RESEARCH NOTE Visual motion combined with base of support width reveals variable field dependency in healthy young adults Jefferson W. Streepey Æ Robert V. Kenyon Æ Emily A. Keshner Received: 1 March 2006 / Accepted: 12 August 2006 / Published online: 28 October 2006 Ó Springer-Verlag 2006 Abstract We previously reported responses to in- duced postural instability in young healthy individuals viewing visual motion with a narrow (25° in both directions) and wide (90° and 55° in the horizontal and vertical directions) field of view (FOV) as they stood on different sized blocks. Visual motion was achieved using an immersive virtual environment that moved realistically with head motion (natural motion) and translated sinusoidally at 0.1 Hz in the fore-aft direc- tion (augmented motion). We observed that a subset of the subjects (steppers) could not maintain continuous stance on the smallest block when the virtual envi- ronment was in motion. We completed a posteriori analyses on the postural responses of the steppers and non-steppers that may inform us about the mechanisms underlying these differences in stability. We found that when viewing augmented motion with a wide FOV, there was a greater effect on the head and whole body center of mass and ankle angle root mean square (RMS) values of the steppers than of the non-steppers. FFT analyses revealed greater power at the frequency of the visual stimulus in the steppers compared to the non-steppers. Whole body COM time lags relative to the augmented visual scene revealed that the time- delay between the scene and the COM was signifi- cantly increased in the steppers. The increased responsiveness to visual information suggests a greater visual field-dependency of the steppers and suggests that the thresholds for shifting from a reliance on visual information to somatosensory information can differ even within a healthy population. Keywords Posture  Virtual reality  Vision  Field dependency  Sensory re-weighting Introduction During upright stance, visual motion increases postural sway (Lishman and Lee 1973; Kawakita et al. 2000; Guerraz et al. 2000) and is further destabilizing when subjects stand on an unstable support surface (Keshner et al. 2004). In a previous study (Streepey et al. 2006), we induced instability by manipulating the size of the base of support (BOS) on which an apparently homogenous group of young, healthy subjects stood while viewing J. W. Streepey (&)  E. A. Keshner SMPP, The Rehabilitation Institute of Chicago, 345 East Superior St., Chicago, IL 60611, USA e-mail: jstreepy@northwestern.edu J. W. Streepey Department of Physical Education and Tourism Management, Indiana University-Purdue University-Indianapolis, 907 W New York Street, Indianapolis, IN 46202, USA e-mail: jwstreep@inpui.edu R. V. Kenyon Department of Computer Science, University of Illinois-Chicago, Chicago, IL, USA E. A. Keshner Department of Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA E. A. Keshner Department of Physical Therapy, College of Health Professions Temple University, Philadelphia, PA, USA e-mail: ekeshner@temple.edu 123 Exp Brain Res (2007) 176:182–187 DOI 10.1007/s00221-006-0677-2