Herschel W. Leibowitz

Peripheral Motion Detection and Refractive Error

Motion thresholds were determined for the fovea and peripheral retina with and without correction for peripheral refractive error. With correction, motion thresholds decreased and inidividual differences disappeared. These results imply that under normal observation conditions, peripheral sensitivity is limited mainly by dioptric rather than retinal variables.

AGE DIFFERENCES IN ESTIMATING VEHICLE VELOCITY

Automobile accidents among older adults may be related to difficulties in judging the speed of other vehicles. To examine this possibility, 3 groups of observers in the young adult, middle-aged, and older adult age ranges were asked to estimate the velocity of an isolated automobile traveling at 15-50 mph (24-80 kph). Across all age groups, perceived and actual velocity were related by a power function with an exponent of 1.36. Age was significantly and positively correlated with intercepts, but negatively correlated with exponents; that is, older observers showed less sensitivity to changes in actual velocity. Results bear on the issues of ontogenetic changes in accident involvement and sensitivity to motion.

Effect of dioptrics on peripheral visual acuity

Abstract The peripheral dioptrics of the eye displays a considerable error of refraction caused primarily by oblique-ray astigmatism. It was hypothesised that this refractive error might contribute to the well known reduction in visual acuity in the peripheral visual field. Independent experiments were carried out in two laboratories, using different methods and different targets; sinusoidal gratings and Landolt rings. A third experiment was run in which two subjects from one laboratory were tested in the other. Measurements of visual acuity were obtained between 0° and 60° of eccentricity along the horizontal meridian both with and without correction of refractive error. The results of all three experiments indicate that the existing errors in refraction do not influence peripheral visual acuity.

Ponzo Perspective Illusion as a Manifestation of Space Perception

The Ponzo perspective illusion, a special case of space perception, is influenced by contextual factors, texture, stereopsis, and familiarity in addition to perspective cues. The importance of familiarity is demonstrated by cultural differences obtained with photographs of natural settings which emphasize depth cues.

The Effect of a Central Task on Luminance Thresholds for Peripherally Presented Stimuli

The effect of a central task on peripheral vision was investigated by obtaining luminance thresholds for stimuli presented from 20° to 90° in the periphery. A control group observed a steady foveal fixation light. For the two experimental groups, the fixation light was interrupted periodically and the subject was required to maintain illumination of the light by pressing a button. Interruption rates of 15 and 53 times per minute were investigated. The control condition with the steady fixation light produced the lowest thresholds. For the interrupted conditions, thresholds were higher for the near periphery but differences disappeared in the far periphery. Luminance threshold values were generally highest for the 15-per-minute interrupted group. The results are interpreted as reflecting the effect of an attention-demanding central task on the size of the functional visual field.

Velocity-time reciprocity in the perception of motion: foveal and peripheral determinations.

Abstract Motion thresholds were determined for durations between 0.025 and 5 sec in the fovea and periphery. For intermediate durations of movement between 0.1 and 1.0 sec. threshold is determined by a constant displacement of the stimulus. For shorter and longer durations, threshold is determined by a fixed velocity. Similar characteristics are displayed by motion detection mechanisms in the fovea and periphery. The close correspondence between resolution and motion thresholds for intermediate durations in both the fovea and the perphery is discussed.

The relation of vergence effort to report of visual fatigue following prolonged near work

Two experiments examined the link between vergence effort and subjective reports of visual fatigue following prolonged near work. In Experiment 1, dark vergence and reports of visual fatigue symptoms associated with computer use were measured in 104 persons. Dark vergence was significantly correlated with four of six symptoms of visual fatigue. In Experiment 2, 16 subjects who exhibited clear “Heuer effects” (systematic gaze-related changes in dark vergence) read from a video display terminal (VDT) at a distance of 20 cm for 60 min under two gaze elevation conditions: 20 deg above and 20 deg below a baseline gaze elevation. This manipulation was designed to vary the amount of vergence effort required to fixate the VDT. Results indicate a positive relation between vergence effort and visual fatigue both within and between subjects, and that prolonged near work leads to decreased vergence accuracy at far distances. Implications for VDT workstation design are discussed.

Oculomotor adjustments in darkness and the specific distance tendency

Two experiments were performed to investigate the relationship between the oculomotor adjustments assumed in total darkness and perceived distance under reduced visual conditions. Experiment I compared the dark focus of accommodation with the perceived distance of a monocular light point presented in a dark environment. Experiment II compared the convergence angle assumed in darkness (dark convergence) with the perceived distance of the light point. Both accommodation and convergence were found to assume intermediate values in darkness. Perceived distance of the monocular light point was significantly correlated with dark convergence and unrelated to the dark focus of accommodation. It was suggested that ocular vergence is a major determinant of perceived distance under reduced visual conditions, and thus provides a possible mechanism for the specific distance tendency.

The effect of pupil size on accommodation

Abstract Accommodation was measured with a laser optometer while viewing letter charts through a series of artificial pupils over a range of distances. As pupil size is reduced, the amplitude of accommodation is diminished, approaching a fixed intermediate resting focus corresponding to approx 1 m. These data are interpreted as supporting the intermediate as opposed to the classical infinity resting focus hypothesis. Implications for instrument myopia and the relationship between perceived distance and accommodation are discussed.

Practice, refractive error, and feedback as factors influencing peripheral motion thresholds*

Motion thresholds were determined in the fovea and in the horizontal peripheral meridian under conditions of practice, correction of refractive error, and feedback. While foveal determinations did not change significantly, peripheral motion thresholds were lowered for all conditions. Both practice and practice with feedback produced significantly better motion discrimination, these effects showing a longevity of several months. However, the largest improvement resulted from correction of peripheral refractive error, which also reduced individual differences. This suggests that peripheral sensitivity to movement is ordinarily limited primarily by dioptric rather than neural factors. Implications for perceptual learning and peripheral vision research are discussed.