Altered age-related alpha and gamma prefrontal-occipital connectivity serving distinct cognitive interference variants

Hemisphere- and condition-specific alpha oscillations support semantic and spatial cognition in aging

Neuroimaging studies have shown age-related alterations in brain structure and function supporting semantic knowledge, although the significance of these is not fully understood. Herein, we report novel temporal, spectral, and spatial information on age-related changes from the largest dynamic functional mapping study of semantic processing. Participants (N = 130, age range 21-87 years, Mage = 51.05, SD = 14.73, 68 females) performed a semantic judgement task during magnetoencephalography (MEG), and significant task-related oscillatory responses were projected into anatomical space using a beamformer. Voxel-wise linear mixed-effects models were performed to assess semantic-related neural oscillations, irrespective of and influenced by age. Mediation analyses were performed to assess if local oscillations mediated the relationship between age and reaction time. Whole-brain analyses revealed stronger left-lateralized alpha/beta oscillations in frontotemporal cortices during semantic trials and stronger right-lateralized alpha/beta responses in temporoparietal regions during length trials (all ps <.001). Older adults showed stronger left temporoparietal alpha and left frontal beta during semantic processing and stronger alpha in the right temporal cortex during the length condition (all ps <.001). Alpha oscillations further mediated the relationship between age and reaction time in a hemisphere- and condition-specific manner, whereby right temporal activity mediated the age-behavior relationship in the length but not semantic condition (Z = 2.01, p =.022), while left temporoparietal activity significantly mediated this relationship in the semantic but not length condition (Z =  - 2.41, p =.008). Altogether, our findings demonstrate accentuated oscillations in aging which are hemisphere- and condition-specific and support compensatory processing to aid in maintaining adequate behavioral performance, lending clear support to leading neuroscientific models of aging.

The polarity of high-definition transcranial direct current stimulation affects the planning and execution of movement sequences

Noninvasive brain stimulation of the primary motor cortex has been shown to alter therapeutic outcomes in stroke and other neurological conditions, but the precise mechanisms remain poorly understood. Determining the impact of such neurostimulation on the neural processing supporting motor control is a critical step toward further harnessing its therapeutic potential in multiple neurological conditions affecting the motor system. Herein, we leverage the excellent spatio-temporal precision of magnetoencephalographic (MEG) imaging to identify the spectral, spatial, and temporal effects of high-definition transcranial direct current stimulation (HD-tDCS) on the neural responses supporting motor control. Participants (N = 67) completed three HD-tDCS visits (anode, cathode, sham), with each involving 20 min of left primary motor cortex stimulation and performance of a simple/complex motor sequencing task during MEG. Whole-brain statistical analyses of beta oscillatory responses revealed stimulation-by-task interaction effects in the left primary motor cortex, right occipitotemporal, and the right dorsolateral prefrontal cortices. Broadly, anodal stimulation induced significantly stronger beta oscillatory responses in these regions during simple movement sequences, while neural responses to complex sequences were not affected by stimulation. En masse, these data suggest that the beta oscillations serving motor planning (i.e., pre-movement) are particularly sensitive to the polarity of noninvasive stimulation and that the impact varies based on the difficulty of the movement sequence.

Aging modulates the impact of cognitive interference subtypes on dynamic connectivity across a distributed motor network

Research has shown age-related declines in cognitive control in the context of interference, but these studies have focused on frontoparietal networks and less is known about impacts on motor response-related dynamics in the face of distractors. Thus, we examined whether healthy aging affected connectivity between attention networks and motor circuitry using a multisource interference task and magnetoencephalography in 72 healthy-aging participants (28-63 years-old). Our results indicated stronger beta connectivity with increasing age between bilateral primary motor (M1) and occipital cortices, as well as stronger gamma fronto-motor connectivity during flanker-type interference. Regarding Simon-type interference, stronger beta interactions were observed between left M1 and right temporal and right M1 and left parietal with increasing age. Finally, the superadditivity effect (flanker + Simon presented simultaneously) indicated weaker beta connectivity between right M1 and left premotor with increasing age. These findings suggest exhaustion of age-related compensatory adaptations in the fronto-parieto-motor network with greater interference.

Modulation of movement-related oscillatory signatures by cognitive interference in healthy aging

Age-related changes in the neurophysiology underlying motor control are well documented, but whether these changes are specific to motor function or more broadly reflect age-related alterations in fronto-parietal circuitry serving attention and other higher-level processes remains unknown. Herein, we collected high-density magnetoencephalography (MEG) in 72 healthy adults (age 28-63 years) as they completed an adapted version of the multi-source interference task that involved two subtypes of cognitive interference (i.e., flanker and Simon) and their integration (i.e., multi-source). All MEG data were examined for age-related changes in neural oscillatory activity using a whole-brain beamforming approach. Our primary findings indicated robust behavioral differences in task performance based on the type of interference, as well as stronger beta oscillations with increasing age in the right dorsolateral prefrontal cortices (flanker and multi-source conditions), left parietal (flanker and Simon), and medial parietal regions (multi-source). Overall, these data indicate that healthy aging is associated with alterations in higher-order association cortices that are critical for attention and motor control in the context of cognitive interference.

Multispectral brain connectivity during visual attention distinguishes controlled from uncontrolled hypertension

Hypertension-related changes in brain function place individuals at higher risk for cognitive impairment and Alzheimer's disease. The existing functional neuroimaging literature has identified important neural and behavioural differences between normotensive and hypertensive individuals. However, previously-used methods (i.e. magnetic resonance imaging, functional near-infrared spectroscopy) rely on neurovascular coupling, which is a useful but indirect measure of neuronal activity. Furthermore, most studies fail to distinguish between controlled and uncontrolled hypertensive individuals, who exhibit significant behavioural and clinical differences. To partially remedy this gap in the literature, we used magnetoencephalography (MEG) to directly examine neuronal activity that is invariant to neurovascular coupling changes induced by hypertension. Our study included 52 participants (19 healthy controls, 15 controlled hypertensives, 18 uncontrolled hypertensives) who completed a modified flanker attention task during MEG. We identified significant oscillatory neural responses in two frequencies (alpha: 8-14 Hz, gamma: 48-60 Hz) for imaging and used grand-averaged images to determine seeds for whole-brain connectivity analysis. We then conducted Fisher-z tests for each pair of groups, using the relationship between the neural connectivity and behavioural attention effects. This highlighted a distributed network of regions associated with cognitive control and selective attention, including frontal-occipital and interhemispheric occipital connections. Importantly, the inferior frontal cortex exhibited a unique neurobehavioural relationship that distinguished the uncontrolled hypertensive group from the controlled hypertensive and normotensive groups. This is the first investigation of hypertension using MEG and identifies critical whole-brain connectivity differences based on hypertension profiles. KEY POINTS: Structural and functional changes in brain circuitry scale with hypertension severity and increase the risk of cognitive impairment and Alzheimer's disease. We harness the excellent spatiotemporal precision of magnetoencephalography (MEG) to directly quantify dynamic functional connectivity in healthy control, controlled hypertensive and uncontrolled hypertensive groups during a flanker task. In the first MEG study of hypertension, we show that there are neurobehavioural relationships that distinguish the uncontrolled hypertensive group from healthy and controlled hypertensive group in the prefrontal cortex. These results provide novel insights into the differential impact of hypertension on brain dynamics underlying selective attention.