Neuronal activity in the subthalamic nucleus (STN) of patients with Parkinson's disease (PD) is characterised by excessive neuronal synchronization, particularly in the beta frequency range. However, less is known about the temporal dynamics of neuronal oscillations in PD. In this respect long-range temporal correlations (LRTC) are of special interest as they quantify the neuronal dynamics on different timescales and have been shown to be relevant for optimal information processing in the brain. While the presence of LRTC has been demonstrated in cortical data, their existence in deep brain structures remains an open question. We investigated (i) whether LRTC are present in local field potentials (LFP) recorded bilaterally from the STN at wakeful rest in ten patients with PD after overnight withdrawal of levodopa (OFF) and (ii) whether LRTC can be modulated by levodopa treatment (ON). Detrended fluctuation analysis was utilised in order to quantify the temporal dynamics in the amplitude fluctuations of LFP oscillations. We demonstrated for the first time the presence of LRTC (extending up to 50 s) in the STN. Importantly, the ON state was characterised by significantly stronger LRTC than the OFF state, both in beta (13-35 Hz) and high-frequency (> 200 Hz) oscillations. The existence of LRTC in subcortical structures such as STN provides further evidence for their ubiquitous nature in the brain. The weaker LRTC in the OFF state might indicate limited information processing in the dopamine-depleted basal ganglia. The present results implicate LRTC as a potential biomarker of pathological neuronal processes in PD.
Cognitive control during conflict monitoring, error processing, and post-error adjustment appear to be associated with the occurrence of midfrontal theta (MFϴ). While this association is supported by correlational EEG studies, much less is known about the possible causal link between MFϴ and error and conflict processing. In the present study, we aimed to explore the role of band-specific effects in modulating the error system during a conflict resolution. In turn, we delivered transcranial alternating current stimulation (tACS) at different frequency bands (delta δ, theta θ, alpha α, beta β, gamma γ) and sham stimulation over the medial frontal cortex (MFC) in 36 healthy participants performing a modified version of the Flanker task. Task performance and reports about the sensations (e.g. visual flickering, cutaneous burning) induced by the different frequency bands, were also recorded. We found that online θ-tACS increased the response speed to congruent stimuli after error execution with respect to sham stimulation. Importantly, the accuracy following the errors did not decrease because of speed-accuracy trade off. Moreover, tACS evoked visual and somatosensory sensations were significantly stronger at α-tACS and β-tACS compared to other frequencies. Our findings suggest that theta activity plays a causative role in modulating behavioural adjustments during perceptual choices in a stimulus-response conflict task.