Biomedical subjects

G J van der Wildt

Publications and source records attributed to G J van der Wildt.

At least 19 recordsLinked to original sources

PubMed1997

Visual field enlargement by neuropsychological training of a hemianopsia patient.

A 58-year old hemianopsia patient was submitted to a two-fold neuropsychological training in order to enhance visual functions in the affected part of his visual field. At first, the visual field was measured perimetrically, to serve as a starting measurement with which after-measurements could be compared. Then, the first training was started: the border area between the intact and the defect visual field was being stimulated by small light spots. The training consisted of repetitive detection threshold measurements. After 27 one-hour sessions, the visual field was being measured again. The visual field appeared to have been enlarged 5 to 12 degrees in the direction of the affected hemifield and contrast-sensitivity thresholds to have been decreased almost at every point in the stimulus-array. Then, a second training started; an eye-movement training. Again, the border area, now shifted outwards, was stimulated. This time, the stimulus concerned a short presentation of light (< 200 msec.) after which the subject, to the best of his abilities had to make an eye-movement to the perceived stimulus-site. Also, he had to categorize the quality of his perception as well as the direction in which the stimulus was thought to be perceived. After 30 sessions, the visual field appeared to have 'grown' just a little bit more, but this seems not to be a significant enlargement. More important, the number of detected stimuli in the supposed 'blind' area had increased, as had the accuracy of the localization of the stimuli. Preliminary results of the detection training of a second subject, also 58 years of age, are presented. Finally, planned actions are discussed.

Eye Movements↗

PubMed1995

'When is VISION asked too much'?

The last two decades a shift took place from substitutional/compensatory training to utilisation of residual vision regarding rehabilitation of the visually impaired. Some of the visually impaired are able to use their visual perception nearly as complete as normal seeing people in spite of a severe visual disability. On the other hand, people with nearly normal functions can be severely visually handicapped. To illustrate this, two cases are presented. The first case is a man, aged 47 years, with a juvenile macular degeneration on both eyes. In spite of a very low visual acuity of less then 0.05, he finished an university education and he is able to maintain himself very well in a leading position in a scientific environment, by using adequate low vision devices. Also for his leisure activities, as photography and speed skating, he relies upon visual perception. The second case is a woman, aged 30 years, with nearly normal visual functions, who is not able to read for longer periods caused by conflicting information from the body- and eye movements, and the visual input. This causes sickness during reading. She is unable to use books for her study and is working with recordings on tape. The results of a comprehensive visual assessment will be related to the specific low vision devices and its use.

Adult↗

PubMed1992

Towards a classification of visual impairment.

For the rehabilitation of people with impaired vision, it is essential to have adequate (preferably quantitative) information about their residual visual functions. Special attention is given to the extra information provided by the results of measurement of the contrast sensitivity, especially in combination with the results of other measurements, such as the visual field, the amount of intraocular straylight and the visual acuity. The value of the contrast sensitivity function as a predictor of the extent of dysfunctioning in the visual activities of everyday life, such as outdoor vision, reading and recognition of faces, will be discussed. As far as this is concerned, a comparison is made between the value of visual acuity and contrast sensitivity as measures of the extent of vision and visual dysfunctioning.

Contrast Sensitivity↗

PubMed1989

Spatial contrast sensitivity in unilateral cerebral ischaemic lesions involving the posterior visual pathway.

Contrast sensitivity function was studied in 16 patients with unilateral ischaemic lesions involving the posterior visual pathway. Sixty-two percent of the patients showed contrast sensitivity loss in at least one eye for horizontal or vertical stimulus orientation. Visual perception was distorted in a qualitatively different way according to the anteroposterior site of the lesion. Patients with occipital or occipitotemporal lesions showed high spatial frequency selective losses and patients with temporal or parietal lesions low frequency selective losses. Stimulus orientation selectivity was observed in patients with lesions of the primary visual cortex as well as in patients with lesions anterior to the striate cortex. Contrast sensitivity orientation-selective losses were demonstrated in 14 of the 17 'affected' eyes.

Aged↗

PubMed1989

Visual contrast sensitivity in drug-induced Parkinsonism.

The influence of stimulus orientation on contrast sensitivity function was studied in 10 patients with drug-induced Parkinsonism. Nine of the 10 patients had at least one eye with contrast sensitivity deficit for vertical and/or horizontal stimuli. Only generalised contrast sensitivity loss, observed in two eyes, was stimulus orientation independent. All spatial frequency-selective contrast deficits in 15 eyes were orientation dependent. The striking similarity between the pattern of contrast sensitivity loss in drug-induced Parkinsonism and that in idiopathic Parkinson's disease, suggests that generalised dopaminergic deficiency, from whatever cause, affects visual function in an analogous way.

Adolescent↗

PubMed1988

Contrast sensitivity for oscillating sine wave gratings during ocular fixation and pursuit.

The influence of pursuit eye movements on the visibility of sine wave gratings was investigated by the simultaneous measurement of eye movements and contrast thresholds under different viewing conditions. It was concluded that: 1. with moving gratings contrast sensitivity during ocular pursuit was equal to contrast sensitivity during maintained fixation on a stationary target, provided that the magnitude of retinal image motion (0-20 deg/sec) was equal in both cases; 2. accurate pursuit eye movements led to a paradoxical suppression of the visibility of low spatial frequency gratings; 3. the visibility of high spatial frequency gratings remained unchanged during accurate pursuit eye movements; 4. iso-contrast-sensitivity resulting from both pursuit and non-pursuit conditions could be used to quantify the ocular pursuit performance.

Contrast Sensitivity↗

PubMed1988

Spatial contrast sensitivity in clinical neurology.

We studied contrast sensitivity function in normal subjects and in three illustrative cases with various neurological disorders. This was done by measuring contrast sensitivity over a range of spatial frequencies for vertical sinewave grating stimuli. It is demonstrated that contrast sensitivity function can give information about visual function not obtainable by conventional test procedures.

Adult↗

PubMed1987

Spatial contrast sensitivity and the diagnosis of amblyopia.

The relationship was studied between spatial contrast sensitivity function and type of pathology of the visual system. Two characteristics were found to be typical for amblyopia: there is a discrepancy between the high-frequency cut-off ('grating acuity') and the Snellen acuity; the contrast sensitivity is strongly dependent on the width of the stimulus. In optic nerve degeneration a decrease in contrast sensitivity is found at low spatial frequencies. Decreased contrast sensitivity at high spatial frequencies is atypical but occurs in those disorders associated with decreased Snellen visual acuity. From these findings a relationship is defined which enables spatial contrast sensitivity tests to be used to differentiate between amblyopia and optic nerve degeneration.

Adult↗

PubMed1986

Minimum procedures for visual electrodiagnostic testing.

A set of procedures for the electroretinogram, electrooculogram, and pattern and luminance visual evoked cortical potentials was defined by representatives from all Dutch University Eye Clinics in order to promote standardization of procedures for routine diagnostic testing. The procedures have been selected to obtain an answer to most electrodiagnostic questions of frequent occurrence with a minimum of equipment and examination time. An individual examiner may add other procedures to the proposed program to extend and diagnostic possibilities.

Electrodiagnosis↗

PubMed1986

Contrast sensitivity in Parkinson's disease.

We studied contrast-sensitivity function in 39 patients with Parkinson's disease. Sixty-four percent of the patients showed contrast-sensitivity loss in one or both eyes. The abnormality was not related to the first symptom or the severity of disease. Sensitivity loss at intermediate frequencies (notch loss) in 30% of the "affected" eyes suggested a cortical component. These findings support the belief that there is a widespread neurotransmitter deficiency in Parkinson's disease.

Adult↗

PubMed1985

Foveal inhibition measured with suprathreshold stimuli.

Foveal inhibition has been studied with elementary stimuli presented at increment threshold luminance and with suprathreshold stimuli. Both kinds of stimuli yield a similar kind of propagating inhibition. The results of this study are compared with previously published data on peripheral inhibition. The inhibition produced with spatially extended stimuli is described in terms of the ideas derived from measurements on elementary stimuli. This model is tested for a metacontrast stimulus consisting of a disk which is followed after a certain delay by a ring surrounding the disk.

Adult↗

PubMed1985

The influence of the background on the generation of inhibition.

The conditions leading to the generation of propagating inhibition have been investigated. The use of elementary point flashes as probes makes it possible to detect both the site and the time of inhibition with a high precision. The background luminance is found to influence the extent of inhibition. When no background illumination is present, no inhibition is detected. When only part of the background is illuminated, inhibition is sometimes found, and sometimes not. Experiments involving systematic buildup and breakdown of a background luminance field showed that inhibition only occurs when luminance is present within the range within which the inhibition effects operate (about 25 min of arc).

Humans↗

PubMed1983

Contrast detection and its dependence on the presence of edges and lines in the stimulus field.

The influence of sharp luminance discontinuities (dark lines and edges) in the stimulus field on the contrast sensitivity has been measured using sinusoidal gratings. Merely by changing the position of edges or lines in the stimulus field, it is possible to alter the contrast sensitivity by a factor of 3 or more. The distance from which such a discontinuity can affect the contrast sensitivity depends on the spatial frequency of the grating itself, and can be up to several degrees for gratings of low spatial frequency. No difference could be found between the effect of edges and the effect of very thin (5 min of arc) dark lines on the contrast sensitivity. The apparently conflicting experimental results in the literature concerning the relationship between the number of periods in a grafting and the contrast sensitivity, and especially concerning the influence that the surround of the grating has on this relationship, can be ascribed to the effect of these luminance discontinuities. In all cases where the number of periods was reported to have a large effect on the contrast sensitivity (up to a factor 10), the position of edges or lines in the stimulus field was also varied when the number of periods was changed. The change in the contrast sensitivity is no more than a factor 2 to 3 when there are no lines or edges in the grating surround.

Form Perception↗

PubMed1982

Propagating inhibition as a function of flash diameter and duration.

Van der Wildt and Vrolijk (1981) Vision Res. 21, 1765-1771, detected propagation of inhibition with small and short flashes, separated in time and space. In this paper, the results measured with flashes of longer duration and/or larger diameter are described and discussed. A model, describing inhibition arising on the flanks of the stimulus; luminance distribution and acting at the top luminance of the probe flash, gives a good description of the results. When non-identical flashes are used, the inhibition proves to be backwards. The propagating inhibition described also seems to give a possible explanation of metacontrast.

Humans↗

PubMed1981