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Biomedical subjects

L B Stelmach

Publications and source records attributed to L B Stelmach.

13 recordsLinked to original sources
PubMed2001

Monovision: a review of the scientific literature.

We reviewed the scientific literature on monovision to compare the visual performance of monovision patients with that of others wearing more traditional prescriptions. We found that visual performance of monovision patients was comparable to that of control patients wearing a balanced binocular correction, provided that reading adds were not greater than about +2.5 D, that illumination was photopic, and that stimuli were presented at supra-threshold levels. Under these conditions, monovision patients were satisfied with their perceptual experience and performed within 2 to 6% of balanced binocular control patients on a range of occupational tasks. It is noteworthy that monovision patients had relatively more difficulty with acuity-based tasks than with tasks demanding good depth perception. With reading adds over +2.5 D, at low levels of illumination, or with near-threshold level stimuli, visual performance of monovision patients was reduced compared with controls. Subjectively, under low levels of illumination, monovision patients experienced problems with glare and halos around point sources of light.

Contact Lenses↗
PubMed2001

Unequal weighting of monocular inputs in binocular combination: implications for the compression of stereoscopic imagery.

For efficient storage and transmission of stereoscopic images over bandwidth-limited channels, compression can be achieved by degrading 1 monocular input of a stereo pair and maintaining the other at the desired quality. The desired quality of the fused stereoscopic image can be achieved, provided that binocular vision assigns greater weight to the nondegraded input. A psychophysical matching procedure was used to determine if such over-weighting occurred when the monocular degradation included blur or blocking artifacts. Over-weighting of the nondegraded input occurred for blur, but under-weighting of the nondegraded input occurred for blockiness. Some participants exhibited ocular dominance, but this did not affect the blur results. The authors conclude that blur, but not blockiness, is an acceptable form of monocular degradation.

Depth Perception↗
PubMed1999

Control' of reflexive and voluntary saccades in the gap effect.

In two experiments, we examined whether voluntary and reflexive saccades shared a common fixation disengagement mechanism. Participants were required to perform a variety of tasks, each requiring a different level of information processing of the display prior to execution of the saccade. In Experiment 1, participants executed either a prosaccade or an antisaccade upon detecting a stimulus array. In Experiment 2, participants executed a prosaccade to a stimulus array only if the array contained a target item. The target could be a line (easy search) or a digit (difficult search). The critical manipulation in both experiments was the relative timing between the removal of the fixation stimulus and the onset of the stimulus array. In both experiments, it was found that saccadic latencies were shortest when the fixation stimulus was removed before the onset of the stimulus array--a gap effect. It was concluded that reflexive and voluntary saccades share a common fixation disengagement mechanism that is largely independent of higher level cognitive processes.

Cognition↗
PubMed1998

Display duration and stereoscopic depth discrimination.

We investigated the role of display duration in stereoscopic depth perception. The display consisted of a dynamic random-dot stereogram, with two disparity-defined squares (1.9 degrees x 1.9 degrees), one on the left and one on the right of a central (Nonius) fixation stimulus. The sign of the disparity (crossed or uncrossed) was always the same for both squares, and the magnitude of disparity was 0.25 degree for one square and either 0.125 degree or 0.375 degree for the other square. Participants indicated which square appeared closer. The display duration was varied adaptively between 20 and 1000 ms until participants performed at 75% accuracy. Results confirmed large individual differences in the display duration required for stereoscopic depth perception. Approximately half of the 100 naive participants were able to perform the task at 20 ms, while the remaining participants required up to 1000 ms to perform at criterion. The present study shows that display duration is a critical variable in explaining wide differences in reported abilities of individuals to process stereoscopic depth information.

Adult↗
PubMed1993

Viewing behavior: ocular and attentional disengagement.

In five experiments, we examined the role of the ocular and attentional systems in determining saccadic latencies. Prior to making a saccade to a target stimulus, subjects were required to direct their attention to a foveal stimulus or to an eccentric stimulus. Either stimulus could be extinguished before the onset of the target. Saccadic latencies were shortest when the foveal stimulus was extinguished, regardless of whether it was attended or not. Control experiments showed that subjects were able to attend properly and that warning, arising from turning off a stimulus before target onset, could not completely account for the results. The results were discussed in terms of ocular disengagement, attentional disengagement, and joint ocular-attentional disengagement. It was concluded that an explanation emphasizing ocular disengagement provided the best account of the data.

Adult↗
PubMed1991

Directed attention and perception of temporal order.

The present research examined the effects of directed attention on speed of information transmission in the visual system. Ss judged the temporal order of 2 stimuli while directing attention toward 1 of the stimuli or away from both stimuli. Perception of temporal order was influenced by directed attention: Given equal onset times, the attended stimulus appeared to occur before the unattended stimulus. Direction of attention also influenced the perception of simultaneity. The findings support the notion that attention affects the speed of transmission of information in the visual system. To account for the pattern of temporal order and simultaneity judgments, a model is proposed in which the temporal profile of visual responses is affected by directed attention.

Adult↗
PubMed1987

On and off systems in human vision.

Three experiments examined the interaction of On and Off responses that were produced by sudden increments and decrements in luminance. All three experiments utilized a masking technique that required observers to detect a signal in a masking field. The mask was produced by brightening or dimming a field of dots, and the signal consisted of the addition or subtraction of a dot. Experiment 1 showed that detection of the signal-dot was more difficult when the luminance of the signal and the mask changed in the same direction (e.g. a new dot added to the field of dots that were being brightened) than when luminance changed in different directions. When the amplitude of signal and mask was varied parametrically (Experiments 2 and 3), accuracy increased with the ratio of amplitudes of signal and mask. But at any given ratio, the signal was more difficult to detect when signal and mask were of the same sign. The greater difficulty encountered in detecting a signal in the context of a "like" mask is ascribed to greater interference between signal and mask when they share the channel.

Humans↗
PubMed1986

Two-pulse temporal resolution in patients with glaucoma, suspected glaucoma, and in normal observers.

We examined the potential usefulness of a two-pulse temporal resolution technique in assessing early glaucomatous damage. Patients with glaucoma and suspected glaucoma and normal observers were studied. The technique produced a high sensitivity and specificity in separating glaucomatous from normal eyes and identified a subgroup of patients with suspected glaucoma for follow-up.

Differential Threshold↗
PubMed1985

Equating visibility of brief decrements: unconfounding duration and luminance.

A common procedure in visual psychophysics involves equating the visual effectiveness of brief luminous displays. It may be equally important to equate the effectiveness of brief interruptions, as when two displays are presented sequentially, separated by a variable interstimulus interval (ISI). For example, in a procedure devised by Phillips and Singer [Expl. Brain Res. 19, 493-506 (1974)], the first display consisted of a random pattern of dots and the second display consisted of the same pattern, but with one added dot. Detectability of the added dot was presumed to be determined by interactions of transient neural events produced at the beginning and end of the ISI. Lengthening the ISI was believed to weaken progressively the magnitude of the neural interactions, resulting in poorer performance. But lengthening the ISI also increased its visual effectiveness (darkness). Using ISIs equated in visual effectiveness for durations from 10 to 320 msec, we found that the visual effectiveness of the interval, not its duration, was the prime determinant of performance. This finding requires a reinterpretation of the neural mechanisms being studied in the Phillips and Singer task.

Female↗
PubMed1984

Does rate of processing determine ease of target detection?

In a complex target identification task, Krumhansl (1982) reported that ease of target detection was inversely related to the exposure duration of contextual elements in the display. This relationship held at brief exposures of the contextual elements. However, in Krumhansl's design, brightness and duration covaried. Hence, the results cannot be ascribed unambiguously to the effect of duration alone. In the present two experiments, brightness and duration were varied independently. The outcome showed that brightness differences were both a necessary and a sufficient condition for producing the relationships observed by Krumhansl. In fact, when brightness was equalized across all durations the relationship was reversed. Although unable to account for the effects of duration, Krumhansl's formal model remains viable as a predictor of the effects of brightness.

Attention↗