[ARTICLE] Driving with Hemianopia V: Do Individuals with Hemianopia Spontaneously Adapt Their Gaze Scanning to Differing Hazard Detection Demands?

Abstract

Purpose: We investigated whether people with homonymous hemianopia (HH) were able to spontaneously (without training or instructions) adapt their blind-side scan magnitudes in response to differing scanning requirements for detection of pedestrians in a driving simulator when differing cues about pedestrian eccentricities and movement behaviors were available in the seeing hemifield.

Methods: Twelve HH participants completed two sessions in a driving simulator pressing the horn when they detected a pedestrian. Stationary pedestrians outside the driving lane were presented in one session and approaching pedestrians on a collision course in the other. Gaze data were analyzed for pedestrians initially appearing at approximately 14° in the blind hemifield. No instructions were given regarding scanning.

Results: After appearing, the stationary pedestrians’ eccentricity increased rapidly to a median of 31° after 2.5 seconds, requiring increasingly larger blind-side gaze scans for detection, while the approaching pedestrians’ eccentricity remained constant at approximately 14°, requiring a more moderate scan (∼14°) for detection. Although median scan magnitudes did not differ between the two conditions (approaching: 14° [IQR 9°–15°]; stationary: 13° [IQR 9°–20°]; P= 0.43), three participants showed evidence of adapting (increasing) their blind-side scan magnitudes in the stationary condition.

Conclusions: Three participants (25%) appeared to be able to apply voluntary cognitive control to modify their blind-side gaze scanning in response to the differing scanning requirements of the two conditions without explicit training.

Translational Relevance: Our results suggest that only a minority of people with hemianopia are likely to be able to spontaneously adapt their blind-side scanning in response to rapidly changing and unpredictable situations in on-road driving.

 

Introduction

Homonymous hemianopia (HH) is the loss of half the field of vision on the same side in both eyes. It is caused by lesions in the postchiasmal visual pathways, primarily due to strokes and, to a lesser extent, trauma and tumors.¹ People with HH may compensate for their hemifield loss by scanning using eye and/or head movements toward the blind hemifield. However, there is accumulating evidence² that many do not compensate well leading to impaired hazard detection in simulated driving,^3–8 in on-road driving,⁹ and in walking tasks.¹⁰

In order to see an object in the blind hemifield, people with complete HH need to scan at least as far into the blind hemifield as the location of the object. However, they receive no visual cues from peripheral vision as to when to scan or how far to scan into the blind hemifield. People with HH are usually aware of their visual field loss (unless they have spatial neglect) and could use voluntary, cognitive control to guide their blind-side scanning. For example, patients with HH may be trained or told to scan to the blind side in order to detect potential hazards.¹¹ However, relatively little is known about the extent to which people with HH use such strategies in real world situations (e.g., driving²) or whether they are able to adapt their scanning patterns in response to differing conditions (e.g., a busy city-center street with frequent hazards versus a quiet rural road with infrequent hazards).

Only a limited number of studies have addressed the question of whether people with HH adapt their scanning strategies. A recent study using a gaze-contingent simulation of HH concluded that efficient search strategies were not spontaneously adopted by the majority of participants; however, a minority (4/20) did modify their search strategy in response to changing task demands. In general, a strategy of searching the seeing side before the blind side was adopted both for difficult search tasks in which each item needed to be viewed in a serial fashion as well as easier tasks in which the target was clearly visible in the periphery and a large saccade toward the blind side would have been the more efficient strategy. Only four participants modified their search to start from the blind field when the task was easy. In another study, Schuett et al.¹²reported that normally sighted participants with simulated HH became more efficient at a dot-counting task (an irregular array of dots) and a reading task after a short period of practice (∼15 minutes) on each of the tasks. However, in follow-up studies they found no evidence of transfer of learning (modification of gaze behaviors) between the two tasks, either for simulated HH¹³ or real HH¹³ (i.e., participants who practiced the dot-couting task did not demonstrate improvements on the reading task and vice versa).

In a study involving detection of peripherally presented moving basketballs within a virtual environment, participants with HH scanned less extensively in the horizontal plane and spent more time looking toward the ground when performing the task while walking than when seated.¹⁰ Thus, when walking, it appears that they modified their gaze behaviors in favor of walking, at the expense of peripheral target detection. In a driving simulator study³ involving detection of stationary pedestrians on the blind and seeing sides, no improvement in blind-side detection rates was found between two simulator sessions, approximately 1 week apart. These results suggested that no learning had occurred, despite 60 minutes of test drives at each session. Moreover, information was available in the seeing hemifield about pedestrian eccentricity, which could have been used as a guide for the scanning behaviors that were needed for detection of blind-side pedestrians. However, gaze movements were not recorded so it was unknown whether there were any changes in gaze behaviors.

In this paper, we report an analysis of gaze behaviors from a driving simulator study⁵ in which we evaluated detection performance of people with HH for pedestrians that were stationary to the side of the driving lane and pedestrians that approached the driving lane, walking or running on a collision course (detection rates and reaction time results were reported previously⁵). The scanning requirements for successful detection of blind-side pedestrians differed for the two pedestrian conditions, providing an opportunity to examine whether participants adapted their scanning behaviors. In the stationary condition the pedestrians did not move after appearing.^3,5 Thus, their eccentricity increased rapidly as the car progressed, moving them farther into the blind hemifield, requiring increasingly larger gaze movements for detection. On the other hand, in the approaching condition, the pedestrians were on a collision course,^5,14 so their eccentricity remained approximately constant as the car progressed. Thus, the magnitude of the gaze scan needed for detection would also have remained approximately constant, and smaller than the gaze scan needed in the stationary condition.

Herein, we address two main questions: (1) did participants with HH adapt their blind-side scan magnitudes (without training or instructions) to match the differing scanning requirements (pedestrian eccentricities) for successful blind-side detection in the stationary and approaching conditions; and (2) were the previously reported⁵ differences in blind-side detection rates between the two conditions accounted for by differences in the scanning requirements? We quantified the magnitudes of scans after the appearance of pedestrians in the blind hemifield and tested the hypothesis that blind-side scans would be larger in the stationary than the approaching condition. We expected to find greater evidence of within-session learning in the stationary condition (where scanning requirements were higher) than in the approaching condition. In addition, we tested the hypothesis that more time would be available after the pedestrian appearance for a moderate-sized gaze scan to reach the pedestrian in the approaching than the stationary condition, resulting in better detection rates but longer response times.[…]

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