Jill C. Slaboda

Visual field dependence influences balance in patients with stroke

To compare the occurrence of visual field independence/dependence in healthy subjects with patients who are post-stroke using the Rod and Frame Test, and determine whether increased visual dependence is reflected in their postural responses when immersed in a moving visual environment. Eight older and twelve young adults, and twelve patients with cortical or sub-cortical stroke, were asked to align a rod enclosed in a tilted frame to vertical and horizontal. Angular deviations of rod position were calculated and compared. Center-of-mass (COM) of the body was calculated for two patients and two young adults standing in the dark and in an immersive virtual environment to examine their postural responses. Balance of the patients did not appear different from healthy subjects when standing in the dark suggesting they were not dependent on the presence of vision, but more rapid and larger COM displacements emerged in the patients when immersed in a moving visual scene. Patients also exhibited greater errors when aligning the rod compared to both healthy groups. Thus, patients with stroke may be more dependent on visual inputs when they are present, and have more difficulty resolving conflict between the visual and somatosensory cues compared to healthy young or older subjects. This impaired conflict resolution may underlie the rapid instability observed in patients when they were placed in a moving visual environment.

Augmenting sensory-motor conflict promotes adaptation of postural behaviors in a virtual environment

We present results from a series of studies that investigated how multimodal mismatches in a virtual environment modified postural response organization. Adaptation of motor commands to functional circumstances is driven directly by error signals. Thus, motor relearning should increase when performing in environments containing sensory mismatch. We hypothesized that kinematics of the response would be linked to specific characteristics of the sensory array. Sensory weighting was varied by: 1) rotating the visual field about the talo-crural joint or the interaural axis, 2) adding stochastic vibrations at the sole of the foot, and 3) combining galvanic vestibular stimulation with rotations of the visual field. Results indicated that postural responses are shaped by the location of a sensory disturbance and also by the processing demands of the environmental array. Sensory-motor demands need to be structured when developing therapeutic interventions for patients with balance disorders.

Time series analysis of postural responses to combined visual pitch and support surface tilt.

The purpose of using time-series analyses is to provide interpretation of information on curves or functions, such as dynamic, biomechanical data. We evaluated the application of one method of time-series analysis for assessing changes in postural responses when exposed to a continuously rotating visual field combined with a tilted support surface. Functional Principal Component Analysis (fPCA) was applied to center of mass (CoM) trajectories collected from 22 young adults (20-39 y.o.) on a fixed surface or following a 3 degree (30°/s) dorsiflexion tilt of the support surface combined with continuous upward or downward pitch rotation of the visual field at 30 and 45°/s. The usefulness of this analytical tool is that each curve is treated as a distinct observation by itself, allowing for traditional PCA to be applied to the analysis of curves. Results of the fPCA highlighted 5 distinct time periods in the CoM curves that explained 91% of the variability in the data. These periods in which the young adults altered their CoM in response to visual field motion would not have been identified if we had relied on the onset and offset of the transient disturbance to distinguish responses. Young adults significantly displaced their CoM in response to visual motion over both the period of support surface tilt and while the support surface returned to a neutral position. Our results indicate that fPCA is a viable method when applied to the small but complex changes that emerge in postural data and might allow for a better understanding of time dependent processes occurring with pathology and intervention.

INFLUENCE OF MOVING VISUAL ENVIRONMENT ON SIT-TO-STAND KINEMATICS IN CHILDREN AND ADULTS'

The effect of visual field motion on the sit-to-stand kinematics of adults and children was investigated. Children (8 tol2 years of age) and adults (21 to 49 years of age) were seated in a virtual environment that rotated in the pitch and roll directions. Participants stood up either (1) concurrent with onset of visual motion or (2) after an immersion period in the moving visual environment, and (3) without visual input. Angular velocities of the head with respect to the trunk, and trunk with respect to the environment, were calculated as was head and trunk center of mass. Both adults and children reduced head and trunk angular velocity after immersion in the moving visual environment. Unlike adults, children demonstrated significant differences in displacement of the head center of mass during the immersion and concurrent trials when compared to trials without visual input. Results suggest a time-dependent effect of vision on sit-to-stand kinematics in adults, whereas children are influenced by the immediate presence or absence of vision.

Postural responses of adults with cerebral palsy to combined base of support and visual field rotation

We employed a virtual environment to examine the postural behaviors of adults with cerebral palsy (CP). Four adults with CP (22-32 years) and nine healthy adults (21-27 years) were tested with a Rod and Frame protocol. They then stood quietly on a platform within a three-wall virtual environment. The platform was either kept stationary or tilted 3 into dorsiflexion in the dark or with pitch up and down visual field rotations at 30 /s and 45 /s. While the visual field rotated, the platform was held tilted for 30 s and then slowly returned to a neutral position over 30 s. Center of pressure (CoP) was recorded and center of mass (CoM) as well as trunk and ankle angles were calculated. Electromyography (EMG) responses of the ankle and the hip muscles were recorded and analyzed using wavelets. Larger angular deviations from vertical and horizontal in the Rod and Frame test indicated that adults with CP were more visually dependent than healthy adults. Adults with CP had difficulty maintaining balance when standing on a stationary platform during pitch upward rotation of the visual scene. When the platform was tilted during visual field rotations, adults with CP took longer to stabilize their posture and had larger CoM oscillations than when in the dark. The inability to compensate for busy visual environments could impede maintenance of functional locomotion in adults with CP. Employing a visual field stimulus for assessment and training of postural behaviors would be more meaningful than testing in the dark.

Efficient marker matching using pair-wise constraints in physical therapy

In this paper, we report a robust, efficient, and automatic method for matching infrared tracked markers for human motion analysis in computer-aided physical therapy applications. The challenges of this task stem from non-rigid marker motion, occlusion, and timing requirements. To overcome these difficulties, we use pair-wise distance constraints for marker identification. To meet the timing requirements, we first reduce the candidate marker labels by proximity constraints before enforcing the pair-wise constraints. Experiments with 38 real motion sequences, our method has shown superior accuracy and significant speedup over a semi-automatic proprietary method and the Iterative Closest Point (ICP) approach.

Visual sensitivity modulates postural sway in a virtual environment in healthy elderly and individuals with stroke

We employed a virtual environment to examine the impact of visual sensitivity on postural behaviors in adults with chronic symptoms of stroke. Six adults at least 1 year post-stroke (52–70 yrs) and 6 healthy adults (50–70 yrs) were tested in a Rod and Frame test. They then stood quietly on a platform within a 3-wall virtual environment. The platform was tilted 3° into dorsiflexion while in the dark, with visual motion matched to head motion, or with pitch up and down visual field rotations at 30 and 45 °/sec. While the visual field rotated, the platform was held tilted for 30 sec and then slowly returned to a neutral position over 30 sec. Center of pressure (COP) was recorded and approximate entropy (ApEn) values were calculated and compared with visual error from the Rod and Frame test. No significant differences in visual errors were detected in the Rod and Frame task between the populations. However, in subjects with large visual errors (>8 deg) strong inverse correlations with ApEn values (r>−0.7) emerged with either a scene referenced to head motion or matched to the velocity of the platform tilt. ApEn values were typically below 1 indicating that COP responses were mostly predictable and reflecting a single input. This low ApEn with increased visual error suggests that the visual field serves as a meaningful reference for postural stabilization in visually dependent adults. Our results support the use of virtual environments to generate adaptive postural behaviors.

Postural behaviors to combined disturbances of the visual field and base of support

This study examines whether combining a sustained mechanical and visual disturbance will modify the automatic postural responses and alter subsequent recovery to vertical orientation in healthy young and older adults and in patients who are post-stroke. Subjects stood on a platform that tilted 3 deg at 30 deg/sec into dorsiflexion while in the dark, with visual motion matched to motion of the head, or with pitch up visual field motion at 30, 45, and 60 deg/sec. The platform was held stable for 30 sec and then returned to neutral at 0.1 deg/sec for 30 sec. EMG responses of lower limb and neck muscles and angular displacement of body segments were recorded. Latencies of the EMG response and magnitudes of muscle and center of mass (COM) motion were calculated. In older adults, EMG response latencies were delayed compared to young adults. In both young and older adults, muscle RMS values were greater with visual field motion than in the dark. The slope and speed of return to a vertical orientation when the base of support was tilted was affected by the presence of visual field motion in all populations. Older subjects demonstrated more sensitivity to the visual inputs and young adults were more sensitive to motion of the base of support. Increased instability in the patients, particularly while the platform returned to neutral, indicates that they were very sensitive to the visual motion when the base of support motion was not reliable. Our data suggest that alterations in the ability to recover an upright position in the presence of visual field motion might underlie the decreased postural stability observed in patients post-stroke and in elderly individuals that exhibit visual sensitivity.

Influence of optic flow on a sit-to-stand task in healthy children and adults

Children and adults stood from a seated position in the dark, with the onset of visual scene motion or after a period of immersion in the virtual reality environment. Multiplanar head and trunk angular velocity were calculated. Both children and adults were found to reduce head and trunk angular velocities when immersed in a moving visual field but not when standing coincident with the onset of visual motion.