Brain oscillations have gained great attention in neuroscience during recent decades as functional building blocks of cognitive‐sensory processes. Research has shown that oscillations in “alpha,” “beta,” “gamma,” “delta,” and “theta” frequency windows are highly modified in brain pathology, including in patients with cognitive impairment like bipolar disorder (BD). The study of changes in brain oscillations can provide fundamental knowledge for exploring neurophysiological biomarkers in cognitive impairment. The present article reviews findings from the role and molecular basis of abnormal neural oscillation and synchronization in the symptoms of patients with BD. An overview of the results clearly demonstrates that, in cognitive‐sensory processes, resting and evoked/event‐related electroencephalogram (EEG) spectra in the delta, theta, alpha, beta, and gamma bands are abnormally changed in patients with BD showing psychotic features. Abnormal oscillations have been found to be associated with several neural dysfunctions and abnormalities contributing to BD, including abnormal GABAergic neurotransmission signaling, hippocampal cell discharge, abnormal hippocampal neurogenesis, impaired cadherin and synaptic contact‐based cell adhesion processes, extended lateral ventricles, decreased prefrontal cortical gray matter, and decreased hippocampal volume. Mechanistically, impairment in calcium voltage‐gated channel subunit alpha1 I, neurotrophic tyrosine receptor kinase proteins, genes involved in brain neurogenesis and synaptogenesis like WNT3 and ACTG2, genes involved in the cell adhesion process like CDH12 and DISC1, and gamma‐aminobutyric acid (GABA) signaling have been reported as the main molecular contributors to the abnormalities in resting‐state low‐frequency oscillations in BD patients. Findings also showed the association of impaired synaptic connections and disrupted membrane potential with abnormal beta/gamma oscillatory activity in patients with BD. Of note, the synaptic GABA neurotransmitter has been found to be a fundamental requirement for the occurrence of long‐distance synchronous gamma oscillations necessary for coordinating the activity of neural networks between various brain regions.

Association between abnormal brain oscillations and cognitive performance in patients with bipolar disorder: Molecular mechanisms and clinical evidence

Background Previous studies have attempted to characterize depression using electroencephalography (EEG), but results have been inconsistent. New noise reduction technology allows EEG acquisition during conversation. Methods We recorded EEG from 40 patients with depression as they engaged in conversation using a single-channel EEG device while conducting real-time noise reduction and compared them to those of 40 healthy subjects. Differences in EEG between patients and controls, as well as differences in patients’ depression severity, were examined using the ratio of the power spectrum at each frequency. In addition, the effects of medications were examined in a similar way. Results In comparing healthy controls and depression patients, significant power spectrum differences were observed at 3 Hz, 4 Hz, and 10 Hz and higher frequencies. In the patient group, differences in the power spectrum were observed between asymptomatic patients and healthy individuals, and between patients of each respective severity level and healthy individuals. In addition, significant differences were observed at multiple frequencies when comparing patients who did and did not take antidepressants, antipsychotics, and/or benzodiazepines. However, the power spectra still remained significantly different between non-medicated patients and healthy individuals. Limitations The small sample size may have caused Type II error. Patients’ demographic characteristics varied. Moreover, most patients were taking various medications, and cannot be compared to the non-medicated control group. Conclusion A study with a larger sample size should be conducted to gauge reproducibility, but the methods used in this study could be useful in clinical practice as a biomarker of depression.

Characteristics of single-channel electroencephalogram in depression during conversation with noise reduction technology

Background and Objectives “Noise power” (NP) is a measure that allows the assessment of the fast-firing synchronization of neural oscillations. We aimed to replicate higher gamma NP values in frontal and midline regions in patients with schizophrenia and re-evaluate its specificity to this disorder. We also aimed to assess the relationship of higher gamma NP values with drug treatment, chronicity, cognition and symptoms. Methods Gamma NP values were obtained from electroencephalograms recorded during an oddball paradigm from 29 patients with schizophrenia, 27 with bipolar disorder and 36 healthy controls. We compared these values between the groups to evaluate the specificity to diagnosis. Altered gamma NP values were compared between the patients who had and had not received different treatments to assess the relationship with drug treatment. We also analyse the correlation between gamma NP values and chronicity, symptoms, and cognition. Results Compared to controls, patients with schizophrenia presented increased gamma NP values in frontal and parietal midline regions, while bipolar patients showed increased gamma NP in the left frontal region. There was no significant relationship between drug treatment or chronicity with altered values. Increased gamma NP correlated with higher negative symptom scores in the schizophrenia group, but not with cognitive impairment in any of the groups of patients. Conclusions We replicated an increase in gamma NP in patients with schizophrenia and found that this alteration was also present in a milder form in bipolar patients. These alterations seem to be independent of pharmacological treatment and illness duration.

María Iglesias

Papers

Altered gamma band noise power in schizophrenia and bipolar patients during a cognitive task