Theta’s functional role in encoding, nocturnal reactivation and retrieval for the formation of declarative memory traces
Final Report Abstract
Overall, our project has shown that it is possible to demonstrate theta’s functional role in the formation of stable memory traces. After the change of some key aspects (reduction to acoustic modality only, bi-hemispheric tACS with 4 electrodes, current oscillating in-phase within one hemisphere), we found a frequency-dependent improvement of behavioral performance compared with a control stimulation in at least one study. Contrary to our expectations, the oscillatory activity showed no tACS-related change right after stimulation, in none of the main frequency bands. In addition, our results did not show any effects on time frequency data so far, which might be caused by an overcharge of the nervous system due to tACS or because we could not reach the assumed theta band generators. However, our approach followed not only the idea of entraining brain activity, but also of synchronising PFC and Hippocampus within one hemisphere. Interestingly, in the encoding study, we were able to show that our set-up increased the coherence between frontal and parietal areas, an aspect that remains to be seen in the other two studies. Based on our results, we conclude that the formation process is highly susceptible to the stimulation itself. We argue that our results do not contradict our hypothesis that theta-tACS increases memory performance compared to control stimulation. The susceptibility of the process to stimulation might require a more precise protocol. Thus, we will conduct at least one more study (as a control for the reactivation study) in which we want to explore if the stimulation applied shortly after word onset leads to the improvement as it was expected with the original protocol. In case of success, this protocol can be used during the encoding of the word pairs as well as in the retrieval study. The COVID-19 pandemic severely hampered our goal to finish all measurements within the time frame. However, we will continue to measure the remaining subjects and conduct the aforementioned control study.
Publications
- (2020). Applying time series analyses on continuous accelerometry data – A clinical example in older adults with and without cognitive impairment. MedRxiv, 1–14
Rackoll, T., Neumann, K., Passmann, S., Grittner, U., Külzow, N., Ladenbauer, J., & Flöel, A.
(See online at https://doi.org/10.1101/2020.03.24.20042226) - (2020). The Modulation of Cognitive Performance with Transcranial Alternating Current Stimulation: A Systematic Review of Frequency-Specific Effects. Brain Sciences
Klink, K., Paßmann, S., Kasten, F. H., & Peter, J.
(See online at https://doi.org/10.3390/brainsci10120932)
