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Biomedical subjects

R B Post

Publications and source records attributed to R B Post.

At least 37 records · Page 2Linked to original sources
PubMed1991

Induced motion of a fixated target: influence of voluntary eye deviation.

Induced motion (IM) was observed in a fixated target in the direction opposite to the real motion of a moving background. Relative to a fixation target located straight ahead, IM decreased when fixation was deviated 10 degrees in the same direction as background motion and increased when fixation was deviated 10 degrees opposite background motion. These results are consistent with a "nystagmus-suppression" hypothesis for subjective motion of fixated targets: the magnitude of illusory motion is correlated with the amount of voluntary efference required to oppose involuntary eye movements that would occur in the absence of fixation. In addition to the form of IM studied, this explanation applies to autokinesis, apparent concomitant motion, and the oculogyral illusion. Accounts of IM that stress visual capture of vection, afferent mechanisms, egocenter deviations, or phenomenological principles, although they may explain some forms of IM, do not account for the present results.

Adult↗
PubMed1991

Observations on the adaptation of induced motion.

Induced motion (IM) was measured before and after a 10-min adaptation period during which subjects viewed the IM display without judging IM magnitude. The inducing stimulus was a rectangle, which contains both horizontal and vertical reference detail. The magnitude of IM was significantly lower following the adaptation period. This result is inconsistent with the hypothesis that adaptation of IM represents an instance of perceptual learning wherein the contribution of relative motion to motion perception is reduced. In a separate study, similar results were obtained when the inducing stimulus was a single vertical bar presented either to the left or to the right of the fixation stimulus. In addition, adaptation was obtained when the location of the inducing bar was changed during test measures, demonstrating that this effect is not specific to the retinal locus of the adaptation stimulus.

Adult↗
PubMed1990

Relationship of induced motion and apparent straight-ahead shifts to optokinetic stimulus velocity.

Induced motion (IM) of a fixated spot stimulus and shifts of the apparent straight-ahead (ASA) from the objective median plane were studied as a function of the velocity of a full-field optokinetic background stimulus. Both IM and ASA were influenced similarly by changes in stimulus velocity. The magnitude of both responses, averaged across subjects, increased to a peak level with background velocities of 40-80 deg/sec and decreased at higher velocities. Individual subjects differed with respect to the precise functions by which IM and ASA shifts were related to stimulus velocity. However, for individual subjects, the effects of velocity on IM and ASA shifts were typically highly correlated. Although IM is correlated with shifts of ASA in the opposite direction, the magnitude of the ASA shift is insufficient to account for the observed IM.

Adult↗
PubMed1989

A reevaluation of the effect of velocity on induced motion.

Induced motion (IM) was measured as a function of the temporal frequency of inducer oscillation. IM magnitude decreased as frequency increased above .5 Hz. Increasing the amplitude of inducer motion, and thereby its velocity, did not influence the temporal frequency dependence of IM. This suggests that it is the duration of inducer motion, rather than its velocity, that is the critical stimulus feature in studies that report decreased IM with higher frequencies of inducer oscillation. In a separate experiment, the optokinetic nystagmus elicited by the inducing stimulus in the absence of a fixation target displayed frequency-response characteristics similar to those of IM. This finding supports the hypothesis that IM magnitude is proportional to the voluntary effort required to suppress reflexive eye movements while maintaining stable fixation.

Acceleration↗
PubMed1988

Induced motion and optokinetic afternystagmus: parallel response dynamics with prolonged stimulation.

Fixation of a stationary target during motion of background contours attenuates optokinetic nystagmus (OKN), while illusory induced motion (IM) of the fixated target occurs opposite the direction of contour motion. It is proposed that IM owes to a perceptually registered efferent signal for ocular pursuit which opposes an unregistered signal for OKN to achieve stable fixation. This proposal predicts parallel changes in the magnitudes of IM and optokinetic reflexes during and after optokinetic stimulation. Accordingly, leftward IM magnitude and rightward slow-phase velocity of optokinetic afternystagmus (OKAN) increased at similar rates across 90 and 160 sec of 60 deg/sec motion of background contours, and decayed at similar rates after stimulus termination. Both responses decayed more deeply following stimulation with, rather than without fixation. Neither retinal image motion nor vection can explain the IM obtained.

Adult↗
PubMed1988

Circular vection is independent of stimulus eccentricity.

The sensation of self-rotation induced by viewing a surround rotating about the observer's vertical axis (circular vection or CV) was investigated with equal-area stimuli located in either the central, the mid-peripheral, or the far-peripheral visual field. Magnitude estimation responses indicated greater CV with larger stimulus area, but no significant differences in CV sensations as a function of stimulus eccentricity. This pattern of results does not support the belief that CV is dominated by peripheral stimulation when equal-area stimuli are compared.

Adolescent↗
PubMed1987

Perceived path of oblique motion: horizontal-vertical and stimulus-orientation effects.

Subjects adjusted the path of moving stimuli to produce apparent slopes of 45 degrees with respect to horizontal. The stimulus was either a single moving dot or a vertical or horizontal bar. In separate experiments either the stimuli were tracked or fixation was maintained on a stationary fixation target positioned 8 deg to the right of the center of stimulus motion. In both experiments the selected path slopes were in general more horizontal than 45 degrees. This pattern indicates that subjects overestimate the vertical component of motion along an oblique path, and is interpreted as a manifestation of the spatial anisometropy generally termed the 'horizontal-vertical illusion'. Additionally, paths selected for horizontal bars were more vertical than those for vertical bars. This finding is interpreted in the context of a previous report of the influence of stimulus orientation on perceived velocity.

Adolescent↗
PubMed1986

Induced motion considered as a visually induced oculogyral illusion.

The possibility that nystagmus suppression contributes to illusory motion was investigated by measuring perceived motion of a stationary stimulus following the removal of an optokinetic stimulus. This was done because optokinetic nystagmus typically outlasts cessation of an optokinetic stimulus. Therefore, it would be expected that a stationary fixated stimulus should appear to move after removal of an optokinetic stimulus if illusory motion results from nystagmus suppression. Illusory motion was reported for a stationary fixation target following optokinetic stimulation. This motion was reported first in the same direction as the preceding induced motion, then in the opposite direction. The two directions of illusory motion following optokinetic stimulation are interpreted as resulting from the use of smooth ocular pursuit to suppress first one phase of optokinetic after nystagmus and then the reverse phase. Implications for the origins of induced motion are discussed.

Fixation, Ocular↗
PubMed1986

Two modes of processing visual information: implications for assessing visual impairment.

The recognition of visual detail and the ability to orient spatially to visual stimuli are apparently subserved by distinct modes of processing, termed "focal" and "ambient," respectively. Current clinical techniques are well suited to evaluate aspects of focal vision, but do little to evaluate ambient visual functions. As a result, the impairment of ambient functions may be difficult to specify or may be overestimated. Implications for the development of assessment techniques for ambient functions are discussed.

Humans↗
PubMed1986

Motion sensitivity in central and peripheral vision.

The visual resolution of motion is important both for indicating the presence of moving objects and for assisting locomotion and spatial orientation. Additionally, motion may enhance the visibility of low contrast features of the environment. For patients with reduced central vision, these functions may be especially significant as the detection of motion is superior to the ability to resolve stationary detail in peripheral vision. The present paper analyzes the effects of specific stimulus variables on motion sensitivity in both central and peripheral vision. Implications of these findings for low vision patients are discussed.

Humans↗
PubMed1985

A revised analysis of the role of efference in motion perception.

The analysis of motion perception historically has included efferent as well as afferent mechanisms to account for the perception of motion during eye movement. The application of efferent mechanisms to motion perception has been limited, however, by several illusions which are apparently inconsistent with the notion that oculomotor mechanisms contribute to motion perception. An alternative account is presented of the manner in which efference may contribute to the perception of motion. It is proposed that distinct smooth eye-movement systems contribute differentially to object motion perception. Specifically, activity in the smooth pursuit system gives rise to the perception of object motion, whereas activity in the smooth component of reflexive eye movements does not. Tracking of a moving object results in object motion perception as a result of efference in the pursuit system. However, the pursuit system may be activated to oppose the smooth component of reflexive eye movements in order to preserve fixation on a stationary object. In such cases neither the fixated object nor the eye is moving but illusory movement results from the efference in the pursuit system. A number of illusory movement phenomena are interpreted in terms of this model.

Eye Movements↗
PubMed1985

Does performance of tasks affect the resting focus of accommodation?

Measurements of the resting focus of accommodation were obtained with an infrared optometer while subjects engaged in four tasks: (1) passive viewing in darkness; (2) passive viewing of a laser speckle pattern; (3) active judgment of motion in a laser speckle pattern; and (4) mental arithmetic in darkness. The results showed resting focus measures to be comparable under all four conditions for some subjects. However, other subjects exhibited consistent increases or decreases in the resting focus position when asked to judge the direction of speckles within the laser test pattern or perform mental arithmetic. This finding suggests that "effort to see" may influence the resting focus of some subjects. Implications for measurement procedures and population estimates of the resting focus are discussed.

Accommodation, Ocular↗
PubMed1985

The California syndrome. Functional visual complaints with potential economic impact.

The records of 84 patients with functional visual loss (59 adults and 25 children) were reviewed over a six-year period. The patients were between 7 and 69 years of age. In the young patients, psychosocial problems relating to parental divorce, poor school performance and attention-getting behavior were common, while in adult patients, financial gain was present in 51 of 59 (86%) individuals. Over 90% of the claimed abnormalities related to the afferent visual system. Fifty-three percent of the patients had underlying organic problems with functional overlay. Inconsistent test results were the most reliable means of identifying the malingering patients. In California, both ophthalmologists and optometrists are able to declare a patient legally blind as well as visually disabled for the purpose of income tax, workers' compensation, and disability benefits. The potential financial impact of malingering patients is discussed.

Adolescent↗
PubMed1984

Correlation of color vision deficits and observable changes in the optic disc in a population of ocular hypertensives.

Both glaucomatous cupping and the presence of acquired color vision deficits have been reported to be precursors to the onset of visual field defects in patients with suspected glaucoma. To examine the relationship between early glaucomatous cupping and acquired color vision anomalies, we performed anomaloscope (Pickford-Nicholson) and Farnsworth-Munsell 100-Hue color vision tests in 48 ocular hypertensive eyes with either clinical evidence of early glaucomatous cupping (group 1) or no evidence of glaucomatous cupping (group 2). All patients had normal visual fields, as determined by extensive static perimetry of the central visual field and kinetic perimetry of the peripheral visual field. Although the overall incidence of blue and blue-green color vision anomalies in the ocular hypertensives was comparable with that reported in previous studies, we found no clear association between early glaucomatous cupping and color vision anomalies. The relationship between these two precursors to visual field loss remains unclear.

Adult↗