Postural stabilization in cerebral palsy adults in response to combined support surface tilt and visual field motion

Abstract

Introduction The relationship between visual field motion and intersegmental behavior during postural destabilization was explored. Individuals with cerebral palsy (CP) were separated through a rod and frame test into those with visual dependence (VD) or independence (VI) to determine whether reliance upon an allocentric reference frame alters the postural organization. We hypothesized that VD reduces intersegmental motion so that whole body responses reflect visual flow direction and velocity; VI would reflect visual flow parameters only at the head. Material and methods Eleven individuals (31 ± 10.5 years) with spastic CP who were VD and 11 with VI, and 18 VI individuals (32 ± 11.5 years) with typical development (TD), stood on either a stationary or 3° dorsiflexion tilted force platform within an immersive, 3-wall virtual environment. Subjects stood quietly for 30 s with eyes closed while the visual scene remained stationary or rotated 15°/s and 30°/s in the pitch-up or pitch-down directions. Motion analysis data were collected and strategies of segmental stabilization were identified with an anchoring index. Group differences and visual and platform conditions were examined with a mixed model, repeated measures ANOVA and simple contrasts (corrected α = 0.01) to determine the effect of condition against stationary visual field motion. Results Either platform or visual motion produced a head locked to upper trunk stabilizing strategy across all subjects (P = 0.015). When the supporting surface was tilted, VI individuals stabilized the thorax to the hip (P = 0.02) whereas VD individuals stabilized the thorax to global space (P = 0.0125). Discussion Both biomechanical and sensory environment motion evoked head to trunk stabilization. CP individuals with visual dependence stabilized the upper trunk to an allocentric rather than egocentric reference frame, thus increasing their potential for instability. Possible causes of an increased reliance on an allocentric frame of reference could be a perceptual inability to suppress external feedback, or diminished signals from internal feedback.