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Biomedical subjects

R B Post

Publications and source records attributed to R B Post.

54 records · Page 3Linked to original sources
PubMed1984

Effects of retinal eccentricity on displacement thresholds for unidirectional and oscillatory stimuli.

Motion sensitivity was determined for line stimuli undergoing unidirectional and oscillatory displacements at various retinal eccentricities. Oscillatory motions of short duration were found to produce consistently greater motion thresholds than unidirectional stimuli at each eccentricity tested. Increasing retinal eccentricity elevated motion thresholds similarly for both forms of stimulus motion. The observed differences between unidirectional and oscillatory stimuli are attributed to differences in the temporal summation of the two forms of stimuli at short duration. These findings also suggest that differences in motion sensitivity between central and peripheral vision are quantitative rather than qualitative.

Fovea Centralis↗
PubMed1984

Short-term variability of the resting focus of accommodation.

Previous investigations have shown that, in dim illumination and empty visual fields, accommodation in most observers is for an intermediate distance referred to as the dark focus or resting focus of accommodation. Dynamic properties of the resting focus were examined in the present study with a high-speed infrared optometer. All subjects displayed fluctuations of accommodation in darkness that were primarily characterized by frequency components below 0.5 Hz. These fluctuations were substantially attenuated during cycloplegia or viewing a bright, high-contrast target. Both the mean resting-focus position and the magnitude of fluctuations varied from one subject to another and from day-to-day. A weak association was found between the mean a standard deviation of accommodation responses in the dark. The present findings suggest that the accommodation mechanism in most observers is somewhat unstable under degraded viewing conditions. In addition, the mean resting-focus value for a large sample of subjects was found to be lower for i.r. optometer measurements than for previous investigations employing a laser optometer.

Accommodation, Ocular↗
PubMed1984

Comparison of laser and infrared techniques for measurement of the resting focus of accommodation: mean differences and long-term variability.

The resting focus of accommodation was measured in the same subjects with both a laser optometer and a high-speed infrared optometer. Although i.r. optometer and laser optometer measures produce similar estimates of the mean resting-focus level in some subjects, others were found to have different levels of resting focus with the two techniques. Control studies demonstrated that these differences were not due to the temporal sampling characteristics of the laser optometer, but resulted instead from making judgments about the direction of speckle motion during the laser optometer procedure. The temporal stability of the resting focus of accommodation was also investigated with i.r. measures obtained several minutes, 1 day, and 1 and 2 weeks apart. Results indicated long-term variability similar to that previously reported with the laser optometer.

Accommodation, Ocular↗
PubMed1984

Adaptation of tonic accommodation.

We compared the resting (dark) focus of accommodation before and after adapting to accommodative stimuli placed nearer or farther from an initial baseline resting focus. Short-term monocular adaptation (less than 2 min) did not result in consistent after-effects that were correlated with the adaptation stimulus. After short-term adaptation, accommodation returned to its resting level in 2-15 s. Long-term monocular adaptation (30 min) to a 6-D near stimulus resulted in a small (0.5-D) average increase in the resting focus of accommodation beyond the normal 2-15-s short-term decay. These observations illustrate a tonic adaptation of accommodation that is small and requires longer durations of adaptation than an analogous adaptation of the fusional vergence system to prism.

Accommodation, Ocular↗
PubMed1983

Effect of X-Chrom lens wear on chromatic discrimination and stereopsis in color-deficient observers.

Four color-deficient observers and one normal trichromatic subject were evaluated with color vision and stereoacuity tests during 1 month of X-Chrom lens wear. For all color tests, performance of the normal subject was unaltered by X-Chrom lens wear. Color-deficient subjects demonstrated improved performance on the Ishihara pseudoisochromatic color plates, but either degraded performance or no change on the Farnsworth-Munsell 100-hue or Pickford-Nicolson red-green anomaloscope test. Three of the five subjects exhibited degraded stereoacuity in conjunction with X-Chrom lens wear. We conclude that the X-Chrom lens does not improve discrimination in color-deficient subjects and may alter stereopsis.

Color Perception Tests↗
PubMed1982

The effect of convergence on the vestibulo-ocular reflex and implications for perceived movement.

The apparent motion of a fixated stimulus during head translation in the dark was found to depend on the magnitude of the vestibulo-ocular reflex (VOR). Absolute convergence level determines VOR magnitude and thereby influences apparent motion during head movement by determining the magnitude and direction of the pursuit effort required to maintain gaze stability. The results are discussed in terms of the biological utility of the coupling between convergence and the VOR in maintaining image stability, and the role of the pursuit eye movement system in illusory movement.

Fixation, Ocular↗
PubMed1982

Autokinesis and peripheral stimuli: implications for fixational stability.

The reduction in autokinesis of a fixated stimulus was determined as a function of the eccentricity of an identical second stimulus. When the stimuli were separated by 1 deg, autokinesis was reduced by approximately 50% but with increasing separation the attenuation was diminished. The results are interpreted in terms of fixational stability models of autokinesis. It is suggested that autokinesis as well as other examples of apparent movement result from fixational instability.

Adult↗
PubMed1982

Monocular deprivation in humans: a study of identical twins.

Ultrasonic axial length measurements and psychophysical vernier acuity thresholds were determined for a pair of identical human twins, one of whom had been monocularly deprived since birth with a congenital lens opacity. Axial lengths of both eyes in the normal twin and the nondeprived eye in the other twin differed by less than 0.2 mm, whereas the axial measurement of the deprived eye was approximately 2.0 mm longer. Monocular vernier acuity thresholds in the nondeprived eye of the one twin were not significantly different from those obtained in the normal twin. Our findings are consistent with previous reports of increased axial length after monocular deprivation deprivation but do not support the concept of enhanced vernier acuity in the nondeprived eye of monocularly deprived humans.

Adult↗
PubMed1980

Saccadic eye movements and body sway.

Different conditions of moving retinal images show differential influences on postural stability. A surrounding pattern moved during steady fixation increases body sway, but similar image motions generated by voluntary saccades do not. Mechanisms for postural control do not respond to visual feedback during saccades. Analogous principles in saccadic suppression and space constancy are discussed.

Biofeedback, Psychology↗
PubMed1980

Independence of radial localization from refractive error.

Localization of single stimuli presented individually on the circumference of an imaginary circle was determined with and without three diopters of refractive error introduced by spherical lenses. The presence of blur had no effect on localization accuracy.

Humans↗
PubMed1979

Correction of empty-field myopia on the basis of the dark-focus of accommodation.

The influence of negative spherical corrections on the detection of a small increment flash presented in a uniform field or Ganzfeld were determined. The range investigated included each observer's normal correction as well as values based on the focus assumed by each observer in total darkness (dark-focus of accomodation). A correction equivalent to the dark-focus resulted in maximum sensitivity. The results suggest a convenient technique for the determination of optimal correction for empty-field viewing conditions.

Accommodation, Ocular↗
PubMed1978

The effect of refractive error on size constancy and shape constancy.

The effect of refractive error was determined for both size constancy and shape constancy. Although an error of 1.5 diopters reduces shape constancy, 3.0 diopters of blur had no effect on size constancy. The results are discussed in terms of the multiple mechanisms subserving the perceptual constancies. The importance of cues from the peripheral visual fields for size constancy and the role of foveally mediated texture and stereopsis cues for shape constancy are emphasized.

Adult↗
PubMedDate not supplied

The effect of alcohol on the vestibulo-ocular reflex and apparent concomitant motion.

Measures of the vestibulo-ocular reflex (VOR) and apparent concomitant motion (ACM), the apparent motion of stationary visual stimuli during head motion, were obtained during sessions in which subjects consumed either alcoholic or placebo beverages. Both VOR and ACM remained constant throughout the placebo sessions. During sessions in which alcohol was consumed, the gain of the VOR was reduced and there was an increased tendency to see ACM in the direction opposite head motion. The effects of alcohol on ACM were greatest 50 min following cessation of drinking, near the maximum of the blood alcohol concentration (BAC). ACM measures then returned towards baseline, whereas the BAC measures remained elevated. VOR gain alterations paralleled the BAC measures. Therefore, reduction of VOR gain with alcohol produces an increase of ACM, but the ACM changes are relatively transient when compared with the altered VOR gain.

Adult↗