Secondary Logo

Journal Logo

Platforms, Thematic Posters, and Posters for CSM 2007: PLATFORM PRESENTATIONS: Research Platform Session II: Postural Control: Saturday 1: 30–3: 00 pm

GAZE STABILIZATION STRATEGIES DURING VOLUNTARY HEAD MOVEMENTS

THE RELATIONSHIP TO FALL RECOVERY

Diehl, D.1; Pidcoe, P.1

Author Information
Journal of Neurologic Physical Therapy: December 2006 - Volume 30 - Issue 4 - p 200
doi: 10.1097/01.NPT.0000281271.03443.4e
  • Free

Purpose/Hypothesis: To investigate the stability of head angular velocity during fixed gaze experiments where subjects systhematically increased the frequency of volitional head movements. In normal gait, sagittal plane head and trunk rotations occur out of phase and act to stabilize the head in space. This promotes gaze stabilization and minimizes the need for contributions from the vestibulo-ocular reflex (VOR). Elderly subjects often segmentally link the head to the trunk during movement. A rigid link between the head and trunk would decrease the stability of the head during normal ambulation. It can be argued that the elderly compensate by ambulating with a slower cadence and decreased stride length to minimize the magnitude of head excursions. Although this strategy seems counter productive during ambulation, it could decrease head angular velocity during a fall and may be a strategy to maximize gaze stabilization in subjects whose vestibular sensitivity is decreased secondary to ageing or pathology. Number of Subjects: Ten visually unimpaired adult subjects (3 male, 7 female; mean age 27 ± 5.1) participated in this research. They had no known visual abnormalities and were in good health. Materials/Methods: Subjects were asked to wear a video-based eye tracker (EyeLink II™, SR Research Ltd.) linked to a kinematic tracking system (MotionMonitor™, Innovative Sports Training). They performed head flexion and extension movements at four metronome driven frequencies while maintaining gaze fxation on an LED target placed 1.5 meters in front of them. Two testing conditions were used: Control-subjects were asked to visually fxate the target while moving the head; Experimental-same as control except that subjects were now trained to move their heads through a 30° arc of motion. The eye tracker was capable of detecting ±30° of horizontal and ±18° of vertical eye movement with a resolution of <;0.05°. The kinematic tracking system had a resolution of 0.5mm in translation and 0.1° in rotation. Bilateral eye and head position data were synchronously collected at 250 Hz during 10 second trials. Point-of-gaze was calculated and geometric relationships were used to determine the position of the target on the retina. A target was considered stable if it was imaged on the fovea. The amount of time the target was foveated was computed for each trial. Results: As pacing frequency increased, the control group consistently decreased head amplitude resulting in a suppression of RMS angular velocity. They demonstrated foveal stabilization 77.6% ± 6.6 of the time. The experimental group consistently maintained 30° head movements. Teir RMS angular velocities increased with increasing frequency. Teir ability to foveate the target fell to 41.8% ± 3.3. Conclusions: Decreased angular head velocities improve target stabilization on the retina. Clinical Relevance: This may begin to explain the head-trunk segmental stifening observed in the elderly, since it may act to decrease head angular velocity during fall and subsequent balance recovery.

© 2006 Neurology Section, APTA