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Biomedical subjects

I J Murray

Publications and source records attributed to I J Murray.

12 recordsLinked to original sources
PubMed1998

Theoretical and practical performance of a concentric bifocal intraocular implant lens.

Some results from a survey of 43 patients who had a monofocal intra-ocular-lens (IOL) in one eye and a concentric bifocal intra-ocular-lens in the fellow eye are reported. Twenty patients with 6/9 or better post-operative visual acuity in both eyes, participated in the main part of the study. Optical transfer functions for the bifocal lens showed that, compared to an optimal single-vision correction, there is a 50% contrast degradation of the distance retinal image across all spatial frequencies above around 3 c/deg. For the patients in the main study, there was a close correspondence between practical measurement of contrast sensitivity and the theoretical predictions of the modulation transfer functions. Measuring contrast sensitivity proved an effective means of assessing misalignment of the bifocal IOLs.

Aged↗
PubMed1998

Scattering properties of Bagolini lenses and their effects on spatial vision.

The effect of a Bagolini lens on spatial vision was investigated by studying its far-field diffraction pattern as produced by a coherent beam of laser light, and its effect on the contrast sensitivity function (CSF) in human subjects. For lenses of the main type studied, which were crossed by a series of slightly-irregular striated bands, each consisting of fine, parallel, etched lines of various widths and separations, the diffraction pattern consisted of undiffracted light giving a bright central spot and wide-angle, diffracted light giving a dim streak. The latter was due to the sum of the diffraction patterns associated by the irregular fine etched lines. The streak produced by a single striated band was modulated by a series of regular maxima and minima related to the width of the band. Analysis of this pattern gave the width of the band as 0.6 mm, in close agreement with direct microscopical measurements. When four bands were illuminated by a beam of about 3 mm diameter, similar to the diameter of the photopic pupil, the diffraction pattern showed no obvious maxima and minima, due to irregularity in the width and separation of the bands. The central spot contained more than 90% of the total light in the diffraction pattern. Thus the Bagolini lens, with its relatively weak far-field diffraction pattern lacking regular maxima and minima when areas > or = 3 mm in diameter were used, was expected to have only a small effect on the apparent contrast of the targets in CSF experiments. This was confirmed by the measurements: Bagolini lenses showed no significant effect on either the monocular or binocular CSF. Further similar measurements with lenses of slightly different design from another manufacturer confirmed these findings. Therefore Bagolini lenses do not disrupt vision when they are used to determine the presence of suppression and anomalous retinal correspondence.

Contrast Sensitivity↗
PubMed1997

Spectral sensitivity in patients with dysthyroid eye disease.

The majority of patients with dysthyroid eye disease have an acquired colour vision defect. However, no psychophysical investigation of selective damage to colour or flicker pathways has been carried out. In order to clarify the nature of the visual pathology, we have used a psychophysical technique (spectral sensitivity) to selectively stimulate the chromatic and achromatic mechanisms. Spectral spots of size 1 degree presented at a rate of 1 Hz on a bright 1000 td white background are detected by the chromatic mechanism but a rate of 25 Hz reveals the achromatic mechanism. Fifteen patients (28 eyes) between the ages of 50-70 years were tested. The study showed that all patients had reduced spectral sensitivity, either 1 Hz, 25 Hz or both. The patients with reduced 1 Hz or 25 Hz spectral sensitivity only had a shorter systemic and ocular duration of the condition, had no proptosis, normal intraocular pressures in primary gaze, slightly higher intraocular pressures on upgaze, normal visual field plots and FM 100-Hue error scores higher than the normal age-matched values. The patients with reduced both 1 Hz and 25 Hz spectral sensitivities had a longer systemic and ocular duration of the condition, had proptosis, normal intraocular pressures in primary position, higher intraocular pressures on upgaze and higher FM 100-Hue error scores than the age-matched normals and those in Groups 1 and 2. A total of 50% of patients in Group 3 had defective visual field plots. These data suggest that there is a damage of the large achromatic fibres and small chromatic fibres in dysthyroid eye disease. The mechanism of the damage could be one of ischaemic or mechanical or both.

Aged↗
PubMed1997

Pattern ERGs from isoluminant gratings; poor selectivity compared with VEPS.

The PERGs elicited by low contrast achromatic (luminance-modulated) and chromatic (isoluminant) gratings were studied as a function of the presentation mode (rapid onset, offset, reversal), departure from isoluminance, contrast and spatial frequency; predominantly phasic-type responses were found in all cases. We propose that the PERGs for low and moderate contrast, achromatic and chromatic grating stimuli, are generated by non-linear, transient retinal cells of the magno-system. Parvo retinal cells seem to contribute to the response only at higher chromatic contrasts. The transient nature of the PERG obtained from isoluminant gratings makes it doubtful that these signals represent the activity of colour-related processes.

Adult↗
PubMed1996

Objective measurement of contrast sensitivity and visual acuity with the steady-state visual evoked potential.

Since the appearance of Campbell and Maffei's and Harter and White's reports it has been well established that the visual evoked potential (VEP) can be used to predict psychophysical contrast sensitivity and visual acuity and is thus suited as an objective technique to assess these fundamental aspects of vision. Nevertheless, the technique has not become a standard diagnostic tool, being too time-consuming to apply and suffering from variable reliability under pathological visual conditions. In addition, there are problems of reliability in normal subjects. By using an unconventional stimulus--temporally sinusoidal 16-Hz on-off modulation of sinewave gratings--we demonstrated that these problems can be alleviated in normal subjects. This stimulus avoids the low signals in the visible range that frequently occur with conventional pattern-reversal stimuli, it leads to high correspondence between normal observers, and it is much faster to apply than are transient VEPs. Initial applications of this stimulus to amblyopes yielded promising results. The steady-state VEP could consequently turn into a viable diagnostic procedure in disturbances of visual contrast perception.

Contrast Sensitivity↗
PubMed1993

Visual safety of quartz linear lamps.

A quartz linear lamp (QLL) is a compact infra-red (i.r.) heating lamp. It consists of a tungsten filament in a quartz envelope, which for space heating purposes is surrounded by a ruby coloured filter. The filament runs at about 2400 K and produces i.r. radiation with a peak output at 1200 nm. The lamps are widely used for space heating and are normally fitted with a parabolic reflector. There are no statutory criteria for potential visual hazards resulting from the use of such lamps in the U.K. The ACGIH has produced a provisional set of rules on light and near i.r. This paper reviews potential hazards to the eye in the context of the ACGIH rules and the output of QLL heaters. The heaters produce no u.v. or blue light hazards. The brightness and heat of the lamp is sufficient to produce an aversion response which will prevent retinal thermal damage from inadvertent viewing. The probability of retinal thermal injury from an i.r.-A image focused on the retina is examined and it is shown that there is no risk at normal viewing distances. To prevent any risk of cataractogenesis, the rules provide for an upper limit of irradiance (TLV) at the eye of 100 W m-2 in the i.r.-A band; the paper examines the potential hazard from longer wavelengths and concludes that they do not present a hazard with this type of heating. The conclusion is that QLL heaters used for space heating in accordance with manufacturer's instructions present no hazard to the eye.

Eye↗
PubMed1991

The visual defect in multiple sclerosis and optic neuritis. A combined psychophysical and electrophysiological investigation.

Chromatic and achromatic visual function were investigated using psychophysical and evoked potential techniques in 32 patients with multiple sclerosis (MS) or optic neuritis. There was no evidence for either chromatic or achromatic vision being selectively damaged. There was, however, some evidence that demyelination of the optic nerve causes a visual defect in which the transmission of high temporal frequencies is imparied. In some patients either a red or blue deficit was evident, but overall there was no evidence of a wavelength specific colour vision defect in our patient group. Reduction of chromatic sensitivity as determined psychophysically had a statistically significant correlation with the amplitude and latency of the chromatic visual evoked potential.

Adult↗
PubMed1986

Visual changes mediated by beer in retrobulbar neuritis--an investigative case report.

A patient with established retrobulbar neuritis and Uhthoff's phenomenon who claimed that his visual acuity improved after drinking beer was assessed by detailed quantitative psychophysical tests. Studies of electrophysiological responses and biochemical levels of blood serum before and after drinking on several occasions were also made. The results of tests of contrast sensitivity, spectral sensitivity, flicker sensitivity, and visually evoked potential confirmed his observation. Some mechanisms for this phenomenon are considered.

Adult↗
PubMed1984

Movement detection and spatial phase.

Contrast thresholds for the detection of movement are determined when a sinusoidal grating pattern is shifted in spatial phase. "True" contrast reversal (180 degrees phase shift) is compared with smaller phase shifts for a range of spatial frequencies. An unexpected increase in movement sensitivity is revealed for intermediate spatial frequencies. This effect is confined to those spatial frequencies where interaction between pattern and movement detectors occurs.

Humans↗
PubMed1978

A pilot study on the use of visual field expanders.

The nature and magnitude of the visual difficulties of five volunteer patients with severely contracted visual field and high visual acuities was investigated by several techniques. These comprised a questionnaire providing a quantitative score of visual impairment, a specially devised visual search task, and an arbitrary method of estimating visual efficiency. Many aspects of visual impairment resulting from contract visual fields were revealed and discussed. Performance scores on the search task indicated that the field expander would probably be useful to a small number of cases for continuous wear, usually as a binocular bioptic system. Other cases might benefit from a clip-on or hand-held device for occasional use. Methods of manufacture and dispensing and difficulty in locating suitable patients were the major limiting factors in this study. The methods of assessment, and the optical system described in this study are worthy of further investigation and development for trial on a larger number of patients.

Eyeglasses↗
PubMedDate not supplied

Amplitude and phase variations of harmonic components in human achromatic and chromatic visual evoked potentials.

Occipital visual evoked potentials (VEPs) were recorded in response to low-contrast, low spatial-frequency chromatic, and achromatic gratings. Fast Fourier Transform (FFT) and time-domain analysis were used to reveal differences in harmonic content and amplitude of chromatic and achromatic response components over a wide range of temporal frequencies. The chromatic ON/OFF VEP is dominated by the fundamental component indicating that onset and offset responses are different. This type of response is typical of neurons with sustained type response characteristics. Conversely, the achromatic onset VEP contains a predominant second harmonic component in addition to the fundamental. This similarity between onset and offset responses suggests that transient mechanisms are responsible for the generation of achromatic components. Frequency analysis of VEPs elicited by phase-reversing stimuli reveals that all of the response energy is concentrated at the second harmonic of the stimulating frequency. The magnitude of the second harmonic component is maximal for achromatic stimuli and undergoes a distinct minimum for isoluminant, chromatic stimuli. This behavior indicates that under the stimulus conditions used, magnocellular neurons with transient characteristics dominate the reversal VEP.

Adult↗