Biomedical subjects

J Wagemans

Publications and source records attributed to J Wagemans.

At least 19 recordsLinked to original sources

PubMed2001

Can neuroimaging really tell us what the human brain is doing? The relevance of indirect measures of population activity.

Neuroimaging studies using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) give an indication towards the localization of mental representations and processes in the human brain. It is not clear to what extent such global measures of neuronal activity, pooling across large populations of neurons, can reveal how certain computations are implemented by the neurons in such population ('computational neuroimaging'). Population activity is related tightly to single-cell activity when all neurons in the population have similar response properties. We describe some evidence from single-cell recordings in monkeys that indicates that neurons with similar response properties are not scattered randomly throughout the visual cortex. Notwithstanding this clustering, populations of nearby neurons are still rather heterogeneous, requiring some prudence in deriving single-cell response properties from population activity. The following review of recent neuroimaging studies of the visual system describes to what degree inferences about computations and representations can be drawn from these studies.

Animals↗

PubMed2001

Different effects of lorazepam and diazepam on perceptual integration.

Recent research has established the detrimental effect of lorazepam, a benzodiazepine, on both implicit and explicit memory. Furthermore, lorazepam is known to affect perceptual integration. Diazepam, on the other hand, though being a benzodiazepine too, only impairs explicit memory, leaving implicit memory fairly intact. Little is known about the effect of diazepam on perceptual integration. The present study aimed at filling in this gap, by comparing the effects of lorazepam and diazepam on the detection of discontinuities in random-shaped outlines. In line with previous findings, the results in a lorazepam-treated group were quite different from the results in a placebo-treated group. The results in a diazepam-treated group were analogous to the results in the placebo-treated group and different from the results in the lorazepam-treated group. This shows that lorazepam and diazepam differ, not only with respect to their effect on implicit memory, but also with respect to their effect on perceptual integration. It is argued that this bears important consequences for memory research that makes use of a pharmacological dissociation rationale.

Adult↗

PubMed2001

Matching multicomponent objects from different viewpoints: mental rotation as normalization?

Novel multicomponent objects were created, and 3 distractors were created for each object by changing the relations between the parts of the object. In a set of 5 experiments, target objects were presented as a motion sequence of multiple views or as a single view. Participants were asked to determine whether an image of an object, viewed from another viewpoint, was the same as the target object. The axis of rotation was aligned with one of the environmental axes or with the main axis of the object. The effects of viewpoint on performance imply that the matching of objects is viewpoint dependent and requires a process of normalization to undo the differences between the perceptual description and the stored object descriptions. The lack of a systematic effect of the axis of rotation, however, suggests that this normalization is best understood as not involving a 3-D transformation of stored 3-D object models.

Adolescent↗

PubMed2001

Inferotemporal neurons represent low-dimensional configurations of parameterized shapes.

Behavioral studies with parameterized shapes have shown that the similarities among these complex stimuli can be represented using a low number of dimensions. Using psychophysical measurements and single-cell recordings in macaque inferotemporal (IT) cortex, we found an agreement between low-dimensional parametric configurations of shapes and the representation of shape similarity at the behavioral and neuronal level. The shape configurations, computed from both the perceived and neuron-based similarities, revealed a low number of dimensions and contained the same stimulus order as the parametric configurations. However, at a metric level, the behavioral and neural representations deviated consistently from the parametric configurations. These findings suggest an ordinally faithful but metrically biased representation of shape similarity in IT.

Action Potentials↗

PubMed2001

The role of the centre of projection in the estimation of slant from texture of planar surfaces.

Displays were presented consisting of a perspective projection of a regular square grid, made up of vertical and horizontal equally spaced white lines, that was slanted in depth. The surface was viewed monocularly, through a circular aperture. A range of slants was shown (0 degree, 10 degrees, 20 degrees, 30 degrees, 40 degrees, 50 degrees, or 60 degrees) and the observers' task was to match the slant by means of a mouse-driven probe. The viewing distance (50, 75, or 100 cm) as well as the focal distance (25, 50, 75, 100, or 125 cm) were varied. We expected the estimation error to be smallest when the viewing distance and the focal distance coincided. This was not the case. Instead, subjects seemed to use the perspective deformation of the texture elements in the stimulus display to make a slant estimation, regardless of the specific combination of viewing distance and focal distance.

Contrast Sensitivity↗

PubMed2001

Visual object categorisation at distinct levels of abstraction: a new stimulus set.

We designed a new stimulus set with 269 line drawings of everyday artifacts and animals. The stimulus set contains several typical exemplars from a sample of 25 basic-level categories. We determined to what extent these stimuli were named at the basic level and at a more subordinate level. An additional experiment showed the validity of this calibration: typicality ratings were correlated significantly with the level of naming. In a final experiment we found that this effect depends largely on the global configuration of a stimulus as it was still apparent with degraded images obtained by locally shifting small fragments of the drawings.

Adult↗

PubMed2001

Multiple routes to object matching from different viewpoints: mental rotation versus invariant features.

Previous research has shown that object recognition from different viewpoints often yields strong effects of viewpoint. However, for some objects and experimental paradigms almost complete viewpoint invariance is obtained. This suggests the existence of multiple routes to object recognition. In this study we further strengthen this notion by designing two different conditions using the same experimental paradigm (simultaneous matching) and highly similar objects (multiblock figures). In the first condition (involving a handedness violation), strong effects of viewpoint were obtained. In the second condition (involving an invariance violation), the effects of viewpoint were negligible. This result illustrates that asking under what circumstances object recognition is viewpoint dependent or independent is more fruitful than attempting to show that object recognition is either viewpoint dependent or independent.

Adult↗

PubMed2000

The representation of shape in the context of visual object categorization tasks.

To investigate the role of human fusiform gyrus in shape processing, we determined the effect of shape degradation on BOLD contrast in this region with fMRI during three tasks requiring subjects to determine either whether two successively presented nonsense shapes had the same global orientation (OR task); whether two successively presented meaningful objects belonged to the same basic level category (CAT task); or whether two successively presented objects represented the same exemplar of a category (EX task). On the behavioral level, shape degradation by locally shifting the pixels constituting the lines of stimuli had no effect on performance in the OR task, while it was detrimental to performance in the CAT and EX tasks. In comparison to the OR task, both the CAT and EX tasks were associated with activations in the occipitotemporal and parietal cortex. When shape degradation was applied, activation in the middle fusiform gyrus was reduced in all tasks. The occurrence of this effect in the OR task indicates that it is independent of memory representations. The persistence of the effect in both tasks that showed a behavioral effect of degradation suggests that it does not reflect the amount of shape processing performed on the stimuli, but rather the specificity of the final perceptual representation that can be built from the shape information that is available. Other studies have shown effects of stimulus familiarity and task requirements in the fusiform gyrus, suggesting that there is no need to assume different modules for perceptual representation and representation in memory.

Adult↗

PubMed2000

Effect of benzodiazepine on temporal integration in object perception.

RATIONALE: Though various psychometrical tests indicate that benzodiazepines affect vigilance, few studies have been conducted to assess the effect of benzodiazepines on attentional processes. OBJECTIVE: We used a RSVP (Rapid Serial Visual Presentation) procedure to investigate the effect of benzodiazepines on the attentional blink effect. It refers to the difficulty in detecting a probe following identification of a target within a temporal window of 500 ms. METHOD: Three experimental groups were tested (placebo, lorazepam and diazepam). Sequences of 15 pictures were centrally displayed for 50 ms each. In a dual-task condition, observers were instructed (1) to identify the target (the single picture on a blue background) and (2) to detect the presence of a probe. In the single-task condition, subjects were asked to detect the probe. The serial position of the probe relative to the target was varied. RESULTS: Performance was equivalent for the three groups in the single-task condition. In the dual-task condition, the attentional blink was increased in magnitude and duration for benzodiazepine-treated subjects, especially diazepam, than for placebo-treated subjects. A large number of intrusions (a tendency to report as target the name of a picture preceding the target) were observed in the benzodiazepine-treated groups. CONCLUSION: The results indicate that benzodiazepines impair visual integration in the temporal domain. This extends previous findings that benzodiazepine impairs visual integration in the spatial domain. The results also suggest that benzodiazepine increase time to disengage attention from a first to a second target.

Adolescent↗

PubMed2000

The viewpoint-dependency of veridicality: psychophysics and modelling.

Human observers were shown projected angles, embedded in solid cross-like figures and were asked whether these projected angles could be the projection of an orthogonal angle in 3-D space (i.e. whether the two legs of the cross were orthogonal to each other). We found that performance depended on the viewpoint at which the angle was viewed: Both slant (i.e. the angle between the normal of the target angle relative to the plane of projection) and roll (i.e. the rotation around the normal of the target angle) had a systematic effect on the proportion of errors when observers were shown non-orthogonal angles. With orthogonal angles, however, this effect was absent (i.e. very low error rate with no systematic effect of slant and roll). Instead of assuming a viewpoint-dependent bias towards orthogonality, a computational analysis of the task, using a Bayesian approach, and a computer simulation showed that the viewpoint-dependency can be modelled by a fixed set of biases in order to constrain the set of possible scenes that could give rise to the projection.

Computer Graphics↗

PubMed2000

Minimal information to determine affine shape equivalence.

Participants judged the affine equivalence of 2 simultaneously presented 4-point patterns. Performance level (d') varied between 1.5 and 2.7, depending on the information available for solving the correspondence problem (insufficient in Experiment 1a, superfluous in Experiment 1b, and minimal in Experiments 1c, 2a, 2b) and on the exposure time (unlimited in Experiments 1 and 2a and 500 ms in Experiment 2b), but it did not vary much with the complexity of the affine transformation (rotation and slant in Experiment 1 and same plus tilt in Experiment 2). Performance in Experiment 3 was lower with 3-point patterns than with 4-point patterns, whereas blocking the trials according to the affine transformation parameters had little effect. Determining affine shape equivalence with minimal-information displays is based on a fast assessment of qualitatively or quasi-invariant properties such as convexity/ concavity, parallelism, and collinearity.

Adult↗

PubMed2000

The influence of two-dimensional stimulus properties in a reconstruction task with minimal information stimuli.

Effects of viewpoint that are typically found in shape recognition tasks have been interpreted in the past as strong evidence against approaches based on the use of geometric invariants by the human visual system. At first sight, the use of such invariants would indeed predict shape recognition to be viewpoint independent. It has been argued before, however, that the visual estimation of invariants need not itself be invariant. Changes of viewpoint introduce changes in 2D stimulus features. The latter may make the estimation of invariants by the visual system more or less difficult. The present study explores how such 2D features influence estimation distributions in a reconstruction paradigm. Subjects were shown four coplanar dots. They were asked to estimate the position of one of the dots relative to the three others on the basis of a slanted version of the configuration. An estimation distribution was obtained for every position of the dot to be estimated. Biases in these distributions indicated that subjects performed the task by using 2D reference axes that were unambiguously identifiable in both the images of the dot pattern.

Form Perception↗

PubMed1999

Visual measurement of relative distances between three collinear dots rotating in a slanted plane.

Displays consisted of orthographic and perspective projections of three collinear dots rotating rigidly around a fixed centre in a plane slanted 45 degrees in depth. Observers were asked to decide whether the middle of the three dots was exactly centred in 3-D space between the other two dots. The visible rotation segments were 120 degrees, 160 degrees, or 200 degrees and the displacements were 2%, 4%, or 6%. Our untrained observers performed more poorly overall than well-practised observers tested earlier by Lappin and Fuqua (1983 Science 221 480-482). Results of additional manipulations suggested that 2-D image properties played a more important role than acknowledged by Lappin and Fuqua. First, displacement size and the position of the rotation centre produced nonlinear effects. Second, the direction of displacement affected performance. Third, projection type and display type (static snapshots versus elliptic traces) were involved in complex interactions. Clearly, performance was affected by 2-D image properties that should be irrelevant to the calculation of 3-D projective invariants.

Cues↗

PubMed1998

On the lawfulness of grouping by proximity.

The visual system groups close things together. Previous studies of grouping by proximity have failed to measure grouping strength or to assess the effect of configuration. We do both. We reanalyze data from an experiment by Kubovy and Wagemans (1995) in which they briefly presented multi-stable dot patterns that can be perceptually organized into alternative collections of parallel strips of dots, and in which they parametrically varied the distances between dots and the angles between alternative organizations. Our analysis shows that relative strength of grouping into strips of dots of a particular orientation approximates a decreasing exponential function of the relative distance between dots in that orientation. The configural or wholistic properties that were varied--such as angular separations of the alternative organizations and the symmetry properties of the dot pattern--do not matter. Additionally, this grouping function is robust under transformations of scale in space (Experiment 1) and time (Experiment 2). Grouping of units which are themselves the result of grouping (i.e., pairs of dots; Experiment 3) also follows our nonconfigural rule.

Adult↗

PubMed1998

Lorazepam but not diazepam impairs identification of pictures on the basis of specific contour fragments.

We tested the effects of lorazepam 0.038 mg/kg and diazepam 0.3 mg/kg on the identification of pictures of everyday objects that were specifically modified to examine the role of different parts of the external contour. By pressing the space bar of a computer keyboard, observers could add 1% of the total contour of each picture until it was recognized. Identification thresholds were measured in three display conditions, depending on where along the contour the addition of contour pixels started. In the minima condition, stimuli were initially displayed with only minima (i.e., locations along the contour where negative curvature is strongest); all parts with negative curvature were then built up gradually from the minima and only later on were the fragments with positive curvature shown until the contour became closed at the maxima (i.e., locations where positive curvature is strongest). In the maxima condition, initially only the maxima were displayed, with all positive contour built up first and then the negative curvature until the minima were reached. In the inflections condition, the points along the contour shown first were inflections (i.e., points where curvature is locally zero because the sign of curvature changes there) and contour was built up by adding parts of positive and negative curvature at both sides of each inflection until the extrema (minima and maxima) were reached to close the contour of the picture. In general, picture identification was more difficult (i.e., a larger portion of the contour was required) in the minima condition than in the maxima and the inflections conditions. The diazepam group did not differ significantly from the placebo group, while the lorazepam group had significantly lower performance in all three display conditions. Results are discussed in relation to previous research showing impaired perceptual integration and impaired implicit memory under lorazepam influence.

Adult↗

PubMed1998

Effects of physical connectivity on the representational unity of multi-part configurations.

While physical connection strongly supports perceptual integration of visual elements or parts into single objects, concavities along an object's contour strongly induce segmentation of objects into different parts. By means of a matching task, we examine whether the segmentation of connected parts and the grouping of disconnected parts imply differences in representational unity. All match trials consisted of either two identical configurations or two 180 degrees rotated versions of an otherwise identical configuration, whereas mismatch trials had one misoriented part. Results show that the 180 degrees matches with the physically disconnected parts were relatively slower than similar matches with physically connected parts, although the connections could be minimal (deep concavities, corner to edge connection). This suggests that the mental operation required for the 180 degrees match has more difficulty with the disconnected parts, indicating a measurably lower degree of representational unity.

Adult↗

PubMed1997

Higher-level cortical processing of colour.

Wavelength information serves both to organise shapes and to provide the appearance of surfaces. These two roles are considered within a model that outlines the role of colour in picture naming (Davidoff and de Bleser, 1993). An introduction to the papers in this issue is given within the framework of the model. The topics in this issue cover the role of colour in scene segmentation, colour constancy and the memory for the colours of objects.

Cerebral Cortex↗

PubMed1997

Perceptual grouping measured by color assimilation: regularity versus proximity.

Two powerful grouping principles in visual perception have been studied rather frequently, namely, regularity and proximity. In many cases, however, the impact of either regularity or proximity was examined. The goal of the present research is to provide and test an experimental paradigm on the basis of which the relative strengths of these grouping factors can be studied. The proposed paradigm is based on early Gestalt observations that colors of local elements within a perceptual whole tend to look more like each other than they actually are. Three experiments were set up to test whether this phenomenon of color assimilation can provide a means to compare perceptual groupings induced by regularity and proximity. In all experiments, the stimuli were linear configurations of colored elements with variable distances between the elements. The task of the participants was to compare the color of a single test element with the colors of two different groups of elements. In Experiment 1, for all colors, the color of the test element was judged to be more similar to the color of the elements with which it can be grouped by means of regularity. In Experiment 2, for specific colors, the color of the test element tended to be more similar to the elements with which it can be grouped by means of proximity. Finally, in Experiment 3, configurations with a greater variety of positions were tested, including those yielding conflicting groupings, that is, on the basis of regularity and proximity different groupings were to be expected. The color judgements for these stimuli clearly reflected the competition between the two factors.

Color Perception↗