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Biomedical subjects

W Jaschinski-Kruza

Publications and source records attributed to W Jaschinski-Kruza.

12 recordsLinked to original sources
PubMed1994

Dark vergence in relation to fixation disparity at different luminance and blur levels.

Fixation disparity was measured psychophysically when subjects viewed a central fusion stimulus at 50 cm distance. Reducing luminance and increasing target blur resulted in an exophoric shift of fixation disparity. Fixation disparity was significantly more exophoric in subjects with distant dark vergence, i.e. the resting position of vergence. This was true for the bright and sharp target, as well as for targets impaired by blurring or dimming. These results are interpreted as a bias of fixation disparity towards the individual resting position of vergence not only for degraded stimuli but also for strong central fusion stimuli.

Accommodation, Ocular↗
PubMed1993

Fixation disparity at different viewing distances of a visual display unit.

Fixation disparity was measured while subjects observed a realistic alphanumeric pattern on the cathode ray tube (CRT) of a visual display unit at viewing distances of 25-85 cm. The psychometric function of fixation disparity was determined by analysing the observers' responses to a series of short (100 ms) flashes of nonius targets that were superimposed on the CRT-image by means of a half-silvered mirror. Test-retest correlation of fixation disparity was high (r = 0.8) at viewing distances of 40 and 82 cm and lower (r = 0.55) at the shorter distance of 26.5 cm. Mean fixation disparity increased to a more exophoric condition as the display approached the eyes. However, this effect was observed only in some of the subjects, so the susceptibility of fixation disparity to viewing distance appears to be an individual characteristic. When subjects observed the VDU screen continuously for 30 min at viewing distances of 85, 47, 31 and 25 cm in different sessions, fixation disparity and accommodation remained unchanged over time.

Accommodation, Ocular↗
PubMed1992

Variability of fixation disparity and accommodation when viewing a CRT visual display unit.

Fixation disparity and accommodation were measured while subjects viewed a target on a cathode ray tube at a 50 cm viewing distance. The psychophysical method of sequential testing was used. For measuring fixation disparity a series of brief dichoptic nonius targets was presented and the direction of perceived misalignment was reported by the subject. From these responses the psychometric function of fixation disparity can be derived which gives not only the mean fixation disparity but also the range of fluctuation during the period of measurement. During a 5 min period, this range was +/- 1.5 min arc averaged over subjects, when expressed as the range in which instantaneous fixation disparity fell with a probability of 68%. Repeated measurements on 5 days within 2 weeks gave a larger range of +/- 2 min arc. However, individual results can differ considerably from these average values. Repeated measurements and the analysis of the psychometric function can provide a more complete description of individual binocular status than is obtained with a single adjustment of continuously visible nonius targets. Laser optometry revealed that, compared to the stimulus distance of 50 cm, accommodation lagged behind in the range 1-2 D. The two display modes of bright characters on dark background and vice versa gave very similar results.

Accommodation, Ocular↗
PubMed1991

On proximal effects in objective and subjective testing of dark accommodation.

This study investigated to what extent objective and subjective measurements of accommodation in darkness are affected by instrumentation or by flashed light stimuli that are arranged close to the subject's eye, and by the instructions given to the subjects. Subjects differed markedly in their susceptibility to these factors. Dark accommodation measured with an infra-red autorefractometer tended to be more distant by 0.2 D on the average if the autorefractometer allowed a free view into a large, but dark, room compared with a closed apparatus. When subjects were instructed to look into far or near distances during objective measurements, mean dark accommodation was more distant by 0.15 D and closer by 1.4 D, respectively, relative to the condition of 'relaxed eyes'. To find a simple screening test of dark accommodation, a 'number test' was investigated: subjects had to report which was the most sharply visible among a series of numbers that were simultaneously flashed at different optical distances in a Badal optometer. Objective measurements revealed that this test induced near shifts of 0.37 D on the average, relative to accommodation in a dark surround. Therefore, the results of the number test were only moderately correlated to objective measurements.

Accommodation, Ocular↗
PubMed1991

Eyestrain in VDU users: viewing distance and the resting position of ocular muscles.

Without fixation stimuli--for instance, in darkness--accommodation and convergence of the eyes reach an intermediate state that is relatively stable within a subject but which ranges from about 30 cm to infinity across subjects. This study investigated whether these oculomotor positions in darkness are related to visual strain. During work at a visual display unit (VDU), visual strain at a viewing distance of 50 cm was stronger than at 100 cm (with characters twice as large). The more distant the individual's dark convergence, the greater was the visual strain at the 50-cm viewing distance. When subjects were free to adjust the viewing distance to their comfort level, they chose distances between 51 and 99 cm (mean 74 cm) with characters 5 mm tall.

Accommodation, Ocular↗
PubMed1990

Effects of stimulus distance on measurements of dark convergence.

Dark convergence is often measured psychophysically by aligning two dichoptic stimuli, which are briefly flashed in a dark surround. In the present study, the effect of different stimulus parameters on dark convergence measurements was investigated. Chief among these was the effect of viewing distance (50, 100 and 500 cm) of the stimulus. Measured dark convergence was biased towards the actual viewing distance of the stimuli; this effect was stronger with bar stimuli than with point-and-line stimuli. Measured dark convergence depended on the viewing distance to a lesser extent if bar stimuli were observed through pin-holes (open accommodative feedback loop). The subject's knowledge of the viewing distance had a small but significant effect; the size of the nonius bars had none. To reduce artefacts due to the stimulus distance on psychophysical measurement of dark convergence, point-and-line stimuli should be used at a viewing distance of about 1 m. In this case, the effect of stimulus distance can be about 0.25 meter angle (ma), averaged over subjects, but it may be as large as 0.5 ms in some individuals.

Adolescent↗
PubMed1988

A hand optometer for measuring dark focus.

To measure the accommodative state of the eye in a stimulus free condition, the so called dark focus, a simple optometer was constructed based on the principle of the polarized vernier optometer originally proposed by Moses (1971). Data obtained with this hand optometer were compared with those of two other subjective techniques, the polarized vernier optometer and the laser optometer, we were found to be similar. The advantage of this hand optometer is that it is a convenient device that allows one to measure the dark focus within minutes, under normal room lighting.

Accommodation, Ocular↗
PubMed1988

Effect of a mental arithmetic task on dark focus of accommodation.

Three subjective techniques for measuring accommodation (polarized vernier optometer, laser optometer, and a hand optometer based on the vernier principle) were used to measure subjects' dark focus while they performed a short-term mental arithmetic task. Compared with the initial values measured without the arithmetic task, significant changes in dark focus occurred when the task was performed. The effects depended on the initial dark focus of the subjects: in far-dark-focus subjects, inward shifts were generally observed; in near-dark-focus subjects, the dark focus shifted in different directions when different measuring techniques were used. The individual changes in dark focus due to the mental task were reproducible when the mean dark focus of the group shifted inward and when the same technique for measuring accommodation was used. The results have implications for the interpretation of dark focus shifts that are found following prolonged visual work at close distances, e.g. at visual display terminals.

Accommodation, Ocular↗
PubMed1984

A multiple-channel model for grating detection.

The multiple-channel model that was proposed in 1974 by MacLeod and Rosenfeld to describe the sensitivity of the visual system to complex gratings has been applied to a variety of complex gratings. The model predicts correctly the contrast sensitivity at both high and low spatial frequencies to square-wave gratings, square-wave with missing fundamental, sawtooth-wave, and trapezoid-wave gratings of different ramp widths. It provides a physiologically reasonable explanation of the detection of luminance gradients without requiring special gradient detectors. The model resembles the threshold behavior of the visual system in that it is sensitive to the relative phase of frequency components at low frequencies, but not at high frequencies.

Form Perception↗