Eye movements of patients with tunnel vision while walking

Abstract

Purpose: To determine how severe peripheral field loss (PFL) affects the dispersion of eye movements relative to the head in patients walking in real environments. This information should help to define the visual field and clearance requirements for head-mounted mobility visual aids.

Methods: Eye positions relative to the head were recorded in five patients with retinitis pigmentosa who had less than 15 degrees of visual field and in three normally sighted people, each walking in varied environments for more than 30 minutes. The eye-position recorder was made portable by modifying a head-mounted system (ISCAN, Burlington, MA). Custom data processing was implemented, to reject unreliable data. Sample standard deviations of eye position (dispersion) were compared across subject groups and environments.

Results: The patients with PFL exhibited narrower horizontal eye-position dispersions than did the normally sighted subjects (9.4 degrees vs. 14.2 degrees , P < 0.0001), and the vertical dispersions of patients with PFL were smaller when they were walking indoors than when walking outdoors (8.2 degrees vs. 10.3 degrees ; P = 0.048).

Conclusions: When walking, the patients with PFL did not increase their scanning eye movements to compensate for missing peripheral vision information. Their horizontal scanning was actually reduced, possibly because of lack of peripheral stimulation. The results suggest that a field of view as wide as 40 degrees may be needed for closed (immersive) head-mounted mobility aids, whereas a much narrower display, perhaps as narrow as 20 degrees , may be sufficient with an open design.

Figure 1

Frame rejection criteria based on pupil diameter. The graphic illustrates the pupil diameters obtained from the ISCAN in each frame during a time segment of approximately 5.5 minutes (each dot represents one data point). The continuous gray line in the middle of the black data represents the running-averaged pupil diameter, and the black dashed lines on the right show the allowed pupil diameter range. The accepted frames (88%) are displayed as black symbols and the discarded ones as gray symbols. The acceptance band is set by the window size (in this case ±200 ISCAN units) centered on the bias-corrected running average.

Figure 2

Examples of bi-dimensional histograms of vertical and horizontal eye position for two subjects and two conditions: a) normally sighted subject (1_NS) indoors and b) outdoors; c) PFL patient (2_RP) indoors, and d) outdoors. Note the change in pattern for the PFL patient between the two environments. Ordinates indicate the percent of valid frames for which the eye was within each bin area. The spatial position on the abscissas is relative to the central fixation point on the portable calibration grid.

Figure 3

Horizontal and vertical eye position dispersions (standard deviations) for normally-sighted subjects (NS) and PFL patients. Each symbol corresponds to the data for a subject in a particular environment (indoors, outdoors, or arcades) collapsed over all segments.

Figure 4

Box plots of angular eye position dispersion in normally-sighted subjects (NS) and PFL patients. Data are segmented by environmental condition: indoors (in), outdoors (out) and overall (total of in and out); and by dispersion component: vertical (Ver.) and horizontal (Hor.). Values for the sample medians and quartiles are displayed in the boxes and the whiskers represent the rest of the sample (unless there are outliers). The few “+” symbols represent outliers, values more than 1.5 times the interquartile range away from the top or bottom of the box.