Daniel S. Marigold

Exercise leads to faster postural reflexes, improved balance and mobility, and fewer falls in older persons with chronic stroke.

Objectives: To determine the effect of two different community‐based group exercise programs on functional balance, mobility, postural reflexes, and falls in older adults with chronic stroke.

Gaze fixation patterns for negotiating complex ground terrain.

We constantly encounter different ground terrain in our environment that we must safely traverse. The visual system is unique, as it is the only sensory system that can provide accurate and precise information about the environment at a distance through a series of fixations directed to salient objects and/or surfaces. However, how the nervous system utilizes visual information regarding complex ground terrain to guide safe foot placement is not known. We had individuals walk across a walkway with varying ground terrain while gaze fixations were monitored. Several findings emerged. First, gaze fixations were highly task-relevant in that they were predominantly made to areas eventually stepped on and their patterns tended to depend on the task instructions. Second, fixations were frequently directed to a transition region between different surfaces in addition to fixations directed to an actual surface. These results suggest that fixations are directed to regions that maximize the amount of information which the nervous system can integrate in order to facilitate safe foot placement. And third, spatial information of the upcoming ground terrain was sampled sequentially in small sections and continuously updated as the individual traversed the challenging ground terrain. This is suggestive of on-line control and may be beneficial to ensure one is able to adapt to stability concerns, unexpected changes in terrain, or sudden changes in the path taken.

Contribution of Muscle Strength and Integration of Afferent Input to Postural Instability in Persons with Stroke

Objective. To determine the relationship of muscle strength to postural sway in persons with stroke under standing conditions in which vision and ankle proprioception were manipulated. Methods. Forty persons with stroke and 40 healthy older adult controls were recruited from the community and underwent balance testing consisting of 6 conditions that manipulate vision and somatosensory information while standing. Postural sway was measured during each condition. In addition, lower extremity joint torques and cutaneous sensation from the plantar surface of the foot were assessed. Results. Postural sway was increased with more challenging standing conditions (i.e., when multiple sensory systems were manipulated) to a greater extent with the group with stroke compared to controls. Muscle strength was only correlated to sway during the most challenging conditions. Furthermore, a greater number of persons with stroke fell during the balance testing compared to controls. Conclusions. Impairments in re-weighting/integrating afferent information, in addition to muscle weakness, appear to contribute to postural instability and falls in persons with stroke. These findings can be used by clinicians to design effective interventions for improving postural control following stroke.

Role of peripheral visual cues in online visual guidance of locomotion.

Vision is normally the predominant sensory system used for guiding locomotion. Online visual control is critical for adjusting lower limb trajectory and ensuring proper foot placement. Research suggests that peripheral visual cues play a large role in this online control, particularly in challenging situations.

Whole-Body Responses: Neural Control and Implications for Rehabilitation and Fall Prevention

Humans are one of the unique species that utilize bipedal gait to ambulate in our environment. Despite this fact, coordination of the arms with the legs and the rest of body is essential for many daily activities. As such, whole-body responses have emerged as the preferred strategy following perturbations to balance during both standing and walking. Complex neural circuitry may allow for this coordination through the use of propriospinal pathways linking lumbar and cervical pattern generators in the spinal cord, with supraspinal centers altering this control depending on the context of the situation. Based on these findings, we argue that whole-body reactions may be exploited for rehabilitation purposes. Preliminary results have indicated training programs designed to elicit whole-body responses are effective in reducing falls and improving functional mobility in older adults with and without neurological impairment.

Taking the next step: cortical contributions to the control of locomotion

The planning and execution of both discrete voluntary movements and visually guided locomotion depends on the contribution of multiple cortical areas. In this review, we discuss recent experiments that address the contribution of the posterior parietal cortex (PPC) and the motor cortex to the control of locomotion. The results from these experiments show that the PPC contributes to the planning of locomotion by providing an estimate of the position of an animal with respect to objects in its path. In contrast, the motor cortex contributes primarily to the execution of gait modifications by modulating the activity of groups of synergistic muscles active at different times during the gait cycle.

Contribution of cells in the posterior parietal cortex to the planning of visually guided locomotion in the cat: effects of temporary visual interruption

In the present study, we determined whether cells in the posterior parietal cortex (PPC) may contribute to the planning of voluntary gait modifications in the absence of visual input. In two cats we recorded the responses of 41 neurons in layer V of the PPC that discharged in advance of the gait modification to a 900-ms interruption of visual information (visual occlusion). The cats continued to walk without interruption during the occlusion, which produced only minimal changes in step cycle duration and paw placement. Visual occlusion applied during the period of cell discharge was without significant effect on discharge frequency in 57% of cells. In the other cells, the visual occlusion produced either significant decreases (18%) or increases (21%) of discharge activity (in 1 cell there was both an increase and a decrease). The mean latency of the changes was 356 ms for decreases and 252 ms for increases. In most neurons, discharge frequency, when modified, returned to the same levels as during unoccluded locomotion when vision was restored. In some cells, there were significant changes in discharge activity after the restoration of vision; these were associated with corrections of gait. These results suggest that the PPC is more involved in the visuomotor transformations necessary to plan gait modifications than in continual sensory processing of visual information. We further propose that cells in the PPC contribute both to the planning of gait modifications on the basis of only intermittent visual sampling and to visually guided online corrections of gait.

Prism adaptation and generalization during visually guided locomotor tasks.

The ability of individuals to adapt locomotion to constraints associated with the complex environments normally encountered in everyday life is paramount for survival. Here, we tested the ability of 24 healthy young adults to adapt to a rightward prism shift (∼11.3°) while either walking and stepping to targets (i.e., precision stepping task) or stepping over an obstacle (i.e., obstacle avoidance task). We subsequently tested for generalization to the other locomotor task. In the precision stepping task, we determined the lateral end-point error of foot placement from the targets. In the obstacle avoidance task, we determined toe clearance and lateral foot placement distance from the obstacle before and after stepping over the obstacle. We found large, rightward deviations in foot placement on initial exposure to prisms in both tasks. The majority of measures demonstrated adaptation over repeated trials, and adaptation rates were dependent mainly on the task. On removal of the prisms, we observed negative aftereffects for measures of both tasks. Additionally, we found a unilateral symmetric generalization pattern in that the left, but not the right, lower limb indicated generalization across the 2 locomotor tasks. These results indicate that the nervous system is capable of rapidly adapting to a visuomotor mismatch during visually demanding locomotor tasks and that the prism-induced adaptation can, at least partially, generalize across these tasks. The results also support the notion that the nervous system utilizes an internal model for the control of visually guided locomotion.

The contribution of vision, proprioception, and efference copy in storing a neural representation for guiding trail leg trajectory over an obstacle

Stepping over obstacles requires vision to guide the leading leg, but direct visual information is not available to guide the trailing leg. The neural mechanisms for establishing a stored obstacle representation and thus facilitating the trail leg trajectory in humans are unknown. Twenty-four subjects participated in one of three experiments, which were designed to investigate the contribution of visual, proprioceptive, and efference copy signals. Subjects stepped over an obstacle with their lead leg, stopped, and straddled the obstacle for a delay period before stepping over it with their trail leg while toe elevation was recorded. Subsequently, we calculated maximum toe elevation and toe clearance. First, we found that subjects could accurately scale trail leg toe elevation and clearance, despite straddling an obstacle for up to 2 min, similar to quadrupeds. Second, we found that when the lead leg was passively moved over an obstacle (eliminating an efference copy signal and altering proprioception) without vision, trail leg toe elevation and clearance were reduced, and variability increased compared with when subjects actively moved their lead leg. Trail leg toe measures returned to normal when vision was provided during the passive manipulation. Finally, we found that altering lead leg proprioceptive feedback by adding mass to the ankle had no effect on trail leg toe measures. Taken together, our results suggest that humans can store a neural representation of obstacle properties for extended periods of time and that vision appears to be sufficient in this process to guide trail leg trajectory.

Chapter 6 - Motor planning of locomotor adaptations on the basis of vision: The role of the posterior parietal cortex

In this chapter, we consider the contribution of the posterior parietal cortex (PPC) to obstacle avoidance behavior and we define a model that identifies the major planning processes that are required for this task. A key aspect of this planning process is the need to integrate information concerning the obstacle, obtained from vision, together with an estimation of body and limb state. We suggest that the PPC makes a major contribution to this process during visually guided locomotion. We present evidence from lesion and single unit recording experiments in the cat that are compatible with this viewpoint.