PubMed Health⌕ Search

Biomedical subjects

A L Duwaer

Publications and source records attributed to A L Duwaer.

11 recordsLinked to original sources
PubMed1983

New measures of fixation disparity in the diagnosis of binocular oculomotor deficiencies.

Two new measures of fixation disparity have been successfully used to evaluate the occurrence of binocular oculomotor deficiencies under (normal) viewing conditions with no interposed prisms. They are the threshold for the perception of misalignment between briefly presented dichoptic nonius lines and the SD of this threshold. The effectiveness of these new measures of fixation disparity for diagnosing binocular oculomotor deficiencies, which are symptomatically manifested as asthenopia, was found to surpass the diagnostic effectiveness of two measures of fixation disparity in current use: the mean amount of fixation disparity at the point where there are no prisms before the eyes and the slope of the fixation disparity curve near this point. The high diagnostic effectiveness of the new measures of fixation disparity, which are related to the spread of the fixation disparity, suggests that the binocular oculomotor deficiency underlying asthenopia is in essence an inaccurate and unstable alignment of the two eyes.

Asthenopia↗
PubMed1982

The effect of presentation time on detection and diplopia thresholds for vertical disparities.

Detection and diplopia thresholds for vertical disparities are found to decrease when the presentation time of the stimulus with disparity is increased between 20 msec and 2 sec. This result supports the following notions previously put forward in literature; (a) the human visual system embodies detection processes which reveal vertical disparities; there is no sensory fusion mechanism in this system that gradually conceals vertical disparities: (b) the detection of vertical disparity in tachistoscopically presented stimuli is not based upon the initial appearance of the stimulus with disparity.

Depth Perception↗
PubMed1982

Detection of vertical disparties.

Psychometric curves determined for the detection of differences between vertical disparities as a function of the reference disparity for three presentation times (20 msec, 160 msec and 2 sec) and for two eccentricites of the stimulus (0.5 degree and 4 degrees) are presented. The results show that the detectability of disparity differences first increases and then decreases when the reference disparity is increased from zero. It further appears that the only effect of the stimulus presentation time and eccentricity on the psychometric curves and their dependency on the reference disparity is that the effective disparities observed are multiplied by a constant factor. The experimental data can be described by a signal detection model with two special features around zero disparity where binocular single vision occurs, viz. Loss of information about the sign of disparity and presence of intrinsic noise at a level up to 25% higher than the minimum level found at intermediate disparities.

Differential Threshold↗
PubMed1982

Assessment of retinal image displacement during head movement using an afterimage method.

An afterimage method has been used to measure retinal image displacement during binocular fixation with active head rotation about a vertical axis. The results confirm previously published reports that fixation accuracy in the horizontal direction deteriorates as a result of a head rotation. However, both retinal image displacements and fixation disparities were found to remain much smaller (by a factor of 3 and 8 respectively) than the values previously reported in the literature for similar head rotations (peak-to-peak amplitude: 20 degrees, frequency: 0.66 Hz). Moreover, our results show that fixation accuracy fell not only in the horizontal direction, but also in the vertical direction by about the same factor. It is concluded that eye-movement compensation of active head rotation may be much better than previously reported in the literature, and good enough to prevent deterioration of vision.

Afterimage↗
PubMed1982

Nonmotor component of fusional response to vertical disparity: a second look using an afterimage method.

An afterimage method has been used to investigate the relative magnitudes of the nonmotor and motor components of the fusional response to vertical disparity in a complex visual stimulus of diameter 57 degrees consisting of 50 horizontal lines and a square of side 2.5 degrees in the middle. The largest vertical disparity that evoked a stable fusional response was found to be in the range 3-6 degrees, of which the nonmotor component amounted only to 8-15', i.e., 2-10% of the total. At these fusional amplitudes, binocular single vision was already disrupted in the foveola. When the 50 horizontal lines were omitted from the stimulus so that only the central square of side 2.5 degrees remained, the fusional amplitudes decreased by only 25% while the absolute level of the nonmotor components remained the same. The nonmotor components found here are much smaller than those (amounting to about 2 degrees, or 25-40% of the total response) reported recently in the literature.

Afterimage↗
PubMed1978

Latency of luminance and contrast evoked potentials in multiple sclerosis patients.

Luminance and contrast visual evoked potentials (VEPs), were studied in 18 multiple sclerosis (MS) patients and 11 healthy subjects. Luminance EPs were recorded to stimulation with noise modulated light (0--60 Hz); contrast EPs were obtained to appearance-disappearance or reversal of a checkerboard pattern with various check sizes and repetition periods. Our results indicate that the apparent latency of luminance EPs to noise modulated light can hardly be used for diagnosis of MS, since the range of normal values scatters widely and greatly overlaps the range of latency values in MS patients (detection rate of 3/13). Our data confirm, on the other hand, that the latency of contrast EPs can be used to discriminate between MS patients and healthy subjects (detection rate of 12/18). We recommend for diagnostic purposes to determine the apparent latency from the phase spectrum of the responses to checkerboard reversal at repetition rates between 5 and 20 Hz, since in this frequency range the failure rate was found to be minimal. The specificity of the apparent latency data can be improved if the wave form of the transient reversal EP at a lower repetition rate (around 2 Hz) is also inspected. We suggest that an increased latency can be ascribed to several causes, only one of these being an increased conduction time due to demyelination. Indirectly this confirms that an increased EP latency is not specific for MS.

Adolescent↗