Biomedical subjects

J M Zanker

Publications and source records attributed to J M Zanker.

At least 19 recordsLinked to original sources

PubMed2002

On temporal hyperacuity in the human visual system.

The spatial grain of the human visual system has always been a central topic for visual sciences, and the optical and physiological basis of perceptual limitations are well described. In particular, we have thorough accounts of spatial hyperacuity, which refers to a precision in the spatial localisation of stimulus contours that is better than the photoreceptor grain that determines spatial resolution. However, although the temporal resolution of the human visual system is comparably well described, we have almost no direct knowledge about the precision of localising visual stimuli in time in the absence of correlated spatial cues. The present study addresses this question by comparing directly the temporal resolution of human observers with their temporal acuity as measured in a temporal bisection task. Despite some improvement with practice, temporal acuity in this task does not fall below 20-30 ms in the best case, which is similar to the temporal resolution limit, and performance does not improve for comparison tasks with multiple stimulus presentations. The absence of visual hyperacuity for purely temporal modulations as tested here contrasts with processing limitations for other types of visual information in comparable tasks, and with other sensory modalities, in particular to those of the auditory system. Such differences can be interpreted in the context of the ecological requirements for organising behaviour, and the functional design of nervous systems.

Adult↗

PubMed2001

Perceptual deformation induced by visual motion.

The perceived position of a moving object can be misleading because the object has advanced while its previous retinal image has been transmitted through the visual stream, leading to a mismatch between actual location and its neural representation. It has been suggested that the human visual system compensates for neural processing delays to retrieve instantaneous position. However, such a mechanism would require a precise measure of the actual delay in order to provide a reliable position estimate. A novel illusory deformation of moving contours demonstrates that humans misjudge the spatial relationship between parts of coherently moving targets, and therefore do not perfectly account for neural delays. The size of this deformation increases with growing speed. In some subjects this illusion can be reversed by varying the luminance of individual dots; a manipulation that affects the neural delays. Our experiments agree with other evidence that the capacity of the visual system to compensate for processing delays is limited.

Decision Making↗

PubMed2001

What determines the perceived speed of dots moving within apertures?

Whereas it is a well known fact that objects appear to move faster in smaller stimulus fields, the reason for such a misjudgement of speed is still a matter of debate. We present four experiments to characterise the stimulus parameters that are important for the apparent speed increase of dots moving behind small apertures. In these experiments we varied the size and the shape of the aperture and its location in the visual field, as well as the stimulus duration. We report that the field-size effect does not depend on the overall duration of the stimulus, which does influence the typical path length of individual dots in the display. It is, however, affected by the shape of the aperture in such a way that the aperture size along the motion path is crucial for the speed misjudgement. The field-size effect furthermore depends on the location of the stimulus in the visual field. Our combined results are best described as an increase in perceived speed that is consistently elicited when a motion sink, i.e. a boundary of disappearing dots, is located close to the fovea. Such a description of the relevant stimulus parameters is discussed with respect to possible high-level mechanisms, relating back to classic Gestalt psychology explanations of the field-size effect, and with respect to well-known aspects of neuronal processing that may underlie speed perception and motion integration.

Adult↗

PubMed2001

The directional tuning of the barber-pole illusion.

In order to study the integration of local motion signals in the human visual system, we measured directional tuning curves for the barber-pole illusion by varying two crucial aspects of the stimulus layout independently across a wide a range in the same experiment. These were the orientation of the grating presented behind the rectangular aperture and the aspect ratio of the aperture, which in combination determine the relative contributions of local motion signals perpendicular to the gratings and parallel to the aperture borders, respectively. The strength of the illusion, ie the tendency to perceive motion along the major axis of the aperture, obviously depends on the spatial layout of the aperture, but also on grating orientation. Subjects were asked which direction they perceived and how compelling their motion percept was, revealing different strategies of the visual system to deal with the barber-pole stimulus. Some individuals respond strongly to the unambiguous motion information at the boundaries, leading to multistable percepts and multimodal distributions of responses. Others tend to report intermediate directions, apparently being less influenced by the actual boundaries. The general pattern of deviations from the motion direction perpendicular to grating orientation--a decrease with aspect ratio approaching unity (ie square-shaped apertures) and with gratings approaching parallel orientation to the shorter aperture boundary--is discussed in the context of simple phenomenological models of motion integration. The best fit between model predictions and experimental data is found for an interaction between two stimulus parameters: (i) cycle ratio, which is the sine-wave gratings equivalent of the terminator ratio for line gratings, describing the effects from the aperture boundaries, and (ii) the grating orientation, responsible for perpendicular motion components, which describes the influence of motion signals from inside the aperture. This suggests that the most simple cycle (terminator) ratio explanation cannot fully account for the quantitative properties of the barber-pole illusion.

Adult↗

PubMed2001

Interaction of first- and second-order direction in motion-defined motion.

Motion-defined motion can play a special role in the discussion of whether one or two separate systems are required to process first- and second-order information because, in contrast to other second-order stimuli, such as contrast-modulated contours, motion detection cannot be explained by a simple input nonlinearity but requires preprocessing by motion detectors. Furthermore, the perceptual quality that defines an object (motion on the object surface) is identical to that which is attributed to the object as an emergent feature (motion of the object), raising the question of how these two object properties are linked. The interaction of first- and second-order information in such stimuli has been analyzed previously in a direction-discrimination task, revealing some cooperativity. Because any comprehensive integration of these two types of motion information should be reflected in the most fundamental property of a moving object, i.e., the direction in which it moves, we now investigate how motion direction is estimated in motion-defined objects. Observers had to report the direction of moving objects that were defined by luminance contrast or in random-dot kinematograms by differences in the spatiotemporal properties between the object region and the random-noise background. When the dots were moving coherently with the object (Fourier motion), direction sensitivity resembled that for luminance-defined objects, but performance deteriorated when the dots in the object region were static (drift-balanced motion). When the dots on the object surface were moving diagonally relative to the object direction (theta motion), the general level of accuracy declined further, and the perceived direction was intermediate between the veridical object motion direction and the direction of dot motion, indicating that the first- and second-order velocity vectors are somehow pooled. The inability to separate first- and second-order directional information suggests that the two corresponding subsystems of motion processing are not producing independent percepts and provides clues for possible implementations of the two-layer motion-processing network.

Adult↗

PubMed2000

Streaming and bouncing: observations on motion defined objects.

When two identical objects move in opposite directions on the same path and at the same speed, they can appear, after crossing over to continue n their original directions (streaming), or to reverse direction (bouncing). In order to be able to man pulate visibility by adding no se, we used objects defined by contrast, flicker, or motion, and thereby extended previous findings on luminance-defined objects. Two identical rectangles (1.1 x 1.4 degrees) composed of random dot patterns moved toward each other at a speed of 3.5 degrees/s. In experiment I we used backgrounds of a grey field, static random dots, or dynamic noise, and examined the effect of introducing a pause in motion and a visual distractor. In experiment 2 we introduced visual noise at four levels. For all three types of motion display, we found an increase in the proportion of the bouncing percept when either a pause in motion or an attentional distractor was introduced. Experiment 2 showed that neither of these effects depends on the visibility of the moving objects. An increase in the bouncing percept, due to a pause in motion or the distraction of attention, can be observed for all types of object definition, and is not affected by decreas ng the visibility of the motion-defined objects.This finding suggests that the role of attention in determining the perception of bouncing does not lie in the modulation of object visibility.

Attention↗

PubMed2000

Mechanisms of human motion perception: combining evidence from evoked potentials, behavioural performance and computational modelling.

Based on single cell recordings in monkey, it has been suggested that neural activity can be related directly to psychophysically measured threshold behaviour. Here, we investigated in humans whether evoked potentials correlate with behavioural measurements like discrimination thresholds and reaction time. Subjects were asked to report the perceived direction of object motion stimuli which contained variable amounts of coherent motion. Simultaneously, we recorded evoked potentials with a multielectrode array, or measured the reaction time. We show here that motion coherence had a strong influence on both amplitude and latency of the evoked potential. Stronger motion signals evoked stronger and faster cortical responses. The latency reduction of the motion onset response with increasing coherence correlated very well with the concurrent decrease in reaction time. Taken together, these results suggest that temporal integration is an important step in analysing motion signals to generate a reliable behavioural response. We stimulated a two-dimensional array of correlation-type motion detectors with the same motion sequences, and analysed the distribution of local motion signals according to signal detection theory. Performance resembled that of human subjects when the decision strategy was optimized so as to exclude small signals and, in particular, when the ideal observer had some knowledge about a region of interest in which the object was to be expected.

Cerebral Cortex↗

PubMed2000

The optomotor response and spatial resolution of the visual system in male Xenos vesparum (Strepsiptera).

The Strepsiptera are an enigmatic group of parasitic insects whose phylogenetic relationships are hotly debated. Male Strepsiptera have very unusual compound eyes, in which each of a small number of ommatidia possesses a retina of at least 60 retinula cells. We analysed the optomotor response of Xenos vesparum males to determine whether spatial resolution in these eyes is limited by the interommatidial angle or by the higher resolution potentially provided by the extended array of retinula cells within each ommatidium. We find that the optomotor response in Strepsiptera has a typical bandpass characteristic in the temporal domain, with a temporal frequency optimum at 1-3 Hz. As a function of spatial wavelength, the optomotor response is zero at grating periods below 12 degrees and reaches its maximum strength at grating periods between 60 degrees and 70 degrees. To identify the combination of interommatidial angles and angular sensitivity functions that would generate such a spatial characteristic, we used motion detection theory to model the spatial tuning function of the strepsipteran optomotor response. We found the best correspondence between the measured response profile and theoretical prediction for an irregular array of sampling distances spaced around 9 degrees (half the estimated interommatidial angle) and an angular sensitivity function of approximately 50 degrees, which corresponds to the angular extent of the retina we estimated at the centre of curvature of the lens. Our behavioural data strongly suggest that, at least for the optomotor response, the resolution of the strepsipteran compound eye is limited by the ommatidial sampling array and not by the array of retinula cells within each ommatidium. We discuss the significance of these results in relation to the functional organisation of strepsipteran compound eyes, their evolution and the role of vision in these insects.

Animals↗

PubMed1999

How to tell circles from ellipses: perceiving the regularity of simple shapes.

Human observers achieve a surprising precision in many visual judgements, such as estimating relative position, colinearity and the regularity of shape. We measured the sensitivity in detecting shape deformations by presenting a square simultaneously with a rectangle of variable aspect ratio, or a circle with an ellipsoid. Weber fractions approach 3-5% and improve to approximately 1% when subjects are asked to tell which of the two objects was 'oriented more vertically', instead of identifying the square or circle. Contour position can be judged with a precision of 10-20 arc s, clearly in the hyperacuity range and also beyond the thresholds known for detecting differences in the curvature of comparable line segments. Our results suggest that detecting deformation in rectangles seems to rely on aspect ratio, whereas performance is improved for ellipsoids by a high sensitivity for changes in local curvature.

Adult↗

PubMed1999

Speed tuning in elementary motion detectors of the correlation type.

A prominent model of visual motion detection is the so-called correlation or Reichardt detector. Whereas this model can account for many properties of motion vision, from humans to insects (review, Borst and Egelhaaf 1989), it has been commonly assumed that this scheme of motion detection is not well suited to the measurement of image velocity. This is because the commonly used version of the model, which incorporates two unidirectional motion detectors with opposite preferred directions, produces a response which varies not only with the velocity of the image, but also with its spatial structure and contrast. On the other hand, information on image velocity can be crucial in various contexts, and a number of recent behavioural experiments suggest that insects do extract velocity for navigational purposes (review, Srinivasan et al. 1996). Here we show that other versions of the correlation model, which consists of a single unidirectional motion detector or incorporates two oppositely directed detectors with unequal sensitivities, produce responses which vary with image speed and display tuning curves that are substantially independent of the spatial structure of the image. This surprising feature suggests simple strategies of reducing ambiguities in the estimation of speed by using components of neural hardware that are already known to exist in the visual system.

Models, Theoretical↗

PubMed1999

Perceptual learning in primary and secondary motion vision.

Specific improvements of perceptual capabilities with practise are thought to give some clues about cortical plasticity and the localisation of cortical processing. In the present study, perceptual learning is used as a paradigm to separate mechanisms underlying the perception of different classes of motion stimuli. Primary motion stimuli (phi-motion), are characterised by displacements of the luminance distribution. However, for secondary motion stimuli the movement is not accompanied by a corresponding luminance shift. Instead, moving objects are defined by their temporal frequency composition (mu-motion) or by motion itself (theta-motion). On theoretical grounds, the perception of secondary motion requires a higher degree of nonlinearity in the processing stream than the perception of primary motion but debate continues as to whether there might be a unique mechanism underlying the perception of both motion classes. In a large group of subjects, coherence thresholds for direction discrimination in random dot kinematograms of phi-, mu-, and theta-motion were repeatedly measured in a staircase paradigm. Training effects were found on different timescales, within short sessions containing multiple staircases and over training periods of several months. They were fairly stable over long breaks without testing. When subjects were trained with two different motion stimuli in a sequence, an asymmetry in the transfer of perceptual learning was revealed: sensitivity increases achieved during practise of theta-motion are largely transferred to phi-motion, but theta-motion perception does not profit from prior exposure to phi-motion. This finding supports the view derived from modelling of motion processing that there must be at least partially separate systems. A primary motion detection mechanism falls short of discriminating direction in secondary motion stimuli, whereas a mechanism able to extract secondary motion will be inherently sensitive to primary motion.

Adolescent↗

PubMed1999

How does noise influence the estimation of speed?

Local motion signals have to be combined in space and time, to yield a coherent motion percept as it is involved in a variety of visual tasks. This combination necessarily means to trade-off between loosing spatio-temporal resolution by pooling local signals and maintaining perceptually significant segmentation between them. When signals are pooled to detect the presence of coherent motion in large amounts of random noise, the question raised is how the noise affects the perceived quality, in particular speed, of the coherent motion. Is there an analogy to the well-known reduction in the perceived speed of moving gratings at low contrast? Using a two-interval forced-choice procedure, we have investigated the assessment of speed in random-dot kinematograms containing different proportions of noise. Under the conditions investigated, there is no strong reduction of perceived speed with increasing noise, as long as coherence levels remain well above the thresholds for directional judgements. This basic result, which could suggest considerable but not perfect segregation of signal and noise motion components in the pooling process leading to speed estimation, is discussed in relation to a model that is designed to decode speed from a population of elementary motion detectors (EMDs) of the correlation type. A strategy to estimate speed from a set of EMDs with a variety of spatio-temporal tuning does not only provide a velocity predictor unambiguous with the spatial structure of the stimulus, but also is largely independent of noise.

Adult↗

PubMed1998

Limiting factors for the detection of orientation.

First steps of visual-information processing in primates are characterised by a highly ordered representation of the outside world on the cortex. Two prominent features of cortical organisation are the retinotopic mapping of position in the visual field on the first stages of the visual stream, and the systematic variation of orientation preference in the same areas. In an attempt to understand the relation of position and orientation representation, we need to know the minimum spatial requirements for orientation detection. In the present paper, the spatial limits for detecting orientation are analysed by simulating simple orientation filters and testing the ability of human observers to detect the orientation of small lines at various positions in the visual field. At sufficiently high contrast levels, the minimum physical length of a line to discriminate orientation differences of 45 degrees-90 degrees is not constant when presented at various eccentricities, but covaries inversely with the cortical magnification factor. In consequence, a line needs to correspond to about 0.2 mm of cortical surface, independently of the actual eccentricity at which the stimulus is presented, in order to allow observers to recognise its orientation. This has consequences for our understanding of orientation detection. (i) In combination with simulation experiments, it becomes clear that the elementary process underlying orientation detection is a local operation, which seems to focus on small regions compared with cortical receptive fields. (ii) With respect to the number of inputs to the visual cortex, the performance of this local operation approaches the physical limits, requiring hardly more than three-four input LGN axons to be activated for detecting the orientation of a highly visible line segment. Comparing these spatial characteristics with the receptive fields of orientation-sensitive neurons in the primate visual system could suggest new insights into the neuronal circuits underlying orientation mapping in the human cortex.

Computer Simulation↗

PubMed1998

Detecting the orientation of short lines in the periphery.

PURPOSE: Visual information processing in the human cortex is based on a highly ordered representation of the surrounding world. In addition to the retinotopic mapping of the visual field, systematic variations of the orientation tuning of neurons have been described in the primary visual cortex. As a step to understanding the relationship between position and orientation representation, we investigated psychophysically the minimum spatial requirements for the determination of orientation at various positions across the visual field. We know that the shortest line whose orientation can be resolved varies with eccentricity, such that its length corresponds to slightly less than 0.2 mm projected onto the cortical surface. Along the horizontal meridian horizontal lines are detected with higher precision than vertical or oblique lines. In the present experiments, we tested whether this is a preference for horizontal lines or for lines that are orientated radially away from the fovea. METHODS: Human observers were tested with lines positioned at one vertical, two horizontal and two oblique meridians at eccentricities between 5 and 25 degrees. RESULTS/CONCLUSION: Three of the four subjects were most sensitive for targets aligned with the meridian of presentation. This suggests that the visual system has the highest resolution in directions radiating from the fovea, which may be particularly useful for the analysis of flow fields resulting from forward translation.

Humans↗

PubMed1998

Motion capture and the 'Ouchi' effect.

PURPOSE: The role of motion contrast and motion capture in the 'Ouchi' effect was investigated in a simplified stimulus. METHODS: Sinewave gratings were rapidly oscillated vertically within two concentric stationary 'apertures', an inner disc surrounded by an annulus. RESULTS: A strong illusion of relative motion between the disc and annulus is observed in the horizontal direction for 8 and 12 cycles/deg given small orientation differences between the two gratings. CONCLUSION: These parameters coincide with those of the 'Ouchi' effect, suggesting a sharing of the same causal mechanism.

Adult↗

PubMed1998

Complex motion stimuli localize higher-order visual processing in normal observers and in patients with parietal lesions.

The present paper illustrates how modern techniques applied in neuroscience can help us to understand the processing of visual information in the human brain and, in turn, how they can be helpful to characterize functional deficits in patients with cortical lesions. Based on theoretical considerations, motion stimuli are developed that require very specific operations to be performed by the visual system. Computational models explaining the processing of these 'Fourier' and 'second-order' motion stimuli are characterized by increasing complexity. The same types of stimuli are used to map the distribution of cortical activity during motion perception by measuring the magnetic and electrical fields on the head surface. Clinical investigations of patients with lesions in the parietal cortex indicate specific deficits in the perception of such stimuli that can be related to the lesion sites.

Action Potentials↗

PubMed1997

Smooth-pursuit eye movements elicited by first-order and second-order motion.

The perception of the displacement of luminance-defined contours (i.e., first-order motion) is an important and well-examined function of the visual system. It can be explained, for example, by the operation of elementary motion detectors (EMDs), which cross-correlate the spatiotemporal luminance distribution. More recent studies using second-order motion stimuli, i.e., shifts of the distribution of features such as contrast, texture, flicker, or motion, extended classic concepts of motion perception by including nonlinear or hierarchical processing in the EMD. Smooth-pursuit eye movements can be used as a direct behavioral probe for motion processing. The ability of the visual system to extract motion signals from the spatiotemporal changes of the retinal image can be addressed by analyzing the elicited eye movements. We measured the eye movement response to moving objects defined by two different types of first-order motion and two different types of second-order motion. Our results clearly showed that the direction of smooth-pursuit eye movements was always determined by the direction of object motion. In particular, in the case of second-order motion stimuli, smooth-pursuit did not follow the retinal image motion. The latency of the initial saccades during pursuit of second-order stimuli was slightly but significantly increased, compared with the latency of saccades elicited by first-order motion. The processing of second-order motion in the peripheral visual field was less exact than the processing of first-order motion in the peripheral field. Steady state smooth-pursuit eye speed did not reflect the velocity of second-order motion as precisely as that of first-order motion, and the resulting retinal error was compensated by saccades. Interestingly, for slow second-order stimuli we observed that the eye could move faster than the target, leading to small, corrective saccades in the opposite direction to the ongoing smooth-pursuit eye movement. We conclude from our results that both visual perception and the control of smooth-pursuit eye movements have access to processing mechanisms extracting first- and second-order motion.

Fixation, Ocular↗

PubMed1997

Is facilitation responsible for the "motion induction" effect.

When a horizontal bar is presented after a single dot is shown at one of its ends, an illusory motion is seen which has been dubbed "motion induction" in the literature. The phenomenon has been attributed to a facilitation process which asymmetrically modulates the inputs to motion detectors, for instance by some sort of changes in processing speed. Computer simulations of motion detector arrays show, however, that this basic effects has to be expected from the properties of simple motion detectors. It has been recently reported that the strength of the illusory motion increases with the subjective salience of the inducing element. New computer simulations demonstrate that this observation can be related to the control of the local gain of motion detector input signals by the feature contrast in a particular region of the stimulus. High-level attentional mechanisms or changes in transmission speed are not required to explain these phenomena. The implications of such local gain-control mechanisms for our understanding of second-order motion perception are discussed.

Attention↗