Robert T. Hennessy

Laser optometer incorporating the Badal principle

The refractive state of the eye can be accurately determined while performing a visual task by introducing the diverged beam of a laser, reflected from a moving drum, into the visual field. The direction of the apparent motion of the resultant interference pattern is a direct indicator of refractive state. By incorporation of the Badal principle into the optical system, a measurement can be completed within approximately 30 sec. The advantage of this method over other techniques is that it permits evaluation of refractive state but does not itself influehce the magnitude of accommodation. A number of applications of this principle are cited.

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.

Oculomotor adjustments and size constancy

The effect on matched size of the oculomotor adjustments was determined by stimulation and relaxation of accommodation and convergence by means of spherical lenses. The normal coupling between accommodation and convergence was maintained by introducing the amount of convergence appropriate to the lens power and each S’s interpupillary distance. Data indicate that the oculomotor adjustments are adequate to account for size constancy up to approximately 1 m, beyond which their effect progressively decreases. The actual accommodation in force was assessed by means of the laser scintillation technique. It was determined that the magnitude of accommodation responds accurately to the spherical lens introduced up to about 1 m observation distance, beyond which underaccommodation was noted. Examination of the matched size as a function of the actual accommodation distance reveals a very close correspondence to the size constancy prediction up to about 1 m.

The effect of a peripheral stimulus on accommodation

Accommodation was measured by the laser scintillation technique while the S viewed a stationary fixation spot through a series of apertures in a screen located at various distances. The magnitude of accommodation was a compromise between the distance of the fixation spot and the screen. Accommodation was affected significantly by the interaction of the distance of the screen with aperture sizes of 1 and 4 deg and distance of the screen with its order of movement from near to far or far to near. The data are interpreted as implying the importance of the peripheral visual field and/or perceptual factors when conflicting cues to distance coexist in the visual field.

The Effect of Heat Stress on Reaction Time to Centrally and Peripherally Presented Stimuli

The effect of heat stress on reaction time to centrally and to peripherally presented stimuli was determined for lean and obese subjects whose work levels on a treadmill in a heat chamber produced either a 2.5 or 5% loss of body weight in six hours. Peripheral reaction time improved with practice, but central reaction time was not affected. Obesity, eccentricity of the peripheral stimulus, and the level of dehydration did not have significant effects on reaction time. It was concluded that, for short periods of time, the effects of heat stress can be overcome for highly motivated and experienced subjects.

The effect of hypnotic ablation of the background on the magnitude of the ponzo perspective illusion

Abstract In an attempt to develop an objective measure of hypnotic perception, and also to test the stability of the Ponzo perspective illusion, negative hallucinations were hypnotically induced to ablate the radiating lines of the Ponzo configuration. 3 groups of Ss (including 1 group of simulators) were tested across 3 conditions—unhypnotized, hypnotized, and hypnotized with hallucination instructions. Results indicated that the suggested hallucination did not affect susceptibility to the illusion. Results also suggested that certain hypnotically susceptible Ss might be more susceptible to the Ponzo under all experimental conditions than non-susceptible Ss. It was concluded that suggested hallucinations do not affect performance on the Ponzo illusion and that the Ponzo is a very stable illusion and appears to be highly resistant to variations in instructions and experimental set, as induced by hypnosis.

Perceived vs retinal relationships in the Ponzo illusion

In order to evaluate the relative contribution of perceived and retinal image relationships as determiners of the Ponzo perspective illusion, the comparison lines were located physically either in the same plane as the background converging lines or in a separate plane nearer to the O. Data from 24 Ss indicated that perceived are more important than retinal image relations but that the latter may contribute to the illusory effect.

Adaptation to heat stress

From the physiological and anatomical point of view, the lateral visual field of the human operator is approximately 180�. Although a number of visual functions including visual acuity and the ability to detect light decrease in the peripheral regions, a normal observer generally makes discriminations out to 80� on either side of the midline and beyond. Such data as these are obtained primarily from standard experiments in which the subject, seated comfortably in an experimental room reports on the presence and nature of stimuli presented to his peripheral visual field. A different situation arises, however, When similar data are taken under non-standard laboratory conditions. If the subject is under physical or emotional stress, frightened, inexperienced, under the effects of drugs, etc., a decrement in the ability to detect peripheral stimuli is observed, This phenomenon has given rise to the concept of the functional visual field implying that although the anatomical and physiological mechanisms are present, stimuli presented to the periphery cannot be detected, or are detected at a lower level of efficiency, under conditions of stress. In connection with an experiment designed to determine the physiological effects of severe water loss as a result of heat stress, an opportunity was provided to also test the subjects ability to detect peripherally presented stimuli. Eight subjects worked on a standard treadmill according to a 20 minutes on-off duty cycle for six hours. The speed and inclination of the treadmill were adjusted so as to produce, on different days, either 5% or 2.5% weight loss. Half the subjects were male, half female, half lean and half obese. In addition, at the end of the 6 hour session, a maximum performance test was given to each subject. Data were obtained not only on the ability to detect peripheral thresholds, but also on central reaction time. The results indicate no effect whatsoever of any of the conditions of the experiment on any of the measurements taken. These data are in marked contrast to those of previous studies in which even the slightest changes in stimulus conditions produced loss of peripheral discrimination. The discrepancy between this and previous studies is attributed to the fact that the subjects had previous training in the use of the apparatus, that they were highly motivated, and that they were required to respond only for short prearranged duty cycles.