Biomedical subjects

B De Bruyn

Publications and source records attributed to B De Bruyn.

16 recordsLinked to original sources

PubMed1998

Human brain activity related to speed discrimination tasks.

The regional cerebral blood-flow (rCBF) pattern of the human brain was measured using positron emission tomography (PET) while subjects viewed, detected, judged the speed of a moving random dot pattern (RDP) or compared speeds of successive RDPs. In all four conditions, retinal input was identical. Two additional conditions, continuous presentation of a moving and a stationary RDP, were included to identify human MT/V5 (hMT/V5). Both speed discrimination tasks involved the right cuneus and right lingual gyrus and to a lesser degree the left lingual gyrus and a more anterior lingual region in the right hemisphere. There was, however, little or no differential activity over hMT/V5 during either speed discrimination. Direct comparison of the two speed discrimination tasks revealed higher activity in the right middle fusiform gyrus, a result reminiscent of that obtained in earlier studies using orientation and direction as the attribute to be discriminated. These results confirm that processing in the human visual cortex is task dependent and underscore the role of the middle fusiform gyrus in temporal comparison of simple attributes.

Adult↗

PubMed1997

Blending transparent motion patterns in peripheral vision.

Human observers are very good at segmenting visual scenes consisting of multiple moving objects. Segregation of transparent motions, however, turns out to be a predominantly central vision process. In peripheral vision transparent motions are blended to form a single novel pattern whose coherent motion corresponds to the average of the separate motions, whilst sensitivity to small differences in coherent motion is maintained. These results point to distinct peripheral processing mechanisms with the advantage of being able to detect changes in motion fields quickly and accurately.

Humans↗

PubMed1996

Task dependency of visual processing in the human visual system.

In this review we contrast passive, attribute driven processing in the visual system with an active, task-dependent view and summarize the evidence from our Positron Emission Tomography (PET) work supporting the task-dependent view. The PET studies involved comparison of regional Cerebral Blood Flow (rCBF) in closely related detection and discrimination tasks. The major finding reported is that the same retinal input or input containing only a single cue activates different extrastriate areas depending on the task.

Adult↗

PubMed1996

The influence of stimulus location on the brain activation pattern in detection and orientation discrimination. A PET study of visual attention.

This PET study concerns changes in regional cerebral blood flow (rCBF) associated with orientation discrimination tasks and with simple detection of the stimulus. The difference in rCBF between discrimination and detection represents the discrimination, or 'task', component of the response. We have examined how such differences are influenced by the position of the visual stimulus and also how they change when a peripheral distractor is added to a relevant central stimulus. We first determined the regions in which the task produced the main effect regardless of stimulus position or distraction with an additional stimulus. In these selected regions, we determined the interactions between task and stimulus position and the interactions between task and stimulus addition. Five regions were more active during orientation discrimination than during stimulus detection: the inferior occipital cortex, the right putamen, the superior parietal lobule, the anterior cingulate cortex and the left lower and the right upper premotor area. Stimulus addition interacted with task only in the lower premotor area. Interactions between the task and stimulus position occurred in the occipital and parietal cortex and in the putamen. When a central stimulus was presented instead of a peripheral one the difference between orientation discrimination and stimulus detection was significantly larger in the inferior occipital lobe and in the right putamen. Conversely, the difference between orientation discrimination and stimulus detection in the superior parietal lobule was significantly larger when a peripheral stimulus was presented instead of a central one.

Adult↗

PubMed1995

A motion area in human visual cortex.

We have localized an area in the human brain involved in the processing of contours defined by motion differences (kinetic contours) by comparing with positron emission tomography the regional cerebral blood flow in tasks performed with kinetic and luminance-defined gratings. These tasks included passive viewing, counting the total number of grating stimuli, and counting the number of gratings of a given orientation. Comparison between the counting tasks and passive viewing with a given type of contour revealed a set of active areas that were similar for both luminance-defined and kinetic contours. Comparisons between these two types of contours revealed a single focus in the right hemisphere that did not overlap with the many regions activated by uniform motion. In particular this "kinetic focus" was clearly separated from the area previously defined as the human homologue of V5/middle temporal. Activity in this kinetic focus was stronger when orientation had to be processed than in the other two tasks. These results and control experiments with uniformly moving random dot patterns suggest the existence of an area in the human visual system that is activated much more by kinetic contours than by luminance contours or uniformly moving random dots. Up to now, such an area has not been described in the monkey visual system.

Animals↗

PubMed1995

Asymmetric spatial distributions of motion vectors yield characteristic errors in direction judgements.

For a continuous flow field depicting a combined translation and expansion, there exists a natural distribution of local motion directions whose mean direction corresponds to the direction of translation. A random sampling of this distribution may introduce spatial asymmetries and thus alter the mean direction. This statistical phenomenon has a perceptual parallel: the spatial distribution of the dots becomes relevant with respect to the perceived global direction. The perceived direction of motion corresponds to the mean direction, not to the actual direction of translation.

Humans↗

PubMed1995

Perceptual latency and complex random-dot stereograms.

Naive observers of random-dot stereograms depicting complex surfaces often find that they require several tens of seconds before the impression of depth emerges. With practice, however, perception times often decrease markedly: perceptual learning occurs. Current explanations of these effects were assessed in two experiments. In the first experiment the perception times of naive observers for random-dot stereograms which depicted the same complex shape but contained different ranges of disparity were measured. In the second experiment the minimum times required by experienced observers to perceive a given complex shape in stereograms that contained different ranges of disparity were determined. Perception times for the naive observers were all very fast (<3 s) and showed no evidence of perceptual learning. There was no effect of disparity range on perception times in either experiment. It was found that very large-disparity (80 min arc) complex stereograms could be perceived quickly, even by naive observers. It is concluded that the long initial latencies previously reported are not due to surface complexity nor to the range of disparities present. Other factors. such as dot size, dot density, and the correlation of the stereo images, appear to be important determinants of efficient stereoscopic performance when viewing complex random-dot stereograms.

Depth Perception↗

PubMed1994

Many areas in the human brain respond to visual motion.

1. The regions of the human brain responsive to motion were mapped using the H2(15)O position emission tomography (PET) activation technique and compared by viewing a moving random dot pattern with a stationary dot pattern. The stimulus was optimized in dot density and 3 degrees in diameter. 2. In addition to bilateral foci at the border between Brodmann areas 19 and 37, a V1/V2 focus and a focus in the cuneus reported earlier, we observed activations in other visual areas (lower BA 19 and the parieto-occipital fissure) in the cerebellum and in two other, presumed vestibular areas, the posterior bank of lateral sulcus and at the border of BA 2/40. 3. Homologies between monkey and human cortex are discussed.

Adult↗

PubMed1993

Segregation of spatially superimposed optic flow components.

The transparency phenomenon, which arises when a radial and a rotational motion pattern are spatially superimposed, suggests that these motion patterns are processed independently. Indeed, for unrestricted stimulus durations, observers could identify the rotational pattern as clockwise or counterclockwise, and the radial pattern as expansion or contraction, even under uncertainty. However, when the time available to process the compound stimulus was equal to the minimum duration required to identify each of the patterns when presented in isolation, identification was impaired. Whereas for spirallike motion patterns the radial and rotational components could be identified, radial and rotational motion patterns were not processed independently when superimposed. Although radial and rotational transformations could not be identified simultaneously, a coherent optic flow pattern could be segregated from another superimposed optic flow component given definite foreknowledge.

Attention↗

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

The importance of velocity gradients in the perception of three-dimensional rigidity.

Sequential presentation of a number of random-dot patterns which when super-imposed yield an expanding flow field leads to the perception of a coherent motion towards the observer. The motion vectors in this type of flow field all radiate from the origin. This percept of a global coherent expanding flow results only when the local speeds (magnitude of the local motion vectors) are zero at the centre and increase linearly towards the periphery. If all the dots radiate outwards but have the same speed, a clear percept of three-dimensional nonrigidity arises.

Acceleration↗

PubMed1990

The role of direction information in the perception of geometric optic flow components.

Theoretically, optic flow, an important source of information for the perception of locomotion and three-dimensional structure of the environment, is described in terms of divergence, curl, and shear components. We measured how the detection of the type of flow field depends on directional information. We manipulated the local directions by rotating them through an angle x relative to the original direction (i.e., the direction of motion at that locus in an unaltered flow field). The results of the first experiments showed that divergence, curl, and shear can be detected even if the directional range of the individual motion vectors is as broad as 180 degrees. Subsequent experiments revealed that the detection of the geometric components of the optic flow field is merely based on the integration of a few (10% of vectors) local directions correctly (within 10 degrees of original direction) specifying the type of flow field. Other directions are irrelevant to this process. This is actually what one would expect if the optic flow is analyzed by special purpose mechanisms that detect and process the geometric components on the basis of the integration of motion information. The results indicate that as far as they integrate motion information, detectors for divergence, curl, and shear operate in a similar manner. Implications of the results for modeling such mechanisms are discussed.

Adult↗

PubMed1989

Discrimination of opposite directions measured with stroboscopically illuminated random-dot patterns.

The aim of the present experiments was to investigate the maximum displacement over which coherent motion can be perceived (Dmax) for a stroboscopically illuminated moving random-dot pattern, both as a function of the size of the pattern and as a function of the spatial-frequency content. Perception of coherent motion was defined operationally as the ability to discriminate opposite directions. The maximum velocity (Vmax) for which the observer could discriminate the direction of motion for a given strobe rate was obtained by using a forced-choice staircase procedure. Vmax was measured as a function of the strobe rate for four different sizes of the pattern. For each pattern size Vmax was limited by the spatial asynchrony between the flashes (Dmax). For a given size of the pattern, Dmax did not depend on the temporal asynchronies tested (10-100 msec). Dmax values were increased with the size of the pattern. Bandpass rectangular filtering was used to change the spatial-frequency content of the pattern. There was a gradual increase in Dmax as the spatial-frequency content shifted to low spatial frequencies. Interestingly, the Dmax that was obtained with the unfiltered pattern was larger than that obtained with the high-pass pattern but also smaller than that obtained with the low-pass pattern. The present results are in agreement with results obtained with a two-frame display. Dmax is not a fixed value: it depends on the size of the pattern as well on the spatial-frequency content.(ABSTRACT TRUNCATED AT 250 WORDS)

Discrimination, Psychological↗

PubMed1988

Human velocity and direction discrimination measured with random dot patterns.

In the present experiments three different motion discrimination tasks were studied using a random dot pattern as stimulus: velocity discrimination, direction discrimination and discrimination of opposite directions. The analysis of the motion of random dot patterns is based on motion sensitive mechanisms without the confounding interference of position sensitive mechanisms (Nakayama and Tyler, 1981). Furthermore, since isotropic random dot patterns contain no dominant orientation, a change in the direction of motion does not parallel a change in orientation. Hence the use of a random dot pattern as stimulus allows velocity and direction discrimination to be compared. Human velocity discrimination displays a U-shaped dependence on the stimulus velocity: the JNDs, expressed as Weber-fractions, are minimal for velocities ranging from 4 to 64 deg.sec-1. The Weber-fractions in velocity, determined with a staircase procedure tracking a 84% correct response level, were about 7% at the optimal speeds. The velocity discrimination curve obtained with the random dot pattern is similar to that obtained with light bars. Human direction discrimination, defined as the smallest difference in direction which can be resolved, also displays a U-shaped dependence on the stimulus velocity. Direction discrimination thresholds decrease up to a velocity of 4 deg.sec-1, they then stay at a constant level up to 128 deg.sec-1. Beyond this velocity the thresholds increase again. The mean direction discrimination threshold was 1.8 deg at optimal speeds. Discrimination of opposite directions, determined for the same conditions as those for which velocity and direction discrimination thresholds were determined, was better than the 90% response level at all speeds. However at low contrast, opposite directions are reliably discriminated only at intermediate speeds. Perceiving a coherent moving random dot pattern is supposed to be based on a cooperation between a large number of local motion detectors. In order to evaluate the importance of detector output pooling, the influence of the size of the pattern and of the presentation time on the three discrimination tasks was measured. The results indicate that the pooling requirements are task dependent. A somewhat larger pooling is required for velocity discrimination than for direction discrimination, whereas for discrimination of opposite directions only a few local motion detectors are involved.

Differential Threshold↗

PubMed1985

Velocity discrimination in central and peripheral visual field.

Just-noticeable differences (jnd's) in velocity were measured as a function of reference velocity for central and peripheral vision. The velocity discrimination curves plotting jnd's in velocity, expressed as Weber fractions, as a function of reference velocity were U shaped at all eccentricities. Under almost every stimulus condition the increase in jnd in velocity with increasing eccentricity was significantly larger at low reference velocities than at high reference velocities. Consequently the shift toward higher velocities with increasing eccentricity was much clearer for the lower end of the velocity-discrimination curve than for the upper end. These results are in agreement with the predictions derived from the response characteristics of velocity-tuned cells. Control experiments involving direction discrimination have shown that the impossibility of making fine velocity judgments at high speeds is due not to too weak a contrast for the stimulus motion to be visible but to a limitation in the neural apparatus analyzing velocity.

Adult↗

PubMedDate not supplied

The kinetic occipital region in human visual cortex.

In the present study we showed that the kinetic occipital (KO) region, located laterally in occipital cortex approximately 20 mm behind human MT/V5, can be strongly and bilaterally activated under passive viewing conditions. We used continuous, randomly changing visual stimulation to compare kinetic gratings to uniform motion and kinetic gratings to luminance defined gratings. The KO activations under these passive conditions are stronger than those observed when the two types of gratings are compared under active conditions, i.e. while subjects perform a task (counting gratings of a given orientation). Region KO was shown to process both shape and motion information, the conjunction of which is typically present in kinetic contours. Area MT/V5 also processes these two aspects of visual stimulation but favors motion signals. Clear segregation of shape and motion processing was observed only in occipitotemporal and parietal regions respectively. Although neurons with properties similar to those derived from the conditions activating the KO region have been documented in the macaque monkey, their location seems inappropriate for them to correspond to the KO activation observed in humans.

Adult↗