Stairway to memory: Left-hemispheric alpha dynamics index the progressive loading of items into a short-term store

Hypertension Impacts the Oscillatory Dynamics Serving the Encoding Phase of Verbal Working Memory

BACKGROUND

Chronic hypertension is known to be a major contributor to cognitive decline, with executive function and working memory being among the domains most commonly affected. Despite the growing literature on such dysfunction in patients with hypertension, the underlying neural processes are poorly understood.

METHODS

In this cross-sectional study, we examine these neural processes by having participants with controlled hypertension, uncontrolled hypertension, and healthy controls perform a verbal working memory task during magnetoencephalography. Neural oscillations associated with the encoding and maintenance components of the working memory task were imaged and statistically evaluated among the 3 groups.

RESULTS

Differences related to hypertension emerged during the encoding phase, where the hypertension groups exhibited weaker α-β oscillatory responses compared with controls in the left parietal cortices, whereas such oscillatory activity differed between the 2 hypertension groups in the right prefrontal regions. Importantly, these neural responses in the prefrontal and parietal cortices during encoding were also significantly associated with behavioral performance across all participants.

CONCLUSIONS

Overall, our data suggest that hypertension is associated with neurophysiological abnormalities during working memory encoding, whereas the neural processes serving maintenance seem to be preserved. The right hemispheric neural responses likely reflected compensatory processing, which patients with controlled hypertension may use to achieve verbal working memory function at the level of controls, as opposed to the uncontrolled hypertension group where diminished resources may have limited such additional recruitment.

The neural oscillations serving task switching are altered in cannabis users

BACKGROUND

Regular cannabis is known to impact higher-order cognitive processes such as attention, but far less is known regarding cognitive flexibility, a component of executive function. Moreover, whether such changes are related to aberrations in the neural oscillatory dynamics serving flexibility remains poorly understood.

AIMS

Quantify the neural oscillatory dynamics serving cognitive flexibility by having participants complete a task-switching paradigm during magnetoencephalography (MEG). Probe whole-brain maps to identify alterations in chronic cannabis users relative to nonusers and determine how these alterations relate to the degree of cannabis use involvement.

METHODS

In all, 25 chronic cannabis users and 30 demographically matched nonuser controls completed neuropsychological testing, an interview regarding their substance use, a urinalysis, and a task switch paradigm during MEG. Time-frequency windows of interest were identified using a data-driven statistical approach and these were imaged using a beamformer. Whole-brain neural switch cost maps were computed by subtracting the oscillatory maps of the no-switch condition from the switch condition per participant. These were examined for group differences.

RESULTS

Cannabis users had weaker theta switch cost responses in the dorsolateral and dorsomedial prefrontal cortices, while nonusers showed the typical pattern of greater recruitment during switch relative to no switch trials. In addition, theta activity in the dorsomedial prefrontal cortex was significantly correlated with cannabis use involvement.

CONCLUSIONS

Cannabis users exhibited altered theta switch cost activity compared to nonusers in prefrontal cortical regions, which are critical for cognitive flexibility. This activity scaled with cannabis use involvement, indicating a link between cannabis use and aberrant oscillatory activity underlying cognitive flexibility.

Regular cannabis use modulates the impact of HIV on the neural dynamics serving cognitive control

BACKGROUND

Cannabis use and HIV are independently associated with decrements in cognitive control. However, the combined effects of HIV and regular cannabis use on the brain circuitry serving higher-order cognition are unclear.

AIMS

Investigate the interaction between cannabis and HIV on neural interference effects during the flanker task and spontaneous activity in regions underlying higher-order cognition.

METHODS

The sample consisted of 100 participants, including people with HIV (PWH) who use cannabis, PWH who do not use cannabis, uninfected cannabis users, and uninfected nonusers. Participants underwent an interview regarding their substance use history and completed the Eriksen flanker task during magnetoencephalography (MEG). MEG data were imaged in the time-frequency domain and oscillatory maps depicting the neural flanker interference effect were probed for group differences. Voxel time series were then assessed for group-level differences in spontaneous activity.

RESULTS

Group differences in behavioral performance were identified along with group differences in theta and alpha neural interference responses in higher-order regions across the cortex, with nonusers with HIV generally exhibiting the most aberrant responses. Likewise, time series analyses indicated that nonusers with HIV also had significantly elevated spontaneous alpha activity in the left inferior frontal and dorsolateral prefrontal cortices (dlPFC). Finally, we found that spontaneous and oscillatory alpha activity were significantly coupled in the inferior frontal cortex and dlPFC among cannabis users, but not nonusers.

CONCLUSIONS

Regular cannabis use appears to suppress the impact of HIV on spontaneous and oscillatory alpha deficits in the left inferior frontal cortex and dlPFC.

Cannabis use impacts pre-stimulus neural activity in the visual cortices of people with HIV

People with HIV (PWH) use cannabis at a higher rate than the general population, but the influence on neural activity is not well characterized. Cannabis use among PWH may have a beneficial effect, as neuroinflammation is known to be a critical problem in PWH and cannabis use has been associated with a reduction in proinflammatory markers. Thus, it is important to understand the net impact of cannabis use on brain and cognitive function in PWH. In this study, we collected magnetoencephalographic (MEG) brain imaging data on 81 participants split across four demographically matched groups (i.e., PWH using cannabis, controls using cannabis, non‐using PWH, and non‐using controls). Participants completed a visuospatial processing task during MEG. Time–frequency resolved voxel time series were extracted to identify the dynamics of oscillatory and pre‐stimulus baseline neural activity. Our results indicated strong theta (4–8 Hz), alpha (10–16 Hz), and gamma (62–72 Hz) visual oscillations in parietal–occipital brain regions across all participants. PWH exhibited significant behavioral deficits in visuospatial processing, as well as reduced theta oscillations and elevated pre‐stimulus gamma activity in visual cortices, all of which replicate prior work. Strikingly, chronic cannabis use was associated with a significant reduction in pre‐stimulus gamma activity in the visual cortices, such that PWH no longer statistically differed from controls. These results provide initial evidence that cannabis use may normalize some neural aberrations in PWH. This study fills an important gap in understanding the impact of cannabis use on brain and cognitive function in PWH.

Event-related synchronization/desynchronization and functional neuroanatomical regions associated with fatigue effects on cognitive flexibility

Cognitive flexibility is an essential prerequisite for goal-directed behavior, and daily observations already show that it deteriorates when one is engaged in a task for a (too) long time. Yet, the neural mechanisms underlying such fatigability effect in cognitive flexibility are poorly understood. We examined how theta, alpha, and beta frequency event-related synchronization and desynchronization processes during cued memory-based task switching are modulated by time-on-task effects. We put special emphasis on the examination of functional neuroanatomical regions being associated with these modulations, using EEG beamforming. We show clear declines in task switching performance (increased switch costs) with time on task. For processes occurring before rule switching or repetition processes, we show that anticipatory attentional sampling and selection mechanisms associated with fronto-parietal structures are modulated by time-on-task effects but sensory areas (occipital cortex) also show fatigability-dependent modulations. After target stimulus presentation, the allocation of processing resources for response selection as reflected by theta-related activity in parietal cortices is compromised with time on task and similarly a concomitant increase in alpha and beta band-related attentional processing or gating mechanisms in frontal and occipital regions. Yet, considering the behavioral data showing an apparent decline in performance, this probably compensatory increase is still insufficient to allow reasonable performance. The same is likely the case for processes occurring before rule switching or repetition processes. Comparative analyses show that modulations of alpha band activity are as strongly modulated by fatigability as theta band activity. Implications of these findings for theoretical concepts on fatigability are discussed.NEW & NOTEWORTHY We examine the neurophysiological and functional neuroanatomical basis of fatigability in cognitive flexibility. We show that alpha and theta modulations in fronto-parietal and primary sensory areas are central for the understanding of fatigability effects in cognitive flexibility.