Biomedical subjects

P De Weerd

Publications and source records attributed to P De Weerd.

11 recordsLinked to original sources

PubMed1995

Responses of cells in monkey visual cortex during perceptual filling-in of an artificial scotoma.

When we view a scene through one eye, we typically do not see the scotomas created by the optic disc and the blood vessels overlying the retinal surface. Similarly, when a texture field containing a hole is steadily viewed in peripheral vision (artificial scotoma), the hole appears to fill in with the surrounding texture in a matter of seconds, demonstrating that the visual system fills in information across regions where no information is available. Here we show that, in monkeys viewing a similar texture field with a hole, the responses of extrastriate visual neurons with receptive fields covering the hole increase gradually to a level comparable to that elicited by the same texture without a hole. The time course of these dynamic changes in activity parallels the time course of perceived filling-in of the hole by human observers, suggesting that this process mediates perceptual filling-in.

Animals↗

PubMed1994

Two stages in visual texture segregation: a lesion study in the cat.

We have used five cats to investigate the effects of two distinct visual cortex lesions on the segregation of two different texture stimuli. The ablation of areas 17 and 18 (tier I) severely impaired the segregation between textures made of line elements differing in orientation, but spared the segregation between annulus and dot textures. In contrast, the ablation of those areas receiving direct afferents from areas 17 and 18 (tier II) destroyed the segregation for both texture stimuli. Strong deficits remained up to 1 year after the lesion, although limited recovery was observed after tier II lesions. We suggest that tier I areas are involved in the local filtering of the texture elements, and that tier II areas compute texture differences on the basis of the filtered image provided by tier I areas. The crucial contribution to texture segregation of visual areas belonging to a second level in the cortical hierarchy challenges the notion that texture segregation is entirely an early process in vision.

Animals↗

PubMed1993

Effects of visual cortex lesions on orientation discrimination of illusory contours in the cat.

We have trained five cats in orientation discrimination using different contours, and compared the deficits caused by lesions of cortical areas 17 and 18 (tier I) to the deficits induced by removal of those areas receiving afferents originating in areas 17 and 18 (tier II). As contour stimuli we used two types of illusory contours and a luminance bar. The two illusory contours were defined by opposed line-ends. One of them coincided with a luminance gradient whereas the other did not. Tier I lesions destroyed the capacity to discriminate the orientation of both illusory contours, and also caused an important, though less severe, deficit in bar orientation discrimination. The deficits induced by tier I lesions were permanent. Tier II lesions also caused significant deficits in orientation discrimination of illusory contours, but only a negligible deficit in bar orientation discrimination, and this result was not a mere consequence of a difference in difficulty between the tasks involving bars and illusory contours. In addition, tier II lesions differentiated between illusory contour types, the deficit being more pronounced for the illusory contour without luminance gradient than for the one with luminance gradient. In contrast to tier I lesions, tier II lesions allowed significant recovery, leading to small final deficits for all contour types tested.

Animals↗

PubMed1993

Occlusion cues contribute to orientation judgments of occlusion-defined contours.

Occlusion cues defining a contour in a 2-D stimulus pattern were shown to contribute to the accuracy of orientation judgments of that contour. The stimulus pattern was altered so that the occlusion cues became ambiguous, by introducing a textured background suggesting transparency of the stimulus pattern. Orientation judgments then became significantly less accurate. This finding shows that occlusion cues in 2-D patterns can be behaviorally relevant, in addition to generating the subjective percept commonly known as an illusory contour. The disruptive effect of the textured background on orientation judgments remained when no texture elements were present in the vicinity of the contour. This suggests that the generation of occlusion-defined contours relies as much on an evaluation of the surfaces at either side of the contour as being opaque as it does on local encoding of occlusion cues close to the contour. Finally, orientation sensitivity measured with contours defined by other than occlusion cues was not altered after the introduction of a textured background.

Female↗

PubMed1992

Texture segregation in the cat: a parametric study.

We have investigated how different texture parameters affect texture segregation in the cat, and which strategies cats use to solve the segregation task. Five cats were presented with stimuli consisting of two adjacent panels. One side contained a square area of a particular texture embedded in a different background texture; the other side was filled with only the background texture. The animal's task was to detect at which side the texture difference was presented. Sensitivity for the texture difference was assessed by making one aspect of the texture (in most instances the size of the texture elements) dependent upon performance by means of a staircase procedure. Among the most prominent parametric effects are those of density and element position randomization. In general, segregation was optimal at intermediate densities and deteriorated at larger and smaller densities. Element position randomization caused a slight but systematic decrease in segregation performance. Furthermore, we found texture elements at the border between different textures to be of primary importance for segregation. Which strategy the animals used for solving the segregation task depended upon the presence of random figure/background reversals in subsequent stimulus presentations during training. The animals learned to detect texture differences if these reversals were present, and without reversals, they learned to identify the particular texture in the target square. Interestingly, parameter dependencies of segregation did not depend upon the detection strategy used. We have speculated that the two different strategies used by the cats to solve the segregation tasks are related to different hierarchical levels of texture segregation which can be traced back to different stages of texture processing in human models of segregation performance.

Animals↗

PubMed1992

Epitope regions on U1 small nuclear RNA recognized by anti-U1RNA-specific autoantibodies.

Autoantibodies specifically directed to U1RNA were found in patients suffering from systemic lupus erythematosus (SLE) overlap syndromes. To obtain more insight in the mechanism responsible for this U1RNA-specific antibody formation and to use the antibodies eventually as a tool to study U1RNA-protein (U1RNP) interactions, the B cell epitopes on U1RNA were mapped. Using in vitro synthesized domains of U1RNA, the main epitope regions were found in stemloops II and IV. Furthermore, 3'-end or 5'-end truncation of both stemloop II and stemloop IV showed that the conformation of the stemloops is critical for antibody recognition. Mutant studies on both stemloops indicated that in the case of stemloop II the stem is the main antigenic region, whereas in stemloop IV, the loop (E-loop) is a main target. The results of this study support the idea that the anti-U1RNA autoantibody could be the result of a process driven by the human U1RNP complex itself (antigen-driven process).

Antibody Affinity↗

PubMed1990

Staircase procedure and constant stimuli method in cat psychophysics.

We measured 73.5% correct just noticeable differences (JNDs) in bar orientation with the method of constant stimuli and with a Wetherill and Levitt staircase procedure, using a total of 25 cats. For the same number of trials per threshold assessment, the variability of the threshold remained independent of the testing method used. However, the JNDs measured using the method of constant stimuli were significantly influenced by the range of the orientation differences (ODs) utilized for measuring the JND. This effect was particularly large in incompletely trained cats, but it also was significant in extensively trained subjects. On the other hand, staircase threshold measurements were not affected by the starting OD, independently of how well the animals had been trained. This shows that the staircase procedure is a more efficient instrument with which to measure JNDs in orientation than is the method of constant stimuli. With the staircase procedure, we found that the JNDs measured at oblique reference orientations did not exceed those measured at principal reference orientations (no oblique effect). Two earlier studies from this laboratory using the method of constant stimuli did report an oblique effect. Our data suggest that this oblique effect might stem from a less efficient training at the right oblique reference orientation in these studies, combined with a relatively inefficient testing procedure such as the constant stimuli method.

Animals↗

PubMed1990

Illusory contour orientation discrimination in the cat.

We present the first evidence that a non-human species (the cat) is able to discriminate the orientation of illusory contours. Following Vogels and Orban45, we used two types of illusory contours. In one type, the illusory contour was defined by a number of contour-inducing semicircles, of which the endpoints were separated by a gap. In the other pattern, the inducing semicircles were shifted in phase along their diameter and their endpoints were aligned along the contour. Just noticeable differences in orientation were measured (at the 73.5% correct level), using a Wetherill and Levitt49 staircase procedure. Values in the order of 11 degrees were obtained when using the first type of illusory contour. Just noticeable differences with the second type were in the order of 17 degrees. Reducing the salience of the illusory contour, whether by scrambling the contour, or by decreasing the number or the contrast of inducing semicircles, systematically increased discrimination thresholds.

Animals↗

PubMed1990

Bar orientation discrimination in the cat.

We have measured orientation-discrimination thresholds of 4 deg in the cat, confirming an earlier study of Vandenbussche and Orban (1983). Unlike Vandenbussche and Orban (1983), we found that orientation-discrimination performance is not better at principal, as compared to oblique, reference orientations (no oblique effect). Despite the absence of the oblique effect, and despite the discrimination thresholds which were elevated by a factor of 4 compared to humans, orientation-discrimination performance of cats and humans is qualitatively similar in a number of aspects. First, orientation-discrimination performance as a function of length and contrast is qualitatively similar to human performance. Second, as in humans, detection and discrimination of the stimuli are closely related. Finally, randomizing the contrast between the stimuli does not affect orientation-discrimination performance. This suggests that similar computations underlay orientation-discrimination performance in both species. In summary, our results confirm that the cat is a useful model for human orientation-discrimination performance.

Animals↗

PubMed1990

Orientation discrimination in the cat: a distributed function.

Cats were trained to make fine orientation discriminations with stimuli similar to those used in physiological experiments--narrow, light bars 12 degrees long--before and after various combinations of lesions of areas 17 and 18. Discrimination thresholds were measured at different contrast levels and different bar widths, both pre- and postoperatively, for up to 1.5 years after the lesion. For high contrast stimuli, lesions restricted to area 17 or area 18 had little effect, but those lesions involving area 17 and a substantial part of area 18 raised thresholds. In the latter case there was a relationship between the amount of area 18 spared and the bar width at which discrimination was impaired. At low contrast deficits were seen only for narrow widths. These results lead to the following conclusions. (i) Orientation discrimination is a function distributed within and across areas 17 and 18. (ii) How this function is distributed in this cortex depends on stimulus width. (iii) The X system does not carry the signal necessary for orientation discrimination. (iv) Cells most narrowly tuned for orientation, which reside in the part of area 17 subserving central vision, do not determine the orientation discrimination threshold.

Animals↗

PubMed1988

Stimulus contrast and visual cortical lesions.

Intact animals can make fine orientation discriminations over a wide range of contrasts. After ablation of area 17 deficits in orientation discrimination are observed only at low contrast. The relevance of this finding for the design of sensitive ablation experiments is discussed.

Animals↗