Visual working memory recruits two functionally distinct alpha rhythms in posterior cortex

Oscillatory activity in the human brain is dominated by posterior alpha oscillations (8-14 Hz), which have been shown to be functionally relevant in a wide variety of cognitive tasks. Although posterior alpha oscillations are commonly considered a single oscillator anchored at an individual alpha frequency (IAF; ∼10 Hz), previous work suggests that IAF reflects a spatial mixture of different brain rhythms. In this study, we assess whether Independent Component Analysis (ICA) can disentangle functionally distinct posterior alpha rhythms in the context of visual short-term memory retention. Magnetoencephalography (MEG) was recorded in 33 subjects while performing a visual working memory task. Group analysis at sensor level suggested the existence of a single posterior alpha oscillator that increases in power and decreases in frequency during memory retention. Conversely, single-subject analysis of independent components revealed the existence of two dissociable alpha rhythms: one that increases in power during memory retention (Alpha1) and another one that decreases in power (Alpha2). Alpha1 and Alpha2 rhythms were differentially modulated by the presence of visual distractors (Alpha1 increased in power while Alpha2 decreased) and had an opposite relationship with accuracy (positive for Alpha1 and negative for Alpha2). In addition, Alpha1 rhythms showed a lower peak frequency, a narrower peak width, a greater relative peak amplitude and a more central source than Alpha2 rhythms. Together, our results demonstrate that modulations in posterior alpha oscillations during short-term memory retention reflect the dynamics of at least two distinct brain rhythms with different functions and spatiospectral characteristics.Significance statementAlpha oscillations are the most prominent feature of the human brain's electrical activity, and consist of rhythmic neuronal activity in posterior parts of the cortex. Alpha is usually considered a single brain rhythm that changes in power and frequency to support cognitive operations. We here show that posterior alpha entails at least two dissociable rhythms with distinct functions and characteristics. These findings could solve previous inconsistencies in the literature regarding the direction of task-related alpha power/frequency modulations and their relation to cognitive performance. In addition, the existence of two distinct posterior alpha rhythms could have important consequences for the design of neurostimulation protocols aimed at modulating alpha oscillations and subsequently cognition.

Oscillatory Properties of Functional Connections Between Sensory Areas Mediate Cross-Modal Illusory Perception

The presentation of simple auditory stimuli can significantly impact visual processing and even induce visual illusions, such as the auditory-induced double flash illusion (DFI). These cross-modal processes have been shown to be driven by occipital oscillatory activity within the alpha band. Whether this phenomenon is network specific or can be generalized to other sensory interactions remains unknown. The aim of the current study was to test whether cross-modal interactions between somatosensory-to-visual areas leading to the same (but tactile-induced) DFI share similar properties with the auditory DFI. We hypothesized that if the effects are mediated by the oscillatory properties of early visual areas per se, then the two versions of the illusion should be subtended by the same neurophysiological mechanism (i.e., the speed of the alpha frequency). Alternatively, if the oscillatory activity in visual areas predicting this phenomenon is dependent on the specific neural network involved, then it should reflect network-specific oscillatory properties. In line with the latter, results recorded in humans (both sexes) show a network-specific oscillatory profile linking the auditory DFI to occipital alpha oscillations, replicating previous findings, and tactile DFI to occipital beta oscillations, a rhythm typical of somatosensory processes. These frequency-specific effects are observed for visual (but not auditory or somatosensory) areas and account for auditory-visual connectivity in the alpha band and somatosensory-visual connectivity in the beta band. We conclude that task-dependent visual oscillations reflect network-specific oscillatory properties favoring optimal directional neural communication timing for sensory binding.SIGNIFICANCE STATEMENT We investigated the oscillatory correlates of the auditory- and tactile-induced double flash illusion (DFI), a phenomenon where two interleaved beeps (taps) set within 100 ms apart and paired with one visual flash induce the sensation of a second illusory flash. Results confirm previous evidence that the speed of individual occipital alpha oscillations predict the temporal window of the auditory-induced illusion. Importantly, they provide novel evidence that the tactile-induced DFI is instead mediated by the speed of individual occipital beta oscillations. These task-dependent occipital oscillations are shown to be mediated by the oscillatory properties of the neural network engaged in the task to favor optimal temporal integration between the senses.

Characteristics of Alpha Band Frequency in Adolescents with Bipolar II Disorder: A Resting-State QEEG Study

OBJECTIVE

The aim of this study was to compare the QEEG of adolescents affected by bipolar II disorder with age and gender matched healthy controls, and to extract the characteristics of the alpha frequency band to better understand this disorder.

METHODS

Twenty one adolescents affected by acute episodes of bipolar II disorder (BMD II), both hypomanic and depressive episodes, were selected via convenience sampling based on DSM IV criteria and child and adolescent psychiatrist diagnosis. Eleven patients were going through a hypomanic episode and 10 patients were going through a depression episode. Of the participants, 18 who were matched with the patient group participated in this study as a normal group. Any major comorbidities and intellectual disabilities were excluded through applying K-SADS-PL and Raven's IQ test for all the patients and the healthy participants. Electroencephalogram signals were obtained according to 10-20 international system by 21 electrodes from participants in open and closed eyes in a resting state. We selected 40 seconds length segments from each recorded EEG signals that had minimal noise and artifacts. Power spectrum density (PSD) was estimated for each segment and extracted alpha band frequency. We used only referential (unipolar) montage for comparison. Eventually, data were analyzed by independent Mann-Whitney test and independent t test.

RESULTS

We observed significant differences in the alpha frequency band in some brain regions. Alpha power increased in the fronto-central region and right parietal lobe in the patients (P < 0.05). In the patients with BMD II, entropy of alpha oscillations was larger than the normal participants in the central region and in the F3, F4 and P4 channels. Also, there were differences in the variance of alpha oscillations in these regions between the two groups (P < 0.05). In the occipital lobe, alpha wave had different skewness between the two groups (P < 0.05).

CONCLUSION

Thalamus as a generator and modulator of at least a part of alpha oscillations may be involved in this disorder and hence this explains the major symptoms like distractibility and inattention in both hypomanic and depressive episodes of bipolar II disorder.