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The Role of Central and Peripheral Optic Flow in the Control of Upright Posture Within a Mixed-Frequency Visual Environment Jeff G. Jasko 1 , Patrick J. Sparto 1,2,3 , Mark S. Redfern 1,3 , and Patrick J. Loughlin 1,4 Departments of Bioengineering 1 , Physical Therapy 2 , Otolaryngology 3 , and Electrical Engineering 4 University of Pittsburgh, Pittsburgh, PA, USA 0.1 Hz 0.25 Hz Peripheral Checkers 0.25 Hz 0.1 Hz Central Target Stimulus 2 Stimulus 1 Stimulus Region
The peripheral stimulus caused significantly more sway than the central stimulus. This suggests that the postural system is more sensitive to anterior-posterior optic flow in the peripheral FOV, regardless of stimulus frequency.
2 test showed a greater number of significant responses to the peripheral stimulus.
Within 3 of the 4 visual/platform conditions, the absolute and normalized sway at the peripheral frequency was significantly greater than the sway at the central frequency.
These findings have implications for the use of virtual environments, showing that head-mounted displays with a limited central FOV do not provide the visual information most important for postural control. Furthermore, the results suggest that rehabilitation therapists should incorporate peripheral movement cues in treatment to enhance balance retraining.
Acknowledgements This research was supported by grants from NIH/NIA-1K25 AG01049, NIH/NIDCD-DC02490, and the Eye and Ear Foundation.
Percival, D.B. (1994). `Spectral Analysis of Univariate and Bivariate Time Series,' in Statistical Methods for Physical Science . Academic Press.
Figure 5 . Comparison of the consistent responders’ normalized sway at one stimulus frequency between the two visual conditions.
The normalized sway at a given frequency was also compared between the two frequency combinations (e.g., Center-0.1 Hz vs. Periphery-0.1 Hz), and in all four cases, the response to a frequency presented in the periphery was significantly greater than when the same frequency was presented in the center (Figure 5).
Low-Frequency Sway During Quiet Stance is a Confounding Factor . Sway obtained during moving-scene trials is a superposition of the response at the stimulus frequencies and quiet-stance (QS) sway (Figure 3). Therefore, absolute RMS sway includes the subjects’ QS sway and their response to the stimulus. The mean QS sway at 0.1 Hz was greater than the QS sway at 0.25 Hz. In order to determine how much the subjects reacted to the stimulus, we normalized their RMS values by subtracting out the QS component at each of the stimulus frequencies (Figure 4). Figure 3 . Mean power spectra of the consistent responders’ sway during quiet stance and during the two stimulus conditions, sway-referenced platform only. Figure 4 . Comparison of the consistent responders’ normalized RMS sway at the two stimulus frequencies within each visual/platform condition. The four pairings correspond to the four conditions. Within each pairing, the left and right bars represent the average ( SEM) normalized sway at the central stimulus and peripheral frequencies, respectively. The solid bars refer to 0.1 Hz, and the textured bars, to 0.25 Hz. Significant differences were observed in the bracketed pairs marked with asterisks (*).
Within 3 of the 4 visual/platform conditions, the normalized sway at the peripheral frequency was significantly greater than the normalized sway at the central frequency (Figure 4).
Table 1. Optic flow conditions. ABSTRACT Several theories have been developed in an attempt to characterize the functional roles of central and peripheral vision in maintaining postural equilibrium (Bardy et al. 1999). While some findings suggest that peripheral vision dominates postural control, others suggest that central vision is equally important in the perception of self-motion. Given these diverging results, further research is needed to clarify this issue. The goal of this study was to investigate the influence of optic flow on upright posture when patterns in the central and peripheral fields of view (FOVs) move simultaneously but at different sinusoidal frequencies. Twenty healthy subjects (9 males, 11 females; ages 21-30) participated in the experiment. They were surrounded by an image encompassing 180 x 70 (horiz. x vert.) FOV. The central stimulus was a pattern of alternating black-and-white concentric rings, and the peripheral stimulus consisted of black-and-white checkers. There were two visual combinations: in one, the central optic flow stimulus moved at 0.1 Hz while the peripheral optic flow moved at 0.25 Hz; in the other, the frequencies were reversed. The peak-to-peak amplitude of all stimuli was 16 cm, in the anterior-posterior direction. In addition, there were 2 support-surface conditions: fixed or sway-referenced. Head position was recorded during the 90-second trials. A statistical test determined whether there was a significant response at either stimulus frequency. Subjects who had significant responses during ¾ of the trials were considered “consistent responders”. Root-Mean-Square (RMS) sway of the responders’ data was calculated at both stimulus frequencies using a bandpass filter, and then normalized to their quiet-stance sway. Within each visual/platform condition, the normalized sway at the peripheral frequency was compared to the normalized sway at the central frequency using paired t-tests. There were more significant responses to the peripheral optic flow stimulus (60/80) than there were to the central optic flow (18/80). Fourteen subjects were classified as responders. In 3 of the 4 visual/platform conditions, the normalized RMS sway at the peripheral frequency was significantly greater than the normalized sway at the central frequency. This suggests that the postural system is more sensitive to optic flow in the peripheral FOV, regardless of frequency. Figure 1 . Visual environment encompassed 180 ° X 70 ° (horiz. X vert.) of the subjects’ field of view.
2 Platform Conditions: Fixed and Sway-Referenced
Recorded head movement with Polhemus Fastrak
Duration of each trial: 90 seconds
20 healthy subjects (mean age: 24 3 years)
Central and peripheral regions of the visual environment moved simultaneously at different frequencies with a peak-to-peak amplitude of 16 cm, in the anterior-posterior direction.
PURPOSE To investigate the influence of optic flow on upright posture when patterns in the central and peripheral fields of view (FOVs) move simultaneously but at different sinusoidal frequencies. METHODS Quiet Stance RESULTS Table 2 . The number of significant responses to the central and peripheral stimulus frequencies during each of the visual/platform conditions, as well as the p-values from the 2 analysis. Includes only the consistent responders (n = 14) . If responses were observed at both stimulus frequencies in one trial, a response was tabulated for each stimulus region.
The number of responses to the periphery was significantly greater than the number of responses to the center, regardless of stimulus frequency.
There were 17 responses to the central stimulus. 15 of these responses were concurrent with responses to the periphery.
In 53 out of 56 trials from this subgroup (consistent responders), there was a significant response to at least one of the stimulus frequencies.
a b Figure 2 . Time series (a) and power spectrum (b) of a subject who responded significantly to both stimulus frequencies during the Sway-Referenced Platform, Central-0.25 Hz/Peripheral-0.1 Hz condition. Within subplot a, the blue trace is the subject’s head motion while the red trace is the sum of the motion of the two stimulus regions, scaled to 1/8 its actual amplitude. Peripheral 0.1 Hz Central 0.25 Hz