Kara M. Hawkins

Neural activity in superior parietal cortex during rule-based visual-motor transformations

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.

The effects of anterior load carriage on lower limb gait parameters during obstacle clearance

The purpose of this study was to assess the effect of anterior load carriage on obstacle-crossing behaviour, with a focus on lower limb gait parameters. Nine male participants (age 23+/-1.8 years, height 176+/-5.0cm) volunteered. Participants either walked without a load (No Load), or carried a load (2KG (empty box), 5KG, 10KG), and stepped over a 20cm obstacle. Vision of the obstacle was obscured 1.0m to 1.3m prior to the obstacle. Significant correlations were found between trail limb toe distance and lead limb toe clearance, in the 2KG, 5KG, and 10KG conditions. Toe clearance increased with load (No Load, 147.3+/-13.9mm; 2KG, 162.5+/-15.6mm; 5KG, 167.6+/-17.6mm; 10KG, 173.9+/-17.5mm; p<0.0001). Trail limb toe distance, trail limb toe distance variability, lead heel distance variability, and lead limb toe clearance variability were greater in the 2KG, 5KG, and 10KG conditions, compared with the No Load condition. Participants adopted a conservative gait pattern during obstacle crossing when carrying a load, evidenced by increasing toe clearance, which may have been influenced by availability of visual information regarding obstacle position. In contrast with previous literature, increased lead limb toe clearance may have been associated with absence of relative surface height difference pre- and post-obstacle crossing.

Decoupling the actions of the eyes from the hand alters beta and gamma synchrony within SPL

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.

Visuomotor impairments in older adults at increased Alzheimer's disease risk

BACKGROUND AND OBJECTIVE Recent evidence suggests that visuomotor behaviors may be disrupted in the very early stages of Alzheimers disease (AD). Here we propose that using kinematic measures under conditions that place demands on visual-spatial and cognitive-motor processing may provide an effective behavioral means to detect subtle changes associated with AD risk. METHODS To this end, we have tested 22 young adults (mean age = 26.4 ± 4.1) and 22 older adults (mean age = 64.3 ± 10.1) at low AD, and 22 older adults (mean age = 67.7 ± 11.3) at high AD risk (i.e., strong family history or diagnosis of mild cognitive impairment). Kinematic measures were acquired on four visuomotor transformation tasks (standard, feedback reversal, plane dissociated, and plane dissociated + feedback reversal) using a dual-touchscreen tablet. RESULTS Comparing participants at increased AD risk with both young and old healthy control groups revealed significant performance disruptions in at-risk individuals as task demands increased. Furthermore, we were able to discriminate between individuals at high and low AD risk with a classification accuracy of 86.4% (sensitivity: 81.8%, specificity: 90.9%). CONCLUSION 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.

Differences in spectral profiles between rostral and caudal premotor cortex when hand-eye actions are decoupled

The aim of this research was to understand how the brain controls voluntary movement when not directly interacting with the object of interest. In the present study, we examined the role of premotor cortex in this behavior. The goal of this study was to characterize the oscillatory activity within the caudal and rostral subdivisions of dorsal premotor cortex (PMdc and PMdr) with a change from the most basic reaching movement to one that involves a simple dissociation between the actions of the eyes and hand. We were specifically interested in how PMdr and PMdc respond when the eyes and hand are decoupled by moving along different spatial planes. We recorded single-unit activity and local field potentials within PMdr and PMdc from two rhesus macaques during performance of two types of visually guided reaches. During the standard condition, a visually guided reach was performed whereby the visual stimulus guiding the movement was the target of the reach itself. During the nonstandard condition, the visual stimulus provided information about the direction of the required movement but was not the target of the motor output. We observed distinct task-related and topographical differences between PMdr and PMdc. Our results support functional differences between PMdr and PMdc during visually guided reaching. PMdr activity appears more involved in integrating the rule-based aspects of a visually guided reach, whereas PMdc is more involved in the online updating of the decoupled reach. More broadly, our results highlight the necessity of accounting for the nonstandard nature of a motor task when interpreting movement control research data.

Diffusion Tensor Imaging Correlates of Cognitive-Motor Decline in Normal Aging and Increased Alzheimer's Disease Risk

Alzheimers 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.

Attentional Demands Associated With Obstacle Crossing While Carrying a Load

ABSTRACT The extent to which different locomotor tasks require cognitive control is not well characterized. In this article, the authors consider the potential increase in attentional demands associated with carrying an anterior load while clearing an obstacle. Nine healthy male volunteers participated in 80 walking trials, 20 in each of 4 conditions: 1 no load condition (NL) and 3 carrying conditions (2KG, 5KG, and 10KG). Of the 20 trials in each condition, 12 included a probe reaction time (PRT) test during lead limb obstacle crossing, which was used to measure cognitive load. A load-dependent increase in PRT was observed, with PRT in the 2KG condition being significantly greater than in the NL condition, and PRT in the 5KG and 10KG conditions being significantly greater than in the 2KG condition. These results suggested that cognitive load was increased when: (a) the obstacle was occluded from vision by the load, and (b) the magnitude of load was increased.

The Contribution of Different Cortical Regions to the Control of Spatially Decoupled Eye–Hand Coordination

Our brains ability to flexibly control the communication between the eyes and the hand allows for our successful interaction with the objects located within our environment. This flexibility has been observed in the pattern of neural responses within key regions of the frontoparietal reach network. More specifically, our group has shown how single-unit and oscillatory activity within the dorsal premotor cortex (PMd) and the superior parietal lobule (SPL) change contingent on the level of visuomotor compatibility between the eyes and hand. Reaches that involve a coupling between the eyes and hand toward a common spatial target display a pattern of neural responses that differ from reaches that require eye–hand decoupling. Although previous work examined the altered spiking and oscillatory activity that occurs during different types of eye–hand compatibilities, they did not address how each of these measures of neurological activity interacts with one another. Thus, in an effort to fully characterize the relationship between oscillatory and single-unit activity during different types of eye–hand coordination, we measured the spike–field coherence (SFC) within regions of macaque SPL and PMd. We observed stronger SFC within PMdr and superficial regions of SPL (areas 5/PEc) during decoupled reaches, whereas PMdc and regions within SPL surrounding medial intrapareital sulcus had stronger SFC during coupled reaches. These results were supported by meta-analysis on human fMRI data. Our results support the proposal of altered cortical control during complex eye–hand coordination and highlight the necessity to account for the different eye–hand compatibilities in motor control research.

Adults at Increased Alzheimer’s Disease Risk Display Cognitive-Motor Integration Impairment Associated with Changes in Resting-State Functional Connectivity: A Preliminary Study

BACKGROUND Many neuroimaging parameters have demonstrated utility as biomarkers in preclinical AD, including resting-state functional connectivity in the default mode network. However, neuroimaging is not a practical, cost effective screening instrument. OBJECTIVE Here we investigate the relationship between performance on a cognitive-motor integration assessment and alterations in resting-state functional connectivity in an at-risk population. METHODS Three groups of ten adults (young: mean age = 26.6 ± 2.7, low AD risk: mean age = 58.7 ± 5.6, and high AD risk: mean age = 58.5 ± 6.9) performed a simple cognitive-motor integration task using a dual-touchscreen laptop and also underwent functional magnetic resonance imaging at rest. RESULTS We found poorer cognitive-motor integration performance in high AD risk participants, as well as an association with lower resting-state functional connectivity in this group. CONCLUSION These findings provide novel insight into underlying AD-related brain alterations associated with a behavioral assessment that can be easily administered clinically.

Sagittal plane lumbar loading when navigating an obstacle and carrying a load

Abstract The current study quantified lumbar loading while carrying an anterior load mass and navigating an obstacle. Eight healthy male participants walked down a walkway and crossed an obstacle under three randomised LOAD conditions; empty-box (2 KG), five kilogram (5 KG) and ten kilogram (10 KG). Each walk was assessed at two events: left foot mid-stance (LMS) and right toe-crossing (TC) to characterise any changes from approach to crossing. Measures of interest included: trunk pitch, L4/L5 joint moment, compression, joint anterior–posterior shear and erector spinae activation. Findings demonstrate that obstacle crossing extended posture by 50, 41, 44%, respectively for each carried load magnitude. Further, these results indicate that shear rather than compressive loading may be an important consideration during crossing due to increase by 8, 9, 22% from LMS to TC for each load magnitude tested. These results provide insight into sagittal lumbar loading when navigating an obstacle while carrying a load. Practitioner Summary: The risk of carrying while navigating obstacles on the lumbar spine is not completely understood. The forces at the lumbar spine while simultaneously carrying and obstacle crossing were analysed. Data indicate that carrying and obstacle crossing influence lumbar shear loads, thereby moderately increasing the relative risk at lumbar spine.