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Biomedical subjects

F W Cornelissen

Publications and source records attributed to F W Cornelissen.

11 recordsLinked to original sources
PubMed1998

When is a background equivalent? Sparse chromatic context revisited.

Jenness and Shevell (Vision Res 1995;35:797-805) reported that a red background with white dots scattered on it has a different influence on a target's apparent colour than an equivalent uniform background. We show that this finding depends on what one considers an equivalent background. Jenness and Shevell averaged the chromaticity and luminance of the background with the dots, and 'superimposed' the target onto this new background. This changed the luminance and chromaticity of both the target and the surround. We show that if only the surround is changed, it is irrelevant whether the latter is red with white dots scattered over it, or a uniform field with the same space averaged chromaticity and luminance. Our findings are consistent with a local contrast mechanism that has a limited spatial resolution.

Adaptation, Ocular↗
PubMed1997

Four issues concerning colour constancy and relational colour constancy.

Four issues concerning colour constancy and relational colour constancy are briefly considered: (1) the equivalence of colour constancy and relational colour constancy; (2) the dependence of relational colour constancy on ratios of cone excitations due to light from different reflecting surfaces, and the association of such ratios with von Kries' coefficient rule; (3) the contribution of chromatic edges to colour constancy and relational colour constancy; and (4) the effects of instruction and observer training. It is suggested that cognitive factors affect colour constancy more than relational colour constancy, which may be an inherently more robust phenomenon.

Color Perception↗
PubMed1997

A binocular fiberscope for presenting visual stimuli during fMRI.

A binocular pair of fiberscopes relays high-resolution images of CRT displays from an adjacent room to an observer lying in a scanner in functional Magnetic Resonance Imaging (fMRI) studies of visual function. We review the problems that must be overcome by any visual display for use in fMRI, present the specific solution we developed, and discuss its merits. Together, the fiberscope and CRT conveniently display accurately controlled high- and low-contrast wide-field images to an observer in an fMRI scanner.

Computer Terminals↗
PubMed1995

Object perception by visually impaired people at different light levels.

We investigated the relationship between the illumination level and the ability of visually impaired subjects to detect and recognize objects in a realistic visual environment. Subjects often continued to show substantial improvement at light levels where normal subjects have reached maximum performance. Integrated contrast sensitivity, a summary measure for the contrast sensitivity function, was better at predicting performance than either visual acuity or peak contrast sensitivity. However, when combined, the latter two predicted performance as well as the former. We conclude that when we try to find the best illumination for orientation and day-to-day activities we should optimize it for both visual acuity and contrast sensitivity.

Contrast Sensitivity↗
PubMed1995

Simultaneous colour constancy revisited: an analysis of viewing strategies.

We examined whether matching instructions influenced the eye movements that subjects made during a colour constancy experiment. The instructions changed the average duration of exposure to the spectrally biased surround. We also measured the influence that small changes in exposure duration have on the perceived colour. Eye movement and adaptation data were combined to predict differences in colour matches. Two of the five subjects showed an instructional effect that was much larger than that predicted. Analysis of the eye movements, and an experiment with dynamic surrounding colours, reveal that several viewing strategies do not account for the influence of the instruction.

Adaptation, Ocular↗
PubMed1994

Interactions between modulated luminance patterns and random-dot patterns.

It has been suggested that density modulated random-dot patterns can be used to study higher order pattern vision [Van Meeteren and Barlow (1981) Vision Research, 21, 765-777]. The high contrast dots of which the pattern is composed, are assumed to be reliably transduced and transmitted by the lower levels of the visual system. Therefore, such dot patterns could offer a way of by-passing the limits set by these earlier steps in the visual system. So, detection performance should reflect the capacity of more central visual mechanisms to combine and compare groups of dots. We test this assumption by selectively desensitizing the spatial frequency channels which are involved in detecting luminance contrast patterns. The results show a selective decrease in sensitivity for modulations in dot density at the adapting spatial frequency. We conclude that detection of differences in dot density is mediated by the same channels that detect luminance contrast. The conclusion by Van Meeteren and Barlow that dot patterns can be used to study higher-order processing in the visual system appears not to be valid. In addition, we present a new type of modulated dot pattern of which the density modulations are shown to be invisible for the spatial frequency channels. This pattern may therefore be used to study higher order visual processing.

Adaptation, Ocular↗
PubMed1992

Development of visually guided behaviour requires oriented contours.

Kittens do not learn to use visual information to guide their behaviour if they are deprived of the optic flow that accompanies their own movements. We show that the optic flow that is required for developing visually guided behaviour is derived from changes in contour orientations, rather than from velocity patterns. We used several tests to assess visually guided behaviour. The performance of kittens that had only been allowed to see isolated dots of light was indistinguishable from that of kittens that had received no visual exposure at all. Kittens that had seen streaks of light performed better on several tasks. We discuss this finding with relation to the visual pathways that are presumably involved.

Animals↗
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↗
PubMed1991

Illumination research as part of a visual assessment of visually impaired individuals.

In this paper we describe a method for the preliminary assessment of the illumination needs of partially sighted individuals. The method is based on determining the smallest readable lettersize under different levels of illumination. Half of the partially sighted in our population had a performance that was dependent upon the level of illumination. At the same time, most of those for whom no improvement in performance was found, did express a preference for a certain level of illumination. Therefore, there remain discrepancies between the objectively determined optimal level and the preference for a certain level. We conclude that the current method is a useful but insufficient method of determining the illumination needs of partially sighted individuals.

Adolescent↗
PubMedDate not supplied

Visual discrimination and short-term memory for random patterns in patients with a focal cortical lesion.

Visual discrimination and short-term recognition memory for computer-generated random patterns were explored in 23 patients with a postsurgical lesion in one of the cortical hemispheres. Their results are compared with those of 23 age-matched volunteers. In a same-different forced-choice discrimination task, d' and log beta (measures of sensitivity and bias), as well as reaction time (RT) were determined. All participants viewed patterns defined either by luminance contrast or isoluminant red-green color contrast, the amplitude of which was adjusted to be 10 times the respective detection threshold level. Block patterns consisting of a 6 x 6 matrix of light and dark (red and green) checks were randomly configured on each presentation. They were presented in pairs, randomly in two visual quadrants for a duration of 200 msec. Three presentation conditions were used: simultaneous presentation of reference and test stimulus, sequential presentation with a short delay (interstimulus interval, ISI = 3 s), and sequential presentation with a long delay (ISI = 6 s). The results indicate that patients with a lesion in the occipitotemporal cortex, the superior temporal cortex and the frontal cortex were significantly impaired on both luminance-contrast and color-contrast pattern discrimination. Patients with damage in the anterior inferotemporal cortex showed no overall impairment. The results suggest that performance in visual discrimination and recognition memory tasks rely on distributed neural processes with more than one neocortical location.

Adult↗