Dependence of Working Memory on Coordinated Activity Across Brain Areas

Neural signatures of working memory (WM) have been reported in numerous brain areas, suggesting a distributed neural substrate for memory maintenance. In the current manuscript we provide an updated review of the literature focusing on intracranial neurophysiological recordings during WM in primates. Such signatures of WM include changes in firing rate or local oscillatory power within an area, along with measures of coordinated activity between areas based on synchronization between oscillations. In comparing the ability of various neural signatures in any brain area to predict behavioral performance, we observe that synchrony between areas is more frequently and robustly correlated with WM performance than any of the within-area neural signatures. We further review the evidence for alteration of inter-areal synchrony in brain disorders, consistent with an important role for such synchrony during behavior. Additionally, results of causal studies indicate that manipulating synchrony across areas is especially effective at influencing WM task performance. Each of these lines of research supports the critical role of inter-areal synchrony in WM. Finally, we propose a framework for interactions between prefrontal and sensory areas during WM, incorporating a range of experimental findings and offering an explanation for the observed link between intra-areal measures and WM performance.

Propofol anesthesia alters cortical traveling waves

Oscillatory dynamics in cortex seem to organize into traveling waves that serve a variety of functions. Recent studies show that propofol, a widely used anesthetic, dramatically alters cortical oscillations by increasing slow-delta oscillatory power and coherence. It is not known how this affects traveling waves. We compared traveling waves across the cortex of non-human primates (NHPs) before, during, and after propofol-induced loss-of-consciousness (LOC). After LOC, traveling waves in the slow-delta (~ 1Hz) range increased, grew more organized, and travelled in different directions relative to the awake state. Higher frequency (8-30 Hz) traveling waves, by contrast, decreased, lost structure, and switched to directions where the slow-delta waves were less frequent. The results suggest that LOC may be due, in part, to changes in slow-delta traveling waves that, in turn, alter and disrupt traveling waves in the higher frequencies associated with cognition.

Modulation of dopamine tone induces frequency shifts in cortico-basal ganglia beta oscillations

AbstractΒeta oscillatory activity (human: 13–35 Hz; primate: 8–24 Hz) is pervasive within the cortex and basal ganglia. Studies in Parkinson’s disease patients and animal models suggest that beta-power increases with dopamine depletion. However, the exact relationship between oscillatory power, frequency and dopamine tone remains unclear. We recorded neural activity in the cortex and basal ganglia of healthy non-human primates while acutely and chronically up- and down-modulating dopamine levels. We assessed changes in beta oscillations in patients with Parkinson’s following acute and chronic changes in dopamine tone. Here we show beta oscillation frequency is strongly coupled with dopamine tone in both monkeys and humans. Power, coherence between single-units and local field potentials (LFP), spike-LFP phase-locking, and phase-amplitude coupling are not systematically regulated by dopamine levels. These results demonstrate that beta frequency is a key property of pathological oscillations in cortical and basal ganglia networks.

Reductions in frontal theta synchrony and fronto-parietal theta-gamma coupling may underlie gaze processing abnormalities in bipolar disorder

Objectives: Impaired social cognition in bipolar disorder (BD) is related to functional outcomes. A critical determinant of social cognition is the ability to discriminate the eye gaze direction of others, and its alteration may play a role in functional impairment in BD. However, the neural mechanisms underlying gaze processing in BD are unclear. Because neural oscillations and their communications through inter-region synchronization are crucial neurobiological mechanisms that support cognition, this study aimed to understand their role in gaze processing in BD.Methods: Electroencephalography (EEG) was recorded during a gaze discrimination task for 38 participants with BD and 34 healthy controls (HC). Time-frequency decomposition of EEG data was used to examine: 1) neural oscillatory power at bilateral parietal and midline frontal locations associated with face processing and higher-level cognition, and 2) feedforward and feedback connectivity between sites via theta-gamma phase-amplitude coupling (PAC).Results: Compared to HC, BD showed reduced theta power at the left parietal and midline frontal sites. BD also showed reduced PAC between bilateral parietal and midline frontal sites. Reduced theta power and PAC were related to poorer gaze discrimination task performance across participants.Conclusions: Altered theta power and feedforward/feedback connectivity between face-processing and midline frontal brain areas may underlie abnormal gaze processing in BD.

A gradient of electrophysiological novelty responses along the human hippocampal long axis.

The hippocampus is implicated in novelty detection, thought to be important for regulating entry of information into long-term memory. Whether electrophysiological responses to novelty differ along the human hippocampal long axis is currently unknown. By recording from electrodes implanted longitudinally in the hippocampus of epilepsy patients, here we show a gradual increase of theta frequency oscillatory power from anterior to posterior in response to unexpected stimuli, superimposed on novelty responses common to all long axis portions. Intracranial event-related potentials (iERPs) were larger for unexpected vs. expected stimuli and demonstrated a polarity inversion between the hippocampal head (HH) and body (HB). We observed stronger theta coherence between HH and hippocampal tail (HT) than between HB and HT, similarly for expected and unexpected stimuli. This was accompanied by theta and alpha traveling waves with surprisingly variable direction of travel characterized by a ~180 degree phase lag between hippocampal poles. Interestingly, this phase lag showed a pronounced phase offset between anterior and middle (HH-HB) hippocampal portions coinciding anatomically with a drop in theta coherence and the novelty iERP polarity inversion. Our findings indicate common response properties along the hippocampal long axis to unexpected stimuli, as well as a multifaceted, non-uniform engagement along the long axis for novelty processing.

Resilience for coupled oscillatory power network via phase difference

This paper investigates the resilience of oscillatory power networks with Kuramoto-type nodal dynamics via phase difference. Here, we propose a general framework to measure the resilience of power network in response to perturbations affecting nodes and transmission lines during operation. Moreover, the assessing resilience approaches provide an estimation of the phase difference and power difference. To identify the resilience of power network that makes stable operation possible, several small-size oscillatory networks are selected to validate the effectiveness and feasibility of the proposed schemes. Simulation results show that the resilience plays a crucial role in the synchronous performance.