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Biomedical subjects

F W Cornelissen

Publications and source records attributed to F W Cornelissen.

17 recordsLinked to original sources
PubMed2002

Grey scales uncover similar attentional effects in homonymous hemianopia and visual hemi-neglect.

Multi-component models of visual hemi-neglect have postulated that visual hemi-neglect is characterised by various attentional deficits. A grey scales task has been developed to quantify the early, automatic, (perhaps obligatory) ipsilesional orienting of visual attention, frequently assumed as the first of these attentional deficits. Explanations for this attentional imbalance are up until now mainly formulated in terms of right hemisphere activation. This lateral attentional bias has also been demonstrated in controls, in whom it is expressed as a leftward perceptual asymmetry. We reproduced previous literature findings on a grey scales task, considering controls and neglect patients. Three patients with neglect showed an extreme ipsilesional lateral bias. This bias did not change during or after cognitive rehabilitation. Additionally, we presented this grey scale task to 32 patients with left- and right-sided homonymous hemianopia (HP). HP is the loss of sight in one visual hemi-field. The HH patients had no clinical signs of impaired lateralised attention. Results revealed that HH patients showed a similar ipsilesional bias, albeit to a lesser degree than in neglect. Left-sided HH patients presented a quantitatively similar, but qualitatively opposite bias than the right-sided HH patients. We suggest that sensory effects can be an alternative source of attentional imbalance, which can interact with the previously proposed (right) hemispheric effects. This suggests that the perceptual asymmetry in the grey scales task is not necessarily an indicator of impaired right hemisphere attention. It rather suggests a pattern of functional cerebral asymmetry, which can also be caused by asymmetric sensory input.

Adolescent↗
PubMed2002

Hemianopic visual field defects elicit hemianopic scanning.

Previous explanations for the variability in success of compensating for homonymous hemianopia (HH) has been in terms of extent of the brain injury. In using on-line eye movement registrations, we simulated HH in 16 healthy subjects and compared their scanning performance on a dot counting task to their own "normal" condition and to real HH patients' performance. We evidenced clear parallels between simulated and real HH, suggesting that hemianopic scanning behaviour is primarily visually elicited, namely by the visual field defect, and not by the additional brain damage. We further observed age-related processes in compensating for the HH.

Adolescent↗
PubMed2001

A girl without a chiasm: electrophysiologic and MRI evidence for the absence of crossing optic nerve fibers in a girl with a congenital nystagmus.

An otherwise healthy 15-year-old girl with a congenital nystagmus was evaluated at our department using visual evoked potential recording and magnetic resonance imaging. She appears to have the unique isolated inborn absence of the optic chiasm, described only once before in two unrelated girls. Unlike these previously described cases, our patient does not seem to display a see-saw nystagmus.

Adolescent↗
PubMed2000

Separate simultaneous processing of egocentric and relative positions.

It is well established that all kinds of visual attributes are processed separately within the brain. This separation is related to differences in the information that is relevant for the different attributes. When attributes differ greatly (such as colour and motion) it is obvious that they must rely on different information. However, separating the processing of different attributes could also allow highly related attributes to evolve independently, so that they end up being judged on the basis of different types of information. Here, we examine the case of egocentric and relative localisation. For judging egocentric positions, the orientation of the eyes has to be taken into account. This is not so for judging relative positions. We demonstrate that these two attributes can be processed separately by showing that simultaneous judgements of relative and egocentric position differ in their dependency on eye orientation. Subjects pursued a moving dot. We flashed either single targets, or pairs of targets with a 67 ms interval between them, directly below the subjects' gaze. As the eyes were moving during the 67 ms interval, the retinal separation between pairs of targets was different from their actual separation. Subjects indicated the position at which they saw the targets with reasonable reproducibility, with a consistent bias in the direction of the eye movement. However, when two targets were flashed, the indicated separation between them usually coincided with their retinal separation, rather than with their actual separation. We conclude that egocentric and relative spatial positions can be estimated separately and simultaneously, on the basis of different types of information.

Eye Movements↗
PubMed2000

Visual memory for random block patterns defined by luminance and color contrast.

We studied the ability of human subjects to memorize the visual information in computer-generated random block patterns defined either by luminance contrast, by color contrast, or by both. Memory performance declines rapidly with increasing inter-stimulus interval, showing a half-life of approximately 3 s. We further show that memory performance declines with eccentricity approximately as a Gaussian function of position. Memory decay functions did not depend on whether the patterns were defined by luminance or color contrast. Changing both luminance and color components of block patterns in conjunction did not improve performance suggesting a single memory mechanism is used to store luminance and color derived pattern information. Our results further suggest that color identity (hue, saturation) and pattern information extracted from color- or luminance-contrast are stored independently of each other.

Adult↗
PubMed2000

Does age change the distribution of visual attention? A comment on McCalley, Bouwhuis, and Juola (1995).

A paper by McCalley, Bouwhuis, and Juola (1995) suggested differences between younger and older adults in the use of visual cues. Furthermore, they reported these differences could largely be attributed to diminished (peripheral) visual processing capacities of elderly adults. Here, we reanalyze the data of McCalley and colleagues emphasizing relative rather than absolute differences. We find that when doing so, the data do not reveal differences in the way older and younger adults transiently allocate attention during visual search. Contrary to the conclusions of McCalley and colleagues, the similarity between the younger and older observers is therefore independent of the characteristics of the visual information. Furthermore, in our view the data suggest that older adults have foveal rather than peripheral visual processing difficulties. The results reemphasize the importance of the analytical approach taken in aging research. We discuss the difficulties and relevance of controlling and separating visual and attentional factors in age-related studies.

Aged↗
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↗