFEMG recording physiologically justified the subtraction of the "peripheral jitter" from the whole cortico-muscular jitter, obtaining an estimation of the actual "central jitter." RESULTS: All subjects completed the study. The peripheral jitter was 28 mus +/- 6 and the cortico-muscular jitter was 344 mus +/- 97. The estimated central jitter was 343 +/- 97 mus. In the patient the central jitter was 846 mus, a value more than twice the central jitter in healthy subjects. CONCLUSION: Current results demonstrate that the evaluation of the central component of the cumulative cortico-muscular latency variability in healthy subjects is feasible with a minimally invasive approach. We present and discuss this methodology and provide a "proof of concept" of its potential clinical applicability in a patient with multiple sclerosis.
Navigated repetitive transcranial magnetic stimulation in stroke rehabilitation (randomize double-blind sham-controlled study)
Transcranial alternating current stimulation (tACS) can be used to modulate brain activity. tACS was shown to induce frequency-, state-, and phase- dependent effects which makes tACS a neurostimulation technique that provides a more valuable predictable outcome. However, the impact of different tACS intensities has not been systematically investigated yet. Here, we proposed to investigate the effects of tACS of the primary motor cortex (M1) delivered at different intensities.
There is a common assumption that application of stimulation for longer duration or for higher intensity leads to more reliable physiological and behavioral effects. However, previous studies performed using different transcranial electrical stimulation methods such as transcranial direct current stimulation (tDCS) and/or at high-frequency such as tACS at ripple range, showed non-monotonic effect of stimulation intensity. Nevertheless, tDCS and high-frequency tACS potentially rely on different mechanisms of neuromodulation with respect to conventional tACS delivered at EEG range (1 – 70 Hz).
In this study we applied 20 Hz tACS to the primary motor cortex (M1) to investigate potential non-monotonic effect of tACS intensities (ranging from 0.25 mA to 2 mA with 0.25 mA interval between conditions) on the M1 excitability measured as the peak-to-peak amplitude of TMS-induced motor evoked potentials (MEPs). As for control, we used 1 mA 10 Hz (alpha) tACS and a no stimulation condition.
Preliminary results (N = 9) showed increase of MEPs for higher intensities (1.5 mA, 2 mA) of stimulation. In addition, an interesting effect emerged for those subjects with a lower motor threshold which showed a higher MEPs modulation effect of beta-tACS