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945 records · Page 53Linked to original sources
PubMedDate not supplied

Psychophysical scaling of circular vection (CV) produced by optokinetic (OKN) motion: individual differences and effects of practice.

Vection (V) refers to the compelling visual illusion of self-motion experienced by stationary individuals when viewing moving visual surrounds. The phenomenon is of theoretical interest because of its relevance for understanding the neural basis of ordinary self-motion perception, and of practical importance because it is the experience that makes simulation, virtual reality displays, and entertainment devices more vicarious. This experiment was performed to address whether an optokinetically induced vection illusion exhibits monotonic and stable psychometric properties and whether individuals differ reliably in these (V) perceptions. Subjects were exposed to varying velocities of the circular vection (CV) display in an optokinetic (OKN) drum 2 meters in diameter in 5 one-hour daily sessions extending over a 1 week period. For grouped data, psychophysical scalings of velocity estimates showed that exponents in a Stevens' type power function were essentially linear (slope = 0.95) and largely stable over sessions. Latencies were slightly longer for the slowest and fastest induction stimuli, and the trend over sessions for average latency was longer as a function of practice implying time course adaptation effects. Test-retest reliabilities for individual slope and intercept measures were moderately strong (r = 0.45) and showed no evidence of superdiagonal form. This implies stability of the individual circularvection (CV) sensitivities. Because the individual CV scores were stable, reliabilities were improved by averaging 4 sessions in order to provide a stronger retest reliability (r = 0.80). Individual latency responses were highly reliable (r = 0.80). Mean CV latency and motion sickness symptoms were greater in males than in females. These individual differences in CV could be predictive of other outcomes, such as susceptibility to disorientation or motion sickness, and for CNS localization of visual-vestibular interactions in the experience of self-motion.

Afterimage↗
PubMed1996

Voluntary suppression of caloric nystagmus under fixation of imaginary or after-image target.

Voluntary suppression of caloric nystagmus was investigated in 10 normal subjects under the following three conditions; i) with visible, earth-fixed target in the dark ("visual"), ii) with imaginary target in the dark ("imaginary"), and iii) an after-image on the fovea in the dark ("after-image") Subjects were able to suppress caloric nystagmus with a visible target very well, but they could only partially suppress caloric nystagmus with imaginary target and with after-image. The degree of suppression in the "imaginary" and "after-image" conditions was significantly less than that in "visual", but was not zero. These results are consistent with those of many reports about visual-vestibular interaction and VOR voluntary modification during rotation, and suggest that smooth pursuit eye movements (SPEM) system plays a major role in visual suppression during caloric nystagmus.

Adult↗
PubMedDate not supplied

Three-dimensional baselines for perceived self-motion during acceleration and deceleration in a centrifuge.

Three-dimensional motion trajectories were computed, representing the motions that would be perceived by a perfect processor of acceleration information during the acceleration and deceleration stages of a centrifuge run. These motions serve as "baselines" for perceived self-motion in a centrifuge, and depend on the initial perception of orientation and velocity immediately preceding the acceleration and immediately preceding the deceleration. The baselines show that a perfect processor of acceleration information perceives self-motion during centrifuge deceleration significantly differently from self-motion during centrifuge acceleration, despite the fact that the angular accelerations have equal magnitude (with opposite direction). At the same time, the baselines can be compared with subjects' reported perceptions to highlight limitations of the nervous system; limitations and peculiarities of the nervous system are identified as deviations from a baseline. As a result, peculiarities of the nervous system are held responsible for any perception of pitch or roll angular velocity or change in tilt of the body-horizontal plane of motion during the centrifuge run. On the other hand, baselines explain perception of tilt position during deceleration, linear velocity, possible lack of significant linear velocity during deceleration, and yaw angular velocity, including on-axis angular velocity during centrifuge deceleration. The results lead to several experimental questions.

Acceleration↗
PubMed1977

The diagnosis of amblyopia and strabismus: a programmed approach.

A simplified method of recording and analyzing amblyopic and strabismic examinations is presented. With this new form, the average practitioner is provided with a relatively standardized set of tests in a logical, organized sequence. Programming the patient's responses into meaningful categories facilitates the formation of a differential diagnosis and also provides valuable insight into the eventual outcome of each case.

Adaptation, Ocular↗
PubMed1997

Laboratory simulator and field pursuit tracking performance with females and males in the presence of laser glare.

BACKGROUND: The developments in laser technology have increased the precision of many tasks and has made the presence of lasers common-place. In the military the pervasive use of laser devices in uncontrolled environments enhances the potential for human exposure. The visual disruption experienced during these exposures could lead to serious injury or disruption of performance. Characterization of changes in visual-motor performance of military personnel exposed to safe levels of laser glare assists in minimizing mission performance decrements. METHODS: There were 18 female and male military personnel who performed a tracking task in the field and in the laboratory. Two systems were used to assess possible gender differences inherent to the operation of each unit. There were six, 3-s laser trials presented at an irradiance of 400 microW.cm-2 during 15 bright light and 15 simulated dawn/dusk trials with each system. The laser beam on the retina was collinear with the image on the sight. Maximum absolute error (MAE) and total time-off-target (TTOT) scores were determined. RESULTS: Analysis showed that after the flash females tended to lead and males lagged behind the target. No significant differences in MAE or TTOT scores attributable to gender were found. Dawn/dusk flash trials produced greater disruption of pursuit tracking than did bright light trials. Repeated flash exposures showed either an adaptive or a cumulative response. CONCLUSIONS: Significant visual disruption was found following exposure to "safe" levels of laser light and this effect was increased during simulated dawn/dusk conditions. The degree of performance decrement was not related to gender.

Adult↗
PubMedDate not supplied

Further evidence for gender differences in circularvection.

This paper reports further evidence that gender is a significant factor in the experience of circularvection (CV), the illusion of self-rotation. Using optokinetic drum velocities between 24 degrees and 92 degrees/s, latency to experience Stage 2 or Stage 3 CV was measured. Males exhibited significantly longer CV latencies than females (P < 0.0001), although the difference was greater for Stage 3 CV than for Stage 2 CV. This result suggests that the potential influence of gender must be carefully controlled in visual-vestibular interaction experiments.

Adult↗
PubMed1998

Myopes show increased susceptibility to nearwork aftereffects.

PURPOSE: Some aspects of accommodation may be slightly abnormal (or different) in myopes, compared with accommodation in emmetropes and hyperopes. For example, the initial magnitude of accommodative adaptation in the dark after nearwork is greatest in myopes. However, the critical test is to assess this initial accommodative aftereffect and its subsequent decay in the light under more natural viewing conditions with blur-related visual feedback present, if a possible link between this phenomenon and clinical myopia is to be considered. METHODS: Subjects consisted of adult late- (n = 11) and early-onset (n = 13) myopes, emmetropes (n = 11), and hyperopes (n = 9). The distance-refractive state was assessed objectively using an autorefractor immediately before and after a 10-minute binocular near task at 20 cm (5 diopters [D]). RESULTS: Group results showed that myopes were most susceptible to the nearwork aftereffect. It averaged 0.35 D in initial magnitude, with considerably faster posttask decay to baseline in the early-onset (35 seconds) versus late-onset (63 seconds) myopes. There was no myopic aftereffect in the remaining two refractive groups. CONCLUSIONS: The myopes showed particularly striking accommodatively related nearwork aftereffect susceptibility. As has been speculated and found by many others, transient pseudomyopia may cause or be a precursor to permanent myopia or myopic progression. Time-integrated increased retinal defocus causing axial elongation is proposed as a possible mechanism.

Accommodation, Ocular↗