Hazard detection with a monocular bioptic telescope

Abstract

Purpose: The safety of bioptic telescopes for driving remains controversial. The ring scotoma, an area to the telescope eye due to the telescope magnification, has been the main cause of concern. This study evaluates whether bioptic users can use the fellow eye to detect in hazards driving videos that fall in the ring scotoma area.

Methods: Twelve visually impaired bioptic users watched a series of driving hazard perception training videos and responded as soon as they detected a hazard while reading aloud letters presented on the screen. The letters were placed such that when reading them through the telescope the hazard fell in the ring scotoma area. Four conditions were tested: no bioptic and no reading, reading without bioptic, reading with a bioptic that did not occlude the fellow eye (non-occluding bioptic), and reading with a bioptic that partially-occluded the fellow eye. Eight normally sighted subjects performed the same task with the partially occluding bioptic detecting lateral hazards (blocked by the device scotoma) and vertical hazards (outside the scotoma) to further determine the cause-and-effect relationship between hazard detection and the fellow eye.

Results: There were significant differences in performance between conditions: 83% of hazards were detected with no reading task, dropping to 67% in the reading task with no bioptic, to 50% while reading with the non-occluding bioptic, and 34% while reading with the partially occluding bioptic. For normally sighted, detection of vertical hazards (53%) was significantly higher than lateral hazards (38%) with the partially occluding bioptic.

Conclusions: Detection of driving hazards is impaired by the addition of a secondary reading like task. Detection is further impaired when reading through a monocular telescope. The effect of the partially-occluding bioptic supports the role of the non-occluded fellow eye in compensating for the ring scotoma.

Keywords: bioptic driving; bioptic telescope; hazard perception; ring scotoma.

Figure 1

(a) An illustration of character placement relative to the hazard (car). Characters are randomly assigned to one of eight directions around the area occupied by the hazard, 15° from the centre of the hazard area and at least 5° from the edge of the screen. (b) Character sequences were positioned 15° from the hazard centre such that when sighted through the telescope, the hazard falls in the monocular ring scotoma area (between the dashed yellow circles). The grey shaded areas mark the binocular scotoma created by the partially occluding VES K device design. The top example shows a lateral hazard (white car) which falls in the binocular scotoma area of the VES K design but only in the monocular scotoma of the non-occluding design. The bottom example shows a vertical hazard (pedestrian) that falls outside the binocular device scotoma. (c) One or two letters appeared amid a sequence of numbers changing every 320 ms for the duration of the hazard.

Figure 2

(a) A person viewing through a 3.0× VES mini bioptic fitted on the left eye; (b) Binocular visual field of a bioptic user viewing through the 3× VES mini left bioptic telescope. The only binocular scotoma is the overlap of the ring scotoma with the physiological blind spot of the fellow eye. The area bound between the two dotted circular regions represents the monocularly measured ring scotoma. (c) A person viewing through a 4× VES K right bioptic telescope. (d) Binocular visual field while viewing through the 4× VES K bioptic. The telescope housing creates a binocular scotoma (shaded grey). The area bound between the two dotted circles represents the monocularly measured ring scotoma and the monocular telescope housing scotoma. The dashed line in (b) and (d) represents the view through a standard car windshield.

Figure 3

Bioptic users’ performance in each of the four conditions. (a) UK point scores, dashed line indicating UK passing mark (b) detection rates (c) reaction times. Thick horizontal line within the box is the median; the vertical extent of the box is the interquartile range (IQR); whiskers represent the data range with outlier excluded; and circles are outliers (1.5–3 IQR).

Figure 4

UK point scores for the main effects. (a) UK scores were significantly lower with the addition of the reading task. (b) UK scores were significantly lower with the non-occluding bioptic design compared to the reading task with no bioptic. (c) UK scores were significantly lower with the partially occluding design compared to the non-occluding bioptic design. Note that higher subject number codes for poorer visual acuity, which is also coded by darkness of the symbols.

Figure 5

UK scores were not significantly different across order tests were administered, suggesting no effects of learning or fatigue for bioptic users or normally sighted controls. The notations in this graph are the same as in Figure 3.