Biomedical subjects

Lawrence R Stark

Publications and source records attributed to Lawrence R Stark.

3 recordsLinked to original sources

PubMed2002

Accommodation to simulations of defocus and chromatic aberration in the presence of chromatic misalignment.

Previous studies have demonstrated that accommodation will respond to sine gratings in which the relative modulations of red, green and blue image components have been altered to simulate the effects of defocus and longitudinal chromatic aberration. The present study aimed to determine the tolerance of the accommodative system to relative phase shifts in those components induced by chromatic misalignment. It was found that accommodation can tolerate moderate amounts of chromatic misalignment (6'), but responds adversely when misalignments are large. Applications to visual display terminals and spectacle lens and instrument design are discussed.

Accommodation, Ocular↗

PubMed2002

Monocular accommodation response for totally occluded objects.

PURPOSE: To investigate the accuracy and stability of the monocular accommodation response when attention is directed toward a totally occluded distant object. METHODS: A static infrared optometer was used to measure the monocular accommodative response in 16 participants while they viewed a distant letter target in the presence or absence of an intervening object that totally occluded the distant target from view. Participants were instructed to attend to and focus for the distant target. RESULTS: Participants differed widely in their accommodative responses for a totally occluded distant object, focusing close to the distant object, close to the intervening object, in between the two objects, and beyond the distant object. Some of this variability could be attributed to individual differences in the dark focus. Accommodation was more variable when the distant target was totally occluded by an obstruction at an intermediate distance (50 cm). CONCLUSIONS: Many individuals focus poorly in the presence of a visual obstruction that totally occludes the object of regard.

Accommodation, Ocular↗

PubMedDate not supplied

Effect of an intervening screen on accommodation to a distant object.

Background: An intervening screen has been suggested to induce an inward shift of accommodation when viewing a distant object. This is an example of the Mandelbaum effect. However, there have been no objective measures of the magnitude of this effect in this particular situation. Methods: Accommodation was recorded with an infra-red optometer, while subjects (n = 16) viewed a distant letter target with or without an intervening screen. Screens were placed near the individual dark focus distance or at 50 cm. In a second experiment the contrast of the distant target was varied and subjects (n = 5) viewed the target directly or through a screen placed near the individual dark focus distance. Results: In the main experiment, the Mandelbaum effect was not significantly different from zero and was less than 0.5 D in every subject. In addition, accommodation was not more variable when viewing through the screen. However, it may be that some subjects do demonstrate a Mandelbaum effect while others do not. The individual dark focus level did not predict susceptibility to the Mandelbaum effect for a screen at the dark focus. Subjects reported their perceptions of the tasks and some noted changes in the perceived distances of objects when viewing through a screen. In the second experiment, the Mandelbaum effect (< 0.6 D) did not vary with distant target contrast. Conclusions: When viewing a distant object through a screen there is a small (< 0.6 D) or negligible inward shift of accommodation.

Journal Article↗