We suggest that the impairments observed in individuals at increased AD risk may reflect inherent brain alteration and/or early neuropathology disrupting the reciprocal communication between hippocampal, parietal, and frontal brain regions required to successfully prepare and update complex reaching movements. Such impairment has the potential to affect activities of daily living, and may serve as a sensitive measure of functional ability in at-risk adults.
Cognition allows for the use of different rule-based sensorimotor strategies, but the neural underpinnings of such strategies are poorly understood. The purpose of this study was to compare neural activity in the superior parietal lobule during a standard (direct interaction) reaching task, with two nonstandard (gaze and reach spatially incongruent) reaching tasks requiring the integration of rule-based information. Specifically, these nonstandard tasks involved dissociating the planes of reach and vision or rotating visual feedback by 180°. Single unit activity, gaze, and reach trajectories were recorded from two female Macaca mulattas. In all three conditions, we observed a temporal discharge pattern at the population level reflecting early reach planning and on-line reach monitoring. In the plane-dissociated task, we found a significant overall attenuation in the discharge rate of cells from deep recording sites, relative to standard reaching. We also found that cells modulated by reach direction tended to be significantly tuned either during the standard or the plane-dissociated task but rarely during both. In the standard versus feedback reversal comparison, we observed some cells that shifted their preferred direction by 180° between conditions, reflecting maintenance of directional tuning with respect to the reach goal. Our findings suggest that the superior parietal lobule plays an important role in processing information about the nonstandard nature of a task, which, through reciprocal connections with precentral motor areas, contributes to the accurate transformation of incongruent sensory inputs into an appropriate motor output. Such processing is crucial for the integration of rule-based information into a motor act.
Alzheimer's disease (AD) is typically associated with impairments in memory and other aspects of cognition, while deficits in complex movements are commonly observed later in the course of the disease. Recent studies, however, have indicated that subtle deteriorations in visuomotor control under cognitively demanding conditions may in fact be an early identifying feature of AD. Our previous work has shown that the ability to perform visuomotor tasks that rely on visual-spatial and rule-based transformations is disrupted in prodromal and preclinical AD. Here, in a sample of 30 female participants (10 young: mean age = 26.6 ± 2.7, 10 low AD risk: mean age = 58.7 ± 5.6, and 10 high AD risk: mean age = 58.5 ± 6.9), we test the hypothesis that these cognitive-motor impairments are associated with early AD-related brain alterations. Using diffusion-weighted magnetic resonance imaging, we examined changes in white matter (WM) integrity associated with normal aging and increased AD risk, and assessed the relationship between these underlying WM alterations and cognitive-motor performance. Our whole-brain analysis revealed significant age-related declines in WM integrity, which were more widespread in high relative to low AD risk participants. Furthermore, analysis of mean diffusivity measures within isolated WM clusters revealed a stepwise decline in WM integrity across young, low AD risk, and high AD risk groups. In support of our hypothesis, we also observed that lower WM integrity was associated with poorer cognitive-motor performance. These results are the first to demonstrate a relationship between AD-related WM alterations and impaired cognitive-motor control. The application of these findings may provide a novel clinical strategy for the early detection of individuals at increased AD risk.
Eye-hand coordination is crucial for our ability to interact with the world around us. However, much of the visually guided reaches that we perform require a spatial decoupling between gaze direction and hand orientation. These complex decoupled reaching movements are in contrast to more standard eye and hand reaching movements in which the eyes and the hand are coupled. The superior parietal lobule (SPL) receives converging eye and hand signals; however, what is yet to be understood is how the activity within this region is modulated during decoupled eye and hand reaches. To address this, we recorded local field potentials within SPL from two rhesus macaques during coupled vs. decoupled eye and hand movements. Overall we observed a distinct separation in synchrony within the lower 10- to 20-Hz beta range from that in the higher 30- to 40-Hz gamma range. Specifically, within the early planning phase, beta synchrony dominated; however, the onset of this sustained beta oscillation occurred later during eye-hand decoupled vs. coupled reaches. As the task progressed, there was a switch to low-frequency and gamma-dominated responses, specifically for decoupled reaches. More importantly, we observed local field potential activity to be a stronger task (coupled vs. decoupled) and state (planning vs. execution) predictor than that of single units alone. Our results provide further insight into the computations of SPL for visuomotor transformations and highlight the necessity of accounting for the decoupled eye-hand nature of a motor task when interpreting movement control research data.
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