Binocular contrast summation and inhibition depends on spatial frequency, eccentricity and binocular disparity.

PURPOSE: When central vision is compromised, visually-guided behaviour becomes dependent on peripheral retina, often at a preferred retinal locus (PRL). Previous studies have examined adaptation to central vision loss with monocular 2D paradigms, whereas in real tasks, patients make binocular eye movements to targets of various sizes and depth in 3D environments. METHODS: We therefore examined monocular and binocular contrast sensitivity functions with a 26-AFC (alternate forced choice) band-pass filtered letter identification task at 2° or 6° eccentricity in observers with simulated central vision loss. Binocular stimuli were presented in corresponding or non-corresponding stereoscopic retinal locations. Gaze-contingent scotomas (0.5° radius disks of pink noise) were simulated independently in each eye with a 1000 Hz eye tracker and 120 Hz dichoptic shutter glasses. RESULTS: Contrast sensitivity was higher for binocular than monocular conditions, but only exceeded probability summation at low-mid spatial frequencies in corresponding retinal locations. At high spatial frequencies or non-corresponding retinal locations, binocular contrast sensitivity showed evidence of interocular suppression. CONCLUSIONS: These results suggest that binocular vision deficits may be underestimated by monocular vision tests and identify a method that can be used to select a PRL based on binocular contrast summation.

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

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.

Visual attention measures predict pedestrian detection in central field loss: a pilot study.

PURPOSE: The ability of visually impaired people to deploy attention effectively to maximize use of their residual vision in dynamic situations is fundamental to safe mobility. We conducted a pilot study to evaluate whether tests of dynamic attention (multiple object tracking; MOT) and static attention (Useful Field of View; UFOV) were predictive of the ability of people with central field loss (CFL) to detect pedestrian hazards in simulated driving. METHODS: 11 people with bilateral CFL (visual acuity 20/30-20/200) and 11 age-similar normally-sighted drivers participated. Dynamic and static attention were evaluated with brief, computer-based MOT and UFOV tasks, respectively. Dependent variables were the log speed threshold for 60% correct identification of targets (MOT) and the increase in the presentation duration for 75% correct identification of a central target when a concurrent peripheral task was added (UFOV divided and selective attention subtests). Participants drove in a simulator and pressed the horn whenever they detected pedestrians that walked or ran toward the road. The dependent variable was the proportion of timely reactions (could have stopped in time to avoid a collision). RESULTS: UFOV and MOT performance of CFL participants was poorer than that of controls, and the proportion of timely reactions was also lower (worse) (84% and 97%, respectively; p = 0.001). For CFL participants, higher proportions of timely reactions correlated significantly with higher (better) MOT speed thresholds (r = 0.73, p = 0.01), with better performance on the UFOV divided and selective attention subtests (r = -0.66 and -0.62, respectively, p<0.04), with better contrast sensitivity scores (r = 0.54, p = 0.08) and smaller scotomas (r = -0.60, p = 0.05). CONCLUSIONS: Our results suggest that brief laboratory-based tests of visual attention may provide useful measures of functional visual ability of individuals with CFL relevant to more complex mobility tasks.

Driving with hemianopia: III. Detection of stationary and approaching pedestrians in a simulator.

PURPOSE: To compare blind-side detection performance of drivers with homonymous hemianopia (HH) for stationary and approaching pedestrians, initially appearing at small (4°) or large (14°) eccentricities in a driving simulator. While the stationary pedestrians did not represent an imminent threat, as their eccentricity increased rapidly as the vehicle advanced, the approaching pedestrians maintained a collision course with approximately constant eccentricity, walking or running, toward the travel lane as if to cross. METHODS: Twelve participants with complete HH and without spatial neglect pressed the horn whenever they detected a pedestrian while driving along predetermined routes in two driving simulator sessions. Miss rates and reaction times were analyzed for 52 stationary and 52 approaching pedestrians. RESULTS: Miss rates were higher and reaction times longer on the blind than the seeing side (P < 0.01). On the blind side, miss rates were lower for approaching than stationary pedestrians (16% vs. 29%, P = 0.01), especially at larger eccentricities (20% vs. 54%, P = 0.005), but reaction times for approaching pedestrians were longer (1.72 vs. 1.41 seconds; P = 0.03). Overall, the proportion of potential blind-side collisions (missed and late responses) was not different for the two paradigms (41% vs. 35%, P = 0.48), and significantly higher than for the seeing side (3%, P = 0.002). CONCLUSIONS: In a realistic pedestrian detection task, drivers with HH exhibited significant blind-side detection deficits. Even when approaching pedestrians were detected, responses were often too late to avoid a potential collision.

Using an eye-tracker to assess the effectiveness of a three-dimensional riding simulator in increasing hazard perception.

A crucial factor contributing to the high rate of road accidents involving young people is inexperience, in particular the inability to promptly identify risky situations. The aim of this study is to test the effectiveness of a riding simulator in improving this skill in young inexperienced riders. We use the first fixation latency to measure the improvement in detecting the hazardous object. Results show that four training sessions can significantly affect promptness in detecting new hazardous objects as they appear, decreasing the time needed to orient the eyes to the hazard.

Hazard perception as a function of target location and the field of view.

A typical hazard perception test presents participants with a single-screen view of the road ahead. This study assessed how increasing this field of view would affect hazard perception abilities. Drivers were shown video clips of driving situations containing at least one hazard either on a single screen, or with the addition of side views on two separate but adjacent screens that extended the perceived worldview to approximately 180 degrees. Mirror information was also included to allow information from behind the vehicle to be attended. Participants were instructed to press a button as soon as they saw a hazard. Faster response times were found for hazards that appeared in the centre of the central screen, than in the periphery of the central screen, with hazards that first appeared in the lateral screens responded to slowest. Additionally, responses to the hazards were faster and were more likely to occur in the three-, as compared to the single-screen condition. These results suggest that providing participants with a wider field of view, which includes more environmental cues that are related to the relevant hazardous situation increases their ability to detect hazards, and some limited support to that providing them with a wider view increases this ability even when all hazard-relevant information appear only in the central screen. A number of reasons for the three-screen advantage are discussed. This study suggests that even responses to central hazards may be under-estimated in a typical single-screen hazard perception test, and that improvements can be made for new hazard perception tests, by including visual information from the side and from behind the driver. This new methodology not only allows testing hazard perception skills in a potentially more immersive and realistic environment, but also enables to create hazard perception clips that cannot be realised in a typical single-screen test.