Biomedical subjects

R S Snelgar

Publications and source records attributed to R S Snelgar.

11 recordsLinked to original sources

PubMed1990

Adaptation to peripheral flicker: relationship to contrast detection thresholds.

The time to disappearance of flicker of a temporally modulated uniform 1 degree field, steadily viewed with the temporal retina at an eccentricity of 12 degrees, was measured as a function of temporal frequency and depth of modulation (contrast). As found by others, for a fixed contrast, adaptation time declined as temporal frequency increased. To check whether this effect was genuinely temporal frequency-dependent, or reflected the amount above threshold of the adapting contrast, measurements were also made at contrasts which were multiples of the contrast threshold or matched across temporal frequencies. The results suggest that both temporal frequency and amount of adapting contrast above threshold are important in determining the speed of adaptation.

Adaptation, Ocular↗

PubMed1990

Optic neuritis: variations in temporal modulation sensitivity with retinal eccentricity.

Temporal modulation sensitivity functions were measured centrally and at eccentricities of 2.5 degrees, 5 degrees and 10 degrees in the temporal visual field of 12 patients with recovered optic neuritis and in a group of matched normal controls. A circular, spatially uniform stimulus of 1 degree angular subtense was presented with sinusoidal modulation at 5, 8, 14 and 23 Hz. The general pattern of results in patients was a loss of sensitivity relative to normal controls at all temporal frequencies at 0 degree and 2.5 degrees eccentricity, with rather greater losses occurring at the medium-to-lower temporal frequencies. At 5 degrees eccentricity, the losses were confined to medium temporal frequencies only, and at 10 degrees eccentricity there was no significant loss at any temporal frequency. These findings may be explained by a greater vulnerability of optic nerve fibers of small diameters to the effects of demyelinating disease.

Adult↗

PubMed1987

Isolation of opponent-colour mechanisms at increment threshold.

An experimental examination was made of some paradigms designed to isolate the opponent-colour system at increment threshold. The effectiveness of a uniform white conditioning field spatially coincident with a 1.05-deg uniform test field was assessed by measuring intensity thresholds for simple detection and for colour discrimination. Values were obtained both by a method of adjustment and by a two-interval forced-choice procedure. For sufficiently high luminances of the conditioning field (3000 td or greater) little or no difference was found between simple-detection and colour-discrimination thresholds over the critical test-flash spectral range 520-620 nm, implying that the paradigm produced almost complete isolation of the opponent-colour system at increment threshold. A control experiment in which thresholds were obtained for a conditioning field larger than the test field gave less satisfactory isolation; near 580 nm the luminance system was found to be at least 0.3 log unit more sensitive than the opponent-colour system. A comparison was also made of the spatially coincident field paradigm with a paradigm in which a modified test stimulus of low temporal and spatial frequency content was presented on a large conditioning field. Test spectral sensitivity curves for simple detection obtained by a method of adjustment showed little difference in effectiveness in opponent-colour isolation.

Adult↗

PubMed1987

Paradoxical effects of temperature in multiple sclerosis.

Six cases of multiple sclerosis are reported in which cold produced a temporary exacerbation of symptoms and signs of the disease. Also, in one case investigated in detail by psychophysical methods, heating produced a paradoxical deterioration in vision and simultaneous improvement in sensory and motor function. The effect of temperature in multiple sclerosis is discussed and a physiological explanation for the paradoxical response to heating is suggested.

Adult↗

PubMed1985

Multiple sclerosis: abnormalities in luminance, chromatic, and temporal function at multiple retinal sites.

Visual function was assessed in a group of patients with multiple sclerosis (MS) and in a group of matched normal controls. In these patients the disease was relatively mild. For each subject, measures of a range of psychophysical visual functions were carried out at multiple sites in each eye. Previous reports have only included some of these functions. Here, luminance threshold, two-flash resolution, perceptual latency, luminance critical flicker frequency (CFF), and chromatic CFF were all measured. Variabilities of these functions and correlation between chromatic and luminance CFFs were also evaluated. For both the MS group and the normal control group, the correlations between pairs of visual parameters were not overall significantly greater than chance level. The MS group did give a significantly reduced value relative to the normal group for luminance CFF and for the gradient of the plot of chromatic CFF against luminance CFF. This group was then subdivided according to history of visual involvement. The subgroup with previous visual symptoms had significant impairment for luminance threshold, variability of luminance threshold, luminance CFF, variability of two-flash resolution, and for the gradient of the plot of chromatic CFF against luminance CFF. The subgroup without previous visual symptoms showed no significant impairment for any individual parameter, although the gradient of the plot of chromatic CFF against luminance CFF was lower than normal.

Adult↗

PubMed1985

Nonselective losses in foveal chromatic and luminance sensitivity in multiple sclerosis.

A psychophysical technique involving simple increment threshold measurements was used to determine foveal chromatic and luminance sensitivity in patients with multiple sclerosis (MS) and in matched normal controls. The patient group showed substantial and nonselective losses in chromatic and luminance sensitivity relative to the normal control group, and these losses were significantly correlated with each other over individual patients. It is suggested that impairment of foveal visual function due to demyelination is not more specific to fibers carrying chromatic information than to fibers carrying luminance information.

Adult↗

PubMed1983

Test and field spectral sensitivities of colour mechanisms obtained on small white backgrounds: action of unitary opponent-colour processes?

It has been shown that for human foveal vision the test spectral sensitivity curve obtained in the presence of a large white background exhibits peaks at about 440, 530 and 610 nm and a small dip or notch at about 580 nm. Additionally, field spectral sensitivity curves for the medium- and long-wavelength sensitive colour mechanisms when derived in the presence of a small monochromatic background (auxiliary field) spatially coincident with the test field are sharper than the corresponding Stiles's II mechanisms and peak at about 530 and 605 nm. The short-wavelength sensitive colour mechanism shows no such effect, the peak remaining at about 440 nm. The test spectral sensitivity curve obtained on a large white background and the sharpened field spectral sensitivity curves obtained on a spatially coincident, monochromatic auxiliary field have each been interpreted in terms of an opponent-process theory of colour vision: the response of the non-opponent luminance system is considered to be reduced by selective achromatic adaptation (with the large white background) or by selective contour masking (with the spatially coincident auxiliary field). The present study combined the two techniques of achromatic and spatial adaptation: a small white auxiliary field spatially coincident with the test field was used in measurements of both test and field spectral sensitivities. Under these conditions, it was found that the test spectral sensitivity curve showed more clearly defined peaks and a deeper notch at about 580 nm than when obtained on the large white background and that the test curve was well fitted by the upper envelope of the short-wavelength and sharpened medium- and long-wavelength field spectral sensitivity curves, suggesting the possibility that three unitary opponent-colour mechanisms may underlie test and field spectral sensitivities. A test of this hypothesis is proposed concerning the effect of auxiliary-field chromaticity on the position of the notch at about 580 nm in the test spectral sensitivity curve.

Adult↗

PubMed1982

Abnormalities of chromatic and luminance critical flicker frequency in multiple sclerosis.

Critical flicker frequency (CFF) was measured for stimuli varying in chromaticity only and in luminance only for patients with multiple sclerosis (MS) and for matched normal controls. The two CFF measurements showed different underlying linear interdependencies for the two groups, consistent with a greater loss of temporal luminance function than of temporal chromatic function in MS patients. These results are discussed in relation to the pathophysiology of demyelinated nerve fibers. It is suggested that demyelination affects all types of nerve fiber unselectively; in particular, no support is found for the notion of a special vulnerability of fibers carrying time-varying chromatic information.

Color Perception↗

PubMed1981

Mapping, in the rat central nervous system, of morphine-induced changes in turnover of 5-hydroxytryptamine.

1. It is known that the full clinical effect of morphine is not seen if those neurones of the central nervous system which contain 5-hydroxytryptamine (5-HT) have been inactivated. Morphine increases 5-HT turnover in brain and cord, and the present work is an attempt at mapping the sites at which turnover is accelerated. 2. The degree of interaction at various sites of the C.N.S. was measured by the increment in the content of 5-hydroxyindol-3-yl acetic acid (5-HIAA) elicited by morphine in rats pre-treated with probenecid. The effect was produced by as little as 3.5 mg morphine hydrochloride/kg; for the sake of convenience this dose was doubled in most experiments. Pre-treatment with probenecid was not necessary for the effect, but a minimum interval of 90 min between injection of morphine and analysis of the brain was essential. 3. Morphine did not act indiscriminately on all 5-HT neurons, as seen by the fact that the increment in 5-HIAA formation was independent of the density of 5-HT neurones in the tissue. Nor was there any relation between basal 5-HT turnover of a region and the size of its response to morphine. 4. Highly reactive sites were found in dorsal cord, medulla, superior colliculi, substantia nigra, thalamus, hypothalamus, amygdala and striatum. Cortical areas were less responsive, but with large differences among themselves. Hippocampus, central grey, olfactory bulb, cerebellum and white matter had lower or negligible increases in 5-HT turnover. There was no increase in the turnover of the (non-neural) 5-HT of the pineal gland. 5. The relation of the reactive sites to their content in endogenous opioids and to the probable localization of the pharmacological actions of morphine is discussed.

Animals↗

PubMedDate not supplied

Effects of 4-aminopyridine in patients with multiple sclerosis.

4-Aminopyridine (4-AP) was administered to two groups of patients with multiple sclerosis (MS). The first group consisted of 5 patients with labile visual symptoms, 2 of whom had arcuate scotomata. 4-AP improved visual performance of most patients in this group and reduced the size of scotomata. The second group consisted of 5 patients with the spinal form of MS who were in a stable state; in this group 4-AP had little effect clinically or on tests of visual function.

4-Aminopyridine↗

PubMedDate not supplied

Early visual processing over the menstrual cycle: the tilt aftereffect.

We have previously shown that the tilt aftereffect (TAE) appears to respond to changes in central dopaminergic function. In this study, the TAE was investigated during the preovulatory and the premenstrual phases of the menstrual cycle. Results in the preovulatory phase mimicked those previously found associated with increased dopaminergic function, compared to data produced in the premenstrual phase, suggesting changes in dopamine function over the cycle.

Adult↗