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Biomedical subjects

R V Abadi

Publications and source records attributed to R V Abadi.

At least 19 recordsLinked to original sources
PubMed1992

Surgery and compensatory head postures in congenital nystagmus. A longitudinal study.

Null zones and head postures were studied in two patients with idiopathic congenital nystagmus who had been carefully selected from a larger group of patients who had nystagmus with compensatory head postures. Oculomotor and sensory features of the two patients indicated that they would have a reasonable chance of profiting from surgery. Neither patient had a periodic alternating or manifest latent nystagmus or any ocular disease of the fixating eye. Postoperative evaluation of the head posture and the nystagmus behavior of both patients was carried out over a period in excess of 6 years. Neither of the patients readopted a head turn, and visual acuity was found to be equivalent to that prior to surgery when using the null zone. In one of the patients the postoperative null zone was located in the primary position, while in the other the null zone was decreased from an eccentricity of 30 degrees to 15 degrees. These results suggest that the position of the postoperative null zone is not always an accurate predictor of head turn.

Adaptation, Physiological↗
PubMed1992

The distribution of macular pigment in human albinos.

This study investigated the variation in density of macular pigment across the central retina in normal and albino subjects. Luminance profiles were measured using a fundus camera and digital video techniques. The normal group had pigment spatial distributions consistent with previous studies. The albinos had no variation in absorbance across the central retina.

Adolescent↗
PubMed1991

The nature of head postures in congenital nystagmus.

We investigated the factors that determine the nature and extent of abnormal head postures in patients with congenital nystagmus. The head positions and eye movements of 16 patients were monitored while they adopted a variety of gaze positions. Five patients displayed a single head posture and four displayed multiple head postures. Six of the nine head postures matched the minimum intensity zone. The extent of the head posture was also found to be dictated by the velocity distribution of the slow phase, the nystagmus beat direction, and the neutral zone. Our results suggest that the surgical management of a head posture should not always be based only on the relocation of the minimum intensity zone to the primary gaze position.

Adolescent↗
PubMed1991

Quick phase programming and saccadic re-orientation in congenital nystagmus.

Horizontal single-step stimuli were presented to 19 subjects with congenital nystagmus (CN). When the stimulus jump was in the same direction as the waveform beat, spatial and temporal measurements of the responses revealed that saccades occurring between 90 and 180 msec after the stimulus were directed to an average of the spatial locations of the initial and final target positions. Transition functions of response end position against delay resembled those collected from normal subjects responding to either double-step staircase or pulse-overshoot stimuli, according to the stimulus direction. An analogy is drawn between the response to a single-step stimulus in the presence of CN and responses made by normal subjects to double-step stimuli. These findings suggest that pathways involved in the computation of amplitude of visually guided saccades also participate in programming CN quick phases and that the time taken to programme CN quick phases is similar to that for voluntary saccades. Evidence of saccadic parallel processing was demonstrated in some, but not all, of the subjects. Responses to steps in the same direction as the nystagmus beat frequently overshot the target. The mean overshoot was proportional to the amplitude and intensity of the nystagmus. Responses to stimuli presented in a direction opposite to the nystagmus beat were as accurate as the responses of a normal control group.

Adolescent↗
PubMed1991

Visual resolution limits in human albinism.

The effects of the involuntary ocular oscillations on visual resolution was examined in 22 albinos and 11 idiopaths with congenital nystagmus. The idiopaths showed a linear relationship between the proportion of the slow phase spent at low velocities (less than or equal to deg/sec) and the log of the minimum angle of resolution; such that long dwell times were compatible with good resolution. For the albinos there appeared to be a critical duration of low retinal slip velocities above which there was no improvement in acuity. This supports the contention that factors other than the congenital nystagmus limit visual resolution in the albino.

Albinism, Ocular↗
PubMed1991

The effects of simultaneous central and peripheral field motion on the optokinetic response.

The effects of central and peripheral retinal stimulation on the optokinetic response were examined using full-field, central field only, peripheral field only and simultaneous central and peripheral field motion. Stimulation of the central field in isolation produced similar responses to those obtained for full-field motion whereas peripheral field stimulation resulted in greatly reduced responses. Central field dominance was also evident during central and peripheral field motion in opposing directions. However with unidirectional, simultaneous central and peripheral stimulation the optokinetic response was determined not by the central stimulus but by whichever stimulus moved the slower.

Adult↗
PubMed1990

Retinal image quality in albinos. A review.

This paper describes two of the factors which affect retinal image quality in oculocutaneous albinos. Contrast detection thresholds were determined to quantify the effects of intra-ocular light scatter and the consequences of continuous retinal image motion. The implications of these results on the limits of visual performance are discussed in the light of the accompanying foveal hypoplasia.

Albinism, Oculocutaneous↗
PubMed1990

Pursuit and optokinetic responses in latent/manifest latent nystagmus.

Abnormalities of foveal smooth pursuit and the monocular optokinetic response (OKR) have often been reported in subjects with latent nystagmus (LN) and manifest latent nystagmus (MLN). This abnormality typically takes the form of a monocular asymmetry with a deficit in the response to nasal-to-temporal (N-T) motion in the visual field. Previous studies have each presented different interpretations of this finding, depending on whether the characteristics of the spontaneous oscillation were considered when analyzing the measured eye movement response: one report has suggested that these asymmetries are in fact the cause of the spontaneous nystagmus. In this study, pursuit and OKRs were examined separately and, when working synergistically and antagonistically, to attempt to overcome this difficulty. Results suggest that pursuit and the OKR could be symmetric in LN and MLN for both binocular and monocular viewing, which leads to the conclusion that the asymmetric patterns of response often reported in LN/MLN result from either shifts in the zone of minimum-intensity oscillation or from non-stimulus-specific increases in the spontaneous nystagmus.

Adolescent↗
PubMed1989

Retinal slip velocities in congenital nystagmus.

An analysis of waveform velocities was performed on 16 individuals with congenital nystagmus. The fast phases were shown to be saccadic in nature and their main sequence peak velocity was significantly slower than that of the normal control group (P less than 0.01). Peak slow phase velocities were found to reach values as high as 180 deg/sec; a relationship between the peak slow phase velocity and the amplitude of the slow phase was demonstrated. The range of the velocities that comprise the slow phase was generally in excess of 100 deg/sec. The correlation between the period of time spent at low retinal slip velocities (less than or equal to 10 deg/sec) and visual acuity was found to be significant (P less than 0.05). Implications of high slow phase velocities are discussed with reference to future experimental design.

Adolescent↗
PubMed1989

Retinal fixation behavior in human albinos.

The retinal fixation behavior of 17 albinos was examined whilst they fixated a stationary target. An optical system was arranged so that the motion of the target on the retina, due to the nystagmus, could be video-recorded. Eye movements were monitored simultaneously. Eight of the 17 subjects were judged to use consistently a retinal location compatible with the anatomical position of the fovea. The implications of this for subjects with foveal hypoplasia are discussed.

Adolescent↗
PubMed1989

The spectral transmittance of hydrogel contact lens filters.

The spectral transmittance characteristics of each of thirty-two solid tint hydrogel contact lenses available in the United Kingdom were investigated using a dual beam spectrophotometer. Good agreement was found with the small number of spectral functions that had previously been published. Lenses with similar colour appearances were seen to differ markedly in terms of their transmittance factors. The importance of visual performance and radiation protection is discussed in the light of the spectral transmittances of the lenses.

Color↗
PubMed1986

Waveform characteristics in congenital nystagmus.

Using infra-red oculography, electro-oculography and fundus video-recordings, waveform characteristics (amplitude, frequency, waveform shape and foveation) were examined in over 150 individuals with congenital nystagmus. For many of the subjects the nystagmus exhibited marked variability in both space and time. The sources of this variability were explored and the state of attention of the observer was found to be a dominant factor. The waveform and precision of foveation were not found to be related to any one classification group (eg. albino or idiopathic).

Adolescent↗
PubMed1985

The influence of preexisting oscillations on the binocular optokinetic response.

Full-field binocular optokinetic responses were examined in patients with congenital nystagmus. If the stimulus moved in the same plane as the congenital nystagmus, a gaze-modulated response was recorded for both idiopathic and albino observers. When the congenital nystagmus was almost totally suppressed, neither a gaze-evoked nor a sawtooth response was seen. For stimulus movements in a plane orthogonal to the congenital nystagmus, normal sawtooth optokinetic responses were exhibited by both groups. Experimental evidence supports the hypothesis that the optokinetic response in the meridian of the spontaneous oscillation has been adapted.

Adult↗
PubMed1985

The influence of nystagmoid oscillation on contrast sensitivity in normal observers.

Normal observers subjected to retinal image motion equivalent to that experienced by congenital nystagmats were found to display a contrast sensitivity function with characteristics similar to those shown by nystagmat observers. The amplitude, frequency and waveform (and hence foveation time) of nystagmic oscillation were varied, with alteration of the waveform producing the most significant effect on the contrast sensitivity function. The pseudocycloid waveform with the longest percentage foveation time per cycle was commensurate with the best visual resolution of medium-to-high spatial frequency targets. For all waveforms the length of the foveation period showed good correlation with visual resolution of the targets.

Adult↗
PubMed1985

Blue light hazard and aniridia.

The fundi of three patients with aniridia were photographed with a 470 nm illuminating light source. No apparent change in contrast was observable throughout the macular region. This would suggest an absence of the macular pigment. The likelihood of aniridics being more susceptible than normal persons to blue light damage is discussed.

Color↗