D. Alfred Owens

A rapid technique to assess the resting states of the eyes and other threshold phenomena: The Modified Binary Search (MOBS)

A technique was developed to automate subjective measurement of the resting states of the eyes. This technique, the Modified Binary Search (MOBS), evolved from the binary search and a commonly used manual bracketing technique. The procedure is mathematically and logically simple, and it requires minimal storage and computation capabilities. Monte Carlo simulations indicate that the MOBS procedure provides more precise measures with fewer stimulus presentations than conventional staircases. It is also relatively insensitive to response errors.

Vertical Gaze Direction and the Resting Posture of the Eyes

With horizontal gaze, the resting posture of binocular vergence typically corresponds to a distance of about 1 m. The effect of vertical direction of gaze on this basic resting posture was investigated. The dark vergence of twenty-four subjects was measured while they fixated a dim monocular light-point at vertical directions ranging from −45° (lowered) to +30° (elevated). In one condition, gaze was varied by changes in eye position with the head held upright; in a second condition, gaze was varied by changes in head inclination with the eyes held in constant (horizontal) position with respect to the head. In both conditions, dark vergence shifted in the convergent (nearer) direction with lowered gaze and in the divergent (farther) direction with elevated gaze. The effect of varied eye inclination was larger, more variable across subjects, and more stable over time than that of varied head inclination. These findings indicate that multiple mechanisms contribute to gaze-related variations of the resting posture of the eyes. They may help to explain the variations of space perception and visual fatigue that are observed with different gaze inclinations.

Standard measures of visual acuity do not predict drivers' recognition performance under day or night conditions.

Purpose. This study investigated whether visual acuity or contrast sensitivity, measured under a range of luminance conditions, could predict drivers’ recognition performance under real-world day and night road conditions. Methods. Twenty-four participants, comprising three age groups (younger, mean = 21.5 years; middle-aged, mean = 46.6 years; and older, mean = 71.9 years), drove around a 1.8-km closed road circuit under day and nighttime conditions. At night, headlight intensity was varied over 1.5 log-units by ND filters mounted on the headlights. Participants drove around the circuit under five light conditions (daytime and four at night) and were asked to report relevant targets, including road signs, large low-contrast road obstacles, and pedestrians who wore retroreflective markings on either the torso or the limb joints (creating “biological motion”). Real-world recognition performance was measured as percent correct recognition and, in the case of low-contrast road obstacles, avoided. Clinical vision tests included high-contrast visual acuity and Pelli-Robson letter contrast sensitivity measured at four luminance levels. Results. Real-world recognition performance of all age groups was significantly degraded under low light conditions, and this impairment was greater for the older participants. These changes in drivers’ recognition performance were more strongly predicted by contrast sensitivity than visual acuity measured under standard photopic conditions. Interestingly, contrast sensitivity was highly correlated with visual acuity measured under low-luminance conditions. Further analyses showed that recognition performance while driving is better predicted by combinations of two tests: either 1) photopic visual acuity and photopic contrast sensitivity, or 2) photopic and mesopic visual acuity. Conclusions. These findings confirm that visibility is seriously degraded during night driving and that the problem is greater for older drivers. These changes in real-world recognition performance were better predicted by a standard test of contrast sensitivity than by visual acuity. Still better predictions can be obtained by the use of two vision tests. The implications of these findings for driver licensing standards are discussed.

Is Accommodation Colorblind? Focusing Chromatic Contours

Two adjacent regions define an edge if they differ in either color or luminance. If the difference is purely chromatic, the edge is said to be isoluminant. Isoluminant contours are often perceptually unstable. Perhaps some of this instability could be explained if isoluminant contours were difficult to bring into focus. To test this hypothesis, a vernier optometer was used to measure the accuracy of steady-state accommodation for the vertical boundary of a red—green bipartite field. This edge was presented at optical distances of 0, 1·5, 3·0, and 4·5 diopters, with brightness contrasts between the two hemifields of 0% (isoluminant), 15%, 58%, and 100%. Accommodation was essentially unresponsive to the isoluminant edge and exhibited increasing focusing accuracy with increased brightness contrast. Control experiments replicated this finding for red—orange, green—blue, and white—white fields. These results imply that luminance contrast is a necessary stimulus for monocular accommodation. Inappropriate accommodation may be a factor contributing to the perceptual instability of isoluminant patterns.

The accuracy of binocular vergence for peripheral stimuli

Two experiments were conducted to investigate the roles of dark vergence and retinal eccentricity as factors influencing binocular vergence responses. A nonius alignment technique was used to measure vergence responses in total darkness (dark vergence) and for dim binocular stimuli presented at retinal eccentricities ranging from 2 degrees to 8 degrees over distances ranging from 28.5 to 342 cm. The results indicate that vergence was progressively less accurate with stimuli at increasing retinal eccentricities, and that errors of fixation were biased toward the individuals dark vergence position. The hypothesis that dark vergence represents the functional resting state of the vergence system which influences oculomotor fusional responses under a variety of conditions is discussed.

The twilight envelope: a user-centered approach to describing roadway illumination at night.

Visual recognition functions, such as acuity and contrast sensitivity, deteriorate rapidly over the declining luminances found during civil twilight. Thus civil twilight, a critical part of the transition between daylight and darkness, represents lighting conditions that may be useful to describe artificial illumination. Automotive headlamps project a three-dimensional beam that ranges from illumination levels comparable to daylight at the vehicle to the dark limit of civil twilight (3.3 lx) at some distance ahead. This twilight envelope is characterized as a distance beyond which foveal visual functions are severely impaired, and thus it provides a general, functional description of the useful extent of the headlamp beam. This user-centered approach to describing illumination is useful for characterizing visibility when driving at night or in other artificially lit environments. This paper discusses the twilight envelope approach and its application to intervehicle variations in headlamp systems. Actual or potential applications of this research include user-centered description of artificial illumination and driver/pedestrian safety education

Nighttime Driving and Visual Degradation

Recent developments which provide new insights into night driving accidents are reviewed. Selective Degradation: The mechanisms subserving steering are different from those underlying hazard recognition and these two modes of processing visual information are selectively impaired at night. Although it is possible to steer a vehicle as well at night as during the day, the ability to recognize and respond to infrequent hazards is seriously degraded. Night Myopia: Many individuals become nearsighted under reduced illumination. The finding that this is a normal consequence of the passive return of the accommodative system to an intermediate resting posdition has led to a procedure to ameliorate this effect. By determining the value of an individuals dark focus it is possible to provide a special night driving prescription which effectively eliminates night myopia.

How do the elderly negotiate stairs

Stair navigation, particularly stair descent, is an extremely challenging and dangerous locomotor task, yet studies suggest that most elderly are unlikely to move to new residences in order to avoid this challenge. The knee and ankle are the key joints where adequate strength and power are required for safe stair descent, and it is not yet clear if sarcopenia in the elderly is likely to result in residual strength below that which is required for successful stair performance. Sensory cues are also critical, and the lack of literature on the specific roles of the various intrinsic and extrinsic factors that affect stair navigation is a clear indication of the need for such research in order to define safer strategies and optimal conditions for elderly individuals to transit between living areas of differing levels.

Predicting Optimal Accommodative Performance from Measures of the Dark Focus of Accommodation

Leibowitz and his colleagues found that accommodation rests at an intermediate distance that shows wide interindividual variation. They proposed that this intermediate dark focus is useful for correcting anomalous refractive errors, but this proposal was later questioned when different measurement techniques yielded discrepant dark focus values. The present study measured dark focus under two levels of visual attentiveness: (a) when performing an open-loop, active viewing task (aDF); and (b) when looking passively into darkness (pDF). These dark focus measures were then compared with an optimal accommodation distance that was derived from accommodative response functions in bright and dim luminance. The aDF measures were found to be more myopic (nearer) than the pDF measures and highly correlated with the optical accommodation distance. No significant relationship was found between pDF and optical accommodation distance. These findings confirm that measures of dark focus are affected by nonoptical aspects of the measurement technique; they also suggest that techniques that demand visual attention (aDF) yield dark focus values that are more useful for optimizing accommodation and potentially reducing fatigue in difficult situations.

Oculomotor tonus and visual adaptation

Abstract The contribution of oculomotor efference to visual perception and performance can be clarified by considering the functions of tonus. Accommodation and vergence typically ‘relax’ at an intermediate distance, reflecting tonic innervation of the ciliary and extraocular muscles, which varies widely among individuals who have normal vision. In many situations, especially when stimulation is degraded, fixation and focusing responses are biased toward the individuals resting state. Moreover, unusual circumstances, such as alteration of the relation of eye position and distance or prolonged exposure to near work, induce adaptive modification of the resting posture. These normal variations of oculomotor toms affect the accuracy and the effort required to fixate objects, and they may help explain problems of space perception and visual fatigue.