Biomedical subjects

R van der Zwan

Publications and source records attributed to R van der Zwan.

16 recordsLinked to original sources

PubMed2006

Auditory motion affects visual biological motion processing.

The processing of biological motion is a critical, everyday task performed with remarkable efficiency by human sensory systems. Interest in this ability has focused to a large extent on biological motion processing in the visual modality (see, for example, Cutting, J. E., Moore, C., & Morrison, R. (1988). Masking the motions of human gait. Perception and Psychophysics, 44(4), 339-347). In naturalistic settings, however, it is often the case that biological motion is defined by input to more than one sensory modality. For this reason, here in a series of experiments we investigate behavioural correlates of multisensory, in particular audiovisual, integration in the processing of biological motion cues. More specifically, using a new psychophysical paradigm we investigate the effect of suprathreshold auditory motion on perceptions of visually defined biological motion. Unlike data from previous studies investigating audiovisual integration in linear motion processing [Meyer, G. F. & Wuerger, S. M. (2001). Cross-modal integration of auditory and visual motion signals. Neuroreport, 12(11), 2557-2560; Wuerger, S. M., Hofbauer, M., & Meyer, G. F. (2003). The integration of auditory and motion signals at threshold. Perception and Psychophysics, 65(8), 1188-1196; Alais, D. & Burr, D. (2004). No direction-specific bimodal facilitation for audiovisual motion detection. Cognitive Brain Research, 19, 185-194], we report the existence of direction-selective effects: relative to control (stationary) auditory conditions, auditory motion in the same direction as the visually defined biological motion target increased its detectability, whereas auditory motion in the opposite direction had the inverse effect. Our data suggest these effects do not arise through general shifts in visuo-spatial attention, but instead are a consequence of motion-sensitive, direction-tuned integration mechanisms that are, if not unique to biological visual motion, at least not common to all types of visual motion. Based on these data and evidence from neurophysiological and neuroimaging studies we discuss the neural mechanisms likely to underlie this effect.

Acoustic Stimulation↗

PubMed2001

Orientation processing mechanisms revealed by the plaid tilt illusion.

The tilt after-effect (TAE) and tilt illusion (TI) have revealed a great deal about the nature of orientation coding of 1-dimensional (1D) lines and gratings. Comparatively little research however has addressed the mechanisms responsible for encoding the orientation of 2-dimensional (2D) plaid stimuli. A multi-stage model of edge detection has recently been proposed [Georgeson, M. A. (1998) Image & Vision Computing, 16(6-7), 389-405] to account for the perceived structure of a plaid stimulus that incorporates extraction of the zero-crossings (ZCs) of the plaid. Data is presented showing that the ZCs of a plaid inducing stimulus can interact with vertical grating test stimulus to induce a standard tilt illusion. However, by considering the second-order structure of a plaid rather than ZCs, it was shown that the perceived orientation of the vertical test grating results from the combination of orientation illusions due to the first- and second-order components of an inducing plaid. The data suggest that the mechanisms encoding the orientation of second-order contours are similar to, and interact directly with, those that encode first-order contours.

Contrast Sensitivity↗

PubMed2001

Perceptual strategies to improve skin cancer discriminations in naive observers.

The aim of these experiments was to assess the effect of an educational brochure on the ability of naive observers to discriminate skin cancers from benign lesions, and to investigate possible new strategies to assist observers in performing this task. A two-alternative forced choice paradigm was used to investigate the ability of observers to correctly identify different types of benign and malignant lesions before and after exposure to an educational brochure. The method of pair comparisons was used to assess the ability of observers to discriminate between benign and malignant lesions under different instruction conditions. Subjects were undergraduate students from Sydney University. An educational brochure did not facilitate the ability to correctly identify malignant lesions, and appeared to result in deterioration of performance in the identification of benign lesions. Similarly, observers were unable to discriminate between benign and malignant lesions on the basis of how dangerous they looked. However, judgements of lesions in terms of perceived distinctiveness resulted in relatively accurate discriminations between benign and malignant lesions. These data suggest current skin cancer detection strategies may be ineffective in improving the ability to visually identify benign and malignant lesions. Discriminating between lesions in terms of how distinctive they appear may form the basis of a new and effective strategy for the detection of skin cancer.

Adult↗

PubMed2000

Tilt aftereffects generated by bilaterally symmetrical patterns.

Tilt aftereffects were generated by bilaterally symmetrical dot patterns. Both expansion and contraction effects, similar in size and magnitude to effects usually reported with luminance contours, were observed after adaptation to symmetrical patterns tilted 15 deg or 75 deg respectively from a vertically oriented test. Large effects were found when both adapting and test stimuli were symmetrical patterns while smaller effects were found when the adapting stimulus was symmetrical and test stimulus was a grating. A third experiment, which manipulated the number of dots near the axis line, confirmed the above findings; expansion and contraction effects were observed again. The results of these experiments suggest that the neural mechanism underlying the perception of luminance contours may be linked to the mechanism for the detection of symmetry.

Adaptation, Ocular↗

PubMed1998

Evidence that both area V1 and extrastriate visual cortex contribute to symmetry perception.

Bilateral symmetry is common in nature and most animals seem able to perceive it. Many species use judgements of symmetry in various behaviours, including mate selection [1-3]. Originally, however, symmetry perception may have developed as a tool for generating object-centered, rather than viewer-centered, descriptions of objects, facilitating recognition irrespective of position or orientation [4]. There is evidence that the visual system treats the orientation of axes-of-symmetry in the same way it treats in orientation of luminance-defined contours [5], suggesting that axes-of-symmetry act as 'processing tokens' [6]. We have investigated the characteristics of neural mechanisms giving rise to the perceived orientation of axes-of-symmetry. We induced tilt aftereffects with symmetrical dot patterns, eliciting perceived angle expansion and contraction effects like those usually observed with luminance-defined contours [7,8]. Induction of aftereffects during binocular rivalry resulted in a reduction of the magnitude of these effects, consistent with the aftereffects being mediated in extrastriate visual cortex, probably between visual areas V2 and MT [9]. In a second experiment in which the aftereffects were induced monocularly, their magnitudes were measured in the unadapted eye. Contraction effects transferred completely, suggesting that they are mediated by binocular cells. Expansion effects did not transfer completely, consistent with their having a monocular component. These data suggest that information about the orientation of axes-of-symmetry may be available as early as area V1, but that processing continues in extrastriate cortex.

Humans↗

PubMed1997

Figure-ground segregation at contours: a neural mechanism in the visual cortex of the alert monkey.

An important task of vision is the segregation of figure and ground in situations of spatial occlusion. Psychophysical evidence suggests that the depth order at contours is defined early in visual processing. We have analysed this process in the visual cortex of the alert monkey. The animals were trained on a visual fixation task which reinforced foveal viewing. During periods of active visual fixation, we recorded the responses of single neurons in striate and prestriate cortex (areas V1, V2, and V3/V3A). The stimuli mimicked situations of spatial occlusion, usually a uniform light (or dark) rectangle overlaying a grating texture of opposite contrast. The direction of figure and ground at the borders of these rectangles was defined by the direction of the terminating grating lines (occlusion cues). Neuronal responses were analysed with respect to figure-ground direction and contrast polarity at such contours. Striate neurons often failed to respond to such stimuli, or were selective for contrast polarity; others were non-selective. Some neurons preferred a certain combination of figure-ground direction and contrast polarity. These neurons were rare both in striate and prestriate cortex. The majority of neurons signalled figure-ground direction independent of contrast polarity. These neurons were only found in prestriate cortex. We explain these responses in terms of a model which also explains neuronal signals of illusory contours. These results suggest that occlusion cues are used at an early level of processing to segregate figure and ground at contours.

Animals↗

PubMed1994

The role of the blobs in determining the perception of drifting plaids and their motion aftereffects.

Motion aftereffects (MAEs) can be induced by adaptation to a pair of differently oriented drifting gratings whether the gratings are presented simultaneously, as a coherent plaid, or in alternation. The fact that the former MAEs were generally larger than the latter led to the suggestion that simultaneous adaptation involved higher-level extrastriate processes not involved in the alternating effects. In the past few years evidence has accumulated that the difference is in fact due to a low-level monocular process which can be termed the 'blob-tracking mechanism'. A review is presented of the evidence on MAEs induced by simultaneous and alternating adaptation, the evidence for the monocularity of the blob-tracking mechanism, the data which implicate the blob mechanism in the determination of MAE magnitude, perceived plaid drift direction, and in perceived plaid coherence.

Humans↗

PubMed1993

Direct evidence for competition between local and global mechanisms of two-dimensional orientation illusions.

Orientation illusions induced by two-dimensional stimuli, such as square outline frames or plaids, have been more or less adequately accounted for in terms of repulsion of the vertical test stimulus from the axis of symmetry nearest vertical of the inducing stimulus, whether that axis is real or virtual. Recently, data have been obtained which directly suggest a more complex mechanism: one in which the observed illusion is the sum of all effects--complementary and antagonistic--induced by all axes flanking vertical which are sufficiently close to vertical to exert a significant effect. Experiments are reported in which this latter hypothesis was directly tested by using nonorthogonal plaid component gratings and varying the real-axis orientations while a virtual plaid axis remained fixed in orientation at 10 degrees from vertical. The data indicate that the real component gratings modulate the virtual-axis effect.

Adult↗

PubMed1991

Local and global mechanisms of one- and two-dimensional orientation illusions.

One-dimensional (1-D) orientation illusions induced on a test grating by a tilted and surrounding 1-D inducing grating have a well-known angular function that exhibits both repulsion and attraction effects. Two-dimensional (2-D) orientation illusions are those induced on a test grating by 2-D image modulation, such as a pair of superimposed inducing gratings at different orientations, usually orthogonal (a plaid). Given the known angular functions induced by the plaid component gratings, two hypotheses were developed that predicted different plaid-induced illusion functions. Hypothesis 1 states that the 1-D component-induced effects simply add linearly; Hypothesis 2 states that there is an additional mechanism that responds to the virtual axes of mirror symmetry of the plaid and adds to the effect. The data of two experiments were consistent with the predictions from the second hypothesis but not the first. Possible neural substrates of mechanisms that extract axes of symmetry are discussed; it is suggested that such global symmetry axes may underlie the perceived orientation of complex shapes.

Adult↗

PubMed1990

Determinants of subjective contour: Bourdon illusions and "unbending" effects.

Wenderoth and O'Connor (1987b) reported that, although matches to the straight edge of two triangles placed apex to apex revealed an apparent bending in the direction of the chevron formed by the hypotenuse pair (the Bourdon effect), no perceptual unbending of the bent chevron occurred. Using subjective contour figures, Walker and Shank (1988b) found large and approximately equal bending and unbending effects, consistent with two theories that they proposed. In Experiment 1, using adjustable chevron matching and subjective contours, we found that Bourdon effects, equivalent in magnitude to those reported by Walker and Shank, were 4-5 times larger than unbending effects. In Experiment 2, we used a variation of Walker and Shank's measurement technique, in which subjects selected a matching angle from a graded series. We obtained Bourdon effects similar to those in Experiment 1, but much larger unbending effects. Nevertheless, Bourdon effects were significantly larger than unbending effects in one set of data; and in another, Bourdon test means were larger than unbending test means. In both data sets, there was a large and significant pretest bending effect, which enhanced the magnitude of unbending test minus pretest scores. These results were consistent with our theory but not the theories of Walker and Shank. The variance of unbending test matches, 3-4 times that of Bourdon test matches, reflected the task difficulty. We propose that subjective obtuse angle contraction that exceeds real obtuse angle contraction explains the fact that unbending effects are larger in subjective than in real contours.

Adult↗

PubMed1989

Two-dimensional tilt illusions induced by orthogonal plaid patterns: effects of plaid motion, orientation, spatial separation, and spatial frequency.

Tilt illusions occur when a drifting vertical test grating is surrounded by a drifting plaid pattern composed of orthogonal moving gratings. The angular function of this illusion was measured as the plaid orientation (and therefore its drift direction) varied over a 180 degrees range. This was done when the test and inducing stimuli abutted and had the same spatial frequency, and when the test and inducing stimuli either differed in frequency by an octave, or were spatially separated by a 2 deg blank annulus, or both differed in frequency and were also separated by the annulus (experiments 1-4). The obtained angular function was virtually identical to that obtained previously with the rod and frame effect and other cases involving orthogonal inducing components, with evidence for illusions induced both by real-line components and by virtual axes of symmetry. Although the magnitude of the illusion was very similar in all four experiments, there was evidence to suggest that largest real-line effects occurred in the abutting same-frequency condition, with a pattern of results similar to that obtained previously with the simple one-dimensional tilt illusion. On the other hand, virtual-axis effects were more prominent with gaps between test and inducing stimuli. A fifth, repeated-measures, experiment confirmed this pattern of results. It is suggested that this pattern-induced tilt effect reflects both striate and extrastriate mechanisms and that the apparent influence of spatially distal virtual axes of symmetry upon perceived orientation implies the existence of AND-gate mechanisms, or conjunction detectors, in the orientation domain.

Attention↗

PubMed1989

Orientation illusions induced by briefly flashed plaids.

Orientation illusions which occur when a vertical grating is surrounded by a plaid can be induced either by one of the plaid's orthogonal component gratings or by a virtual axis of symmetry of the plaid, whichever is nearest vertical. In six experiments in which such illusion displays were flashed for durations between 15 and 405 ms, it was found that when these two-dimensional illusions are induced by a component grating (direct effects) the illusions increase monotonically as duration decreases, from 1 degree-2 degrees to about 6 degrees-7 degrees, over this range. Effects induced by axes of symmetry (indirect effects), in contrast, begin to occur only at longer durations: for short exposures, illusions are directionally opposite and large, similar to direct effects. These results suggest that only plaid-component-selective mechanisms operate at the shortest exposure durations and that additional time is required to extract more global higher-order pattern structure. The data are discussed in relation to sustained and transient mechanisms and also with respect to recent reports of more global processing mechanisms in extrastriate cortex and related data on one-dimensional tilt illusions.

Adult↗

PubMed1989

The effects of exposure duration and surrounding frames on direct and indirect tilt aftereffects and illusions.

Direct and indirect tilt illusions (TIs) have been shown to have different mechanisms, because spatial parameters that affect the one do not affect the other, and vice versa. The indirect TI, for example, is reduced markedly by a surrounding vertical square frame, a manipulandum that has no effect on the direct TI (Wenderoth & Johnstone, 1988a). In six experiments, we show that both direct and indirect TIs are enhanced in magnitude with short (10-60 msec) exposures; that tilt aftereffects (TAEs) induced with short test exposures are entirely comparable in magnitude; that a surrounding square frame reduces indirect TAEs but not direct TAEs, just as occurs with the TI; and that when the surrounding frame is present during adaptation only, test only, and both or neither, the greatest indirect TAE reduction occurs when the frame is present during the test. These results are consistent with the view (Wenderoth & Johnstone, 1987, 1988a, 1988b) that indirect TIs and TAEs may not reflect temporary neural modification based on V1 lateral inhibitory processes but rather the operation of more global, possibly extrastriate, orientation-constancy mechanisms.

Adult↗

PubMedDate not supplied

Psychophysical evidence for area V2 involvement in the reduction of subjective contour tilt aftereffects by binocular rivalry.

Previous research suggests binocular rivalry disrupts extrastriate, but not striate processes, although the locus along the visual pathway at which such disruption first occurs is uncertain. It has been argued that subjective contours arise via a two-stage process in which end-stopped cells feed into orientation-sensitive neurones in V2, and that orientation aftereffects induced with subjective contours are the product of mechanisms similar to those giving rise to real contour aftereffects. If binocular rivalry disrupts the acquisition of subjective contour aftereffects, then it follows from this model that rivalry disrupts processing in V2. Experiments reported here confirm this and provide evidence which suggests binocular rivalry arises through interactions between binocular neurones, rather than via some type of specialized binocular rivalry mechanism.

Afterimage↗

PubMedDate not supplied

Reduction of a pattern-induced motion aftereffect by binocular rivalry suggests the involvement of extrastriate mechanisms.

Previous research suggests that plaid-induced motion aftereffects (MAEs) involve extrastriate mechanisms (Wenderoth et al., 1988). There is evidence also that binocular rivalry occurs beyond V1 and that it disrupts the processing of MAEs which are believed to be based upon extrastriate mechanisms (e.g. the spiral MAE) but not MAEs, such as linear MAE induced by a drifting grating, which are thought to arise in striate cortex (Wiesenfelder & Blake, 1990). The logical inference is that binocular rivalry during drifting plaid-induced adaptation should reduce the MAEs which result. We report experiments which confirm this prediction.

Adaptation, Ocular↗

PubMedDate not supplied

Global form perception: interactions between luminance and texture information.

PURPOSE: This experiment reports the independence of first- and second-order processing mechanisms in form perception. METHODS: Symmetrical dot patterns were created using either luminance-increment dots (luminance above background), or texture-defined dots (average luminance equal to background). The proportion of luminance increment or texture dots defining each pattern was varied among fields of noise dots of the same type to determine symmetry detection thresholds. RESULTS: Differences in detection thresholds were found between luminance- and texture-defined patterns. Further, symmetry detection thresholds for luminance-increment dot patterns were resistant to noise defined by dots of opposite contrast polarity (luminance-decrement dots) or texture, while texture-defined patterns were resistant to neither texture nor luminance-decrement noise. CONCLUSIONS: These data suggest that symmetry perception, along with other types of form perception, use both first- and second-order processing mechanisms. The data are compatible with a second-order system that includes a negative half-wave rectifying non-linearity.

Contrast Sensitivity↗