Klyuchnikov, V. | Search bioRxiv

bioRxiv2022

Ambiguous tDCS: transcranial direct current stimulation effects on performance of monetary incentive delay task reinforcement learning

Transcranial electrical stimulation (tES) serves as a powerful technique for assessing the causal role of specific brain regions in behavior, including decision-making. While tES studies investigating sensorimotor function yield unequivocal results, cognitive research reveals considerable ambiguity and diversity in stimulation-related effects. The consequences of transcranial direct current stimulation (tDCS) on cognitive functioning are not consistently predictable based on the current direction (cathodal or anodal), which limits its applicability in cognitive research. In the present study, we further explored tES effect ambiguity in cognitive tasks by modulating medial frontal cortex (MFC) activity in an auditory monetary incentive delay (MID) task, where participants responded to acoustic cues encoding expected monetary losses using cathodal tDCS. We analyzed feedback-related negativity (FRN), reflecting prediction error processing when participants encountered losses compared to no losses during two subsequent MID task sessions, and exogenous P2 response to stimulus onset unrelated to anticipated monetary incentives. We anticipated an inhibitory cathodal tDCS effect on both P2 response and FRN. Contrary to our expectations, we observed a facilitatory effect of cathodal tDCS on FRN, replicating our earlier results (Gorin et al., 2022). No effect of stimulation was observed on P2; however, tDCS influenced the learning effect of P2. The difference in P2 amplitude between the first and second sessions, observed in the sham group, was absent in the group that received cathodal tDCS. We offered the interpretation of the complex picture of tES effects during auditory MID performance in light of brain plasticity theory for P2 and reward-learning mechanisms for FRN. Importantly, our findings regarding the multidirectionality of tDCS effects on cognitive function challenge the utility of tES as a readily employable method for testing brain causality in highly complex neurocognitive events, such as decision-making.

neuroscience↗

bioRxiv2019

Diverse genetic origins of medieval steppe nomad conquerors

Over millennia, steppe nomadic tribes raided and sometimes overran settled Eurasian civilizations. Most polities formed by steppe nomads were ephemeral, making it difficult to ascertain their genetic roots or what present-day populations, if any, have descended from them. Exceptionally, the Khazar Khaganate controlled the trade artery between the Black and Caspian Seas in VIII-IX centuries, acting as one of the major conduits between East and West. However, the genetic identity of the ruling elite within the polyglot and polyethnic Khaganate has been a much-debated mystery; a controversial hypothesis posits that post-conversion to Judaism the Khazars gave rise to modern Ashkenazim. We analyzed whole-genome sequences of eight men and one woman buried within the distinctive kurgans of the Khazar upper (warrior) class. After comparing them with reference panels of present-day Eurasian and Iron Age populations, we found that the Khazar political organization relied on a polyethnic elite. It was predominantly descended from Central Asian tribes but incorporated genetic admixture from populations conquered by Khazars. Thus, the Khazar ruling class was likely relatively small and able to maintain a genetic identity distinct from their subjugated populations over the course of centuries. Yet, men of mixed ancestry could also rise into the warrior class, possibly providing troop numbers necessary to maintain control of their large territory. However, when the Khaganate collapsed it left few persistent genetic traces in Europe. Our data confirm the Turkic roots of the Khazars, but also highlight their ethnic diversity and some integration of conquered populations.View Full Text

paleontology↗