Peripheral Prisms Improve Obstacle Detection during Simulated Walking for Patients with Left Hemispatial Neglect and Hemianopia

Abstract

Significance: The first report on the use of peripheral prisms (p-prisms) for patients with left neglect and homonymous visual field defects (HVFDs).

Purpose: The purpose of this study was to investigate if patients with left hemispatial neglect and HVFDs benefit from p-prisms to expand the visual field and improve obstacle detection.

Methods: Patients (24 with HVFDs, 10 of whom had left neglect) viewed an animated, virtual, shopping mall corridor and reported if they would have collided with a human obstacle that appeared at various offsets up to 13.5° from their simulated walking path. There were 40 obstacle presentations on each side, with and without p-prisms. No training with p-prisms was provided, and gaze was fixed at the center of expansion.

Results: Detection on the side of the HVFD improved significantly with p-prisms in both groups, from 26 to 92% in the left-neglect group and 43 to 98% in the non-neglect group (both P < .001). There was a tendency for greater improvement in the neglect patients with p-prisms. For collision judgments, both groups exhibited a large increase in perceived collisions on the side of the HVFD with the prisms (P < .001), with no difference between the groups (P = .93). Increased perceived collisions represent a wider perceived safety margin on the side of the HVFD.

Conclusions: Within the controlled conditions of this simulated, collision judgment task, patients with left neglect responded well to initial application of p-prisms exhibiting improved detection and wider safety margins on the side of the HVFD that did not differ from non-neglect patients. Further study of p-prisms for neglect patients in free-gaze conditions after extended wear and in real-world mobility tasks is clearly warranted.

Figure 1.

Standard unilateral fitting of oblique 57Δ peripheral prisms (p-prisms) on the left eye for left homonymous visual field defect (HVFD) with the segments laterally centered on the visual axis in primary gaze (indicated by the dashed arrow). The red dot is the position of the visual axis at the spectacle plane (picture was taken from slightly off to the left and up from the photographer’s perspective). The p-prisms straddle the border of the HVFD such that half of the prism is within the intact (right) visual field when in primary position of gaze.

Figure 2.

Virtual shopping mall experimental set-up photographed from above. Patients were seated with head fixed in a head-chin rest and eyes fixed on a central cross. Optic flow was generated to simulate walking and then a single realistically-sized human obstacle (0.7 m (8°) wide x 0.7 m deep x 2 m tall (23°) (as shown here on the left) would suddenly appear and then disappear after 1s, at which time the simulated walking would stop and the patient would be asked for a response of “collision”, “no collision”, or “no obstacle” if no human obstacle was seen. The human obstacle appeared at offsets from the walking path/fixation point ranging from 0° (directly in the walking path) up to 13.5° of visual angle to the left and right, measured from the inside edge of the obstacle. There were 88 trials (40 on each side and 8 catch trials where no obstacle appeared).

Figure 3.

Scatter plot of detection for each patient on the side with homonymous visual field defect (HVFD) without (x-axis) and with (y-axis) peripheral prisms (p-prisms). “Non” refers to the non-neglect group for those with complete HVFD and incomplete HVFD. One patient with left neglect and one without neglect did not improve substantively (data points close to the unity line).

Figure 4.

Average proportion detected by obstacle offset without and with peripheral prisms (p-prisms) for the non-neglect (n=14) and left-neglect (n = 10) groups. The shaded area with negative x-axis values represents the homonymous visual field defect (HVFD) side. Gaze was fixed throughout the testing. Without p-prisms there was equally poor HVFD side detection in the non-neglect and left neglect groups (p = 0.77) and both groups improved significantly with p-prisms (p < 0.001). There was a tendency for the improvement in HVFD side detection with the p-prisms to be higher in the neglect group (p = 0.08).

Figure 5.

Average collision judgments by offset of the obstacle from the walking path without and with peripheral prisms (p-prisms) for the non-neglect and left-neglect groups. On the affected (homonymous visual field defect (HVFD)) side (negative x-axis values), p-prisms improved (p < 0.001) but exaggerated the safety margin an equal amount in left-neglect and non-neglect groups, p = 0.90 (i.e. patients report most obstacles detected via the p-prisms as an impending collision).