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J M Zanker

Publications and source records attributed to J M Zanker.

17 recordsLinked to original sources
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↗
PubMed1997

A glimpse into crabworld.

Almost all known arthropod compound eyes exhibit regional variations of resolving power, absolute light, spectral and polarisation sensitivity which are likely to be matched to the probability of significant events and the availability of cues in the visual world. To understand the signal processing requirements that have led to the evolution of matched sensory and neural filters, we thus need a detailed description of the input signals to a visual system and of the tasks to be performed under natural operating conditions. We report here on the first steps we took in an attempt to reconstruct an animal's specific visual world with emphasis on the motion domain. Fiddler crabs (genus Uca) live in burrows on sand- and mudflats and are active during low tide. They carry their eyes on long, vertically oriented stalks and use vision to detect predators and conspecific signals generated by males waving one massively enlarged claw. The crabs sit on the ground plane of a flat world, where significant events are most likely to occur in a narrow band around the horizon. We recorded scenes in a crab colony with a video camera at crab eye height. The salience of relevant features in the spatial, spectral and polarisation domains was analysed in digitised video images and short sequences of film were processed by a two-dimensional network of motion detectors at various spatial scales. The output of the network provides us with histograms of the direction and strength of motion signals in various spatio-temporal frequency bands. We discuss our results in terms of detection problems, predictability of events, global vs local information content and higher level motion processing involved in intraspecific communication.

Animals↗
PubMed1997

Second-order motion perception in the peripheral visual field.

In motion perception, luminance-defined stimuli (first-order motion) are distinguished from stimuli defined by more complex attributes (second-order motion), because they differ in their processing requirements. For instance, a two-layer model with the output of an array of elementary motion detectors (EMD's) feeding into a second array of EMD's has been proposed to account for seeing the movement of motion-defined objects. The question is raised whether this processing scheme is operating across the whole visual field or whether second-order motion perception is restricted to the fovea. The detection, orientation discrimination, and motion direction discrimination of oblique, vertically moving bars was tested at horizontal eccentricities between 0 degree and 16 degrees. Bars were defined on a dynamic noise background by an area of static dots (drift-balanced motion) or by coherent dot motion either in the direction of the bar motion (Fourier motion) or in the orthogonal direction (theta motion). Coherence thresholds for direction discrimination are severely impaired in the periphery for both types of second-order motion but not for Fourier motion, whereas orientation discrimination and detection marginally decline for all three bar types when the stimuli are presented further out in the periphery. In a control experiment it is shown that this result cannot be due entirely to the changes in spatial scale of the peripheral visual system. The facts that motion-defined objects can be detected in the periphery and that their orientation can be detected, but not their direction of motion, supports the view that the two-layer system suggested for the processing of theta motion is restricted to the central region of the visual field.

Adult↗
PubMed1996

On the elementary mechanism underlying secondary motion processing.

The movement of luminance-defined targets can be easily extracted by elementary motion detectors (EMDs) of the correlation type which often are referred to as Reichardt-detectors. In contrast to such 'primary motion', in 'secondary motion' the moving target is defined by more complex features, like changes in texture, flicker, or local contrast. Such stimulus attributes have to be extracted from the retinal intensity distribution by some nonlinear preprocessing, before they are fed into motion detectors. An intriguing case is the perception of the movement of the motion signal, as is present in theta motion, where an object moves in a different direction than the texture on its surface. A two-layer model of hierarchically organised EMDs has been postulated to account for such motion extraction. Other than for the first layer, the computational nature of the mechanism underlying motion processing in the second layer so far is a matter of speculation, and is therefore characterized here by means of computer simulations and psychophysical experiments. Random dot kinematograms were generated in which sinusoidally modulated vertical dot motion defined gratings, and coherence thresholds were measured for the direction discrimination of a horizontally travelling modulation function. This was done for a variety of spatial frequencies and speeds of the modulation sinusoid. Thresholds turn out to be lowest not for a particular speed, but for a fixed temporal frequency of the modulation function (about 1 cycle per second), when various combinations of fine and coarse, and fast and slow secondary gratings are tested. This result favours a correlation-type mechanism over a gradient-type scheme which should lead to a speed-optimum independent of spatial frequency.

Adult↗
PubMed1996

The cortical representation of object motion in man is interindividually variable.

Cortical areas processing visual motion have been well investigated in monkeys, but comparatively little is known about these areas in man. In order to define such cortical areas in the brains of individuals, the magnetic field was recorded while subjects were watching motion-defined static and moving objects. The magnetic response showed a transient component with a clear dipolar magnetic field followed by a sustained component which exhibited some variation in magnetic field structure over time. For the transient component, the single equivalent current dipoles superimposed upon magnetic resonance images for individual subjects were clearly localized outside the primary visual areas. In most cases the neural generator was found in the region of the temporo-parieto-occipital junction of the lateral cortex. The results also suggest that the activated cortical area show interindividual variations in location.

Adult↗
PubMed1996

Hyperacuity for spatial localization of contrast-modulated patterns.

The acuity for localizing the position of a grating and other first order patterns which are defined directly by the luminance distribution, is much higher than the resolution for such gratings. This well-described phenomenon usually is referred to as hyperacuity, and is regarded as a cortical function which is not limited by the optics and the sampling properties of the eye. Second order patterns which can be defined by the distribution of local contrast gained some interest because they require more complex processing mechanisms than first order patterns. We investigated how well gratings and bars which are exclusively defined by the variation of the local contrast of static random dot patterns can be localized in space. In this case localization acuity does not reach the precision which is known for first order patterns. However, the localization of contrast-modulated patterns can be almost one order of magnitude better than second order grating resolution, and therefore reaches into the hyperacuity range. In combination with findings for motion-defined or stereo-defined patterns it is concluded that the brain mechanisms responsible for the localization of features in the visual scene have not only access to first order information which is available immediately from the retinal image, but in addition, to second order information which has to be extracted from the retinal intensity distribution by some sort of nonlinear processing.

Adult↗
PubMed1995

Does motion perception follow Weber's law?

The subjective strength of a percept often depends on the stimulus intensity in a nonlinear way. Such coding is often reflected by the observation that the just-noticeable difference between two stimulus intensities (JND) is proportional to the absolute stimulus intensity. This behaviour, which is usually referred to as Weber's Law, can be interpreted as a compressive nonlinearity extending the operating range of a sensory system. When the noise superimposed on a motion stimulus is increased along a logarithmic scale (in order to provide linear steps in subjective difference) in motion-coherency measurements, observers often report that the subjective differences between the various noise levels increase together with the absolute level. This observation could indicate a deviation from Weber's Law for variation of motion strength as obtained by changing the signal-to-noise ratio in random-dot kinematograms. Thus JNDs were measured for the superposition of uncorrelated random-dot patterns on static random-dot patterns and three types of motion stimuli realised as random-dot kinematograms, namely large-field and object 'Fourier' motion (all or a group of dots move coherently), 'drift-balanced' motion (a travelling region of static dots), and paradoxical 'theta' motion (the dots on the surface of an object move in opposite direction to the object itself). For all classes of stimuli, the JNDs when expressed as differences in signal-to-noise ratio turned out to increase with the signal-to-noise ratio, whereas the JNDs given as percentage of superimposed noise appear to be similar for all tested noise levels. Thus motion perception is in accordance with Weber's Law when the signal-to-noise ratio is regarded as stimulus intensity, which in turn appears to be coded in a nonlinear fashion. In general the Weber fractions are very large, indicating a poor differential sensitivity in signal-to-noise measurements.

Acceleration↗
PubMed1994

Interaction between primary and secondary mechanisms in human motion perception.

Two layers of information processing can be distinguished as being involved in human motion perception. The primary motion detection stage processes displacements of the luminance distribution across space, such as experienced in natural scenes during the pursuit of moving targets. Primary motion detection is often investigated with artificial motion stimuli realized as random-dot kinematograms (RDKs). Such stimuli belong to the class of "Fourier motion", and their perception can be easily explained by means of elementary motion detectors (EMDs) of the correlation type. Other tasks require the comparison of motion signals from neighbouring areas in the visual field. The perception of the displacement of the motion distribution, for instance, has been accounted for by a secondary motion processing stage. In order to understand the principles of interaction between the motion in neighbouring areas of the visual field, we investigated the sensitivity of the human visual system for moving objects which are defined by moving dots in variable directions. These experiments lead to "secondary tuning curves" of direction discrimination for secondary motion as function of primary motion direction. A base level of sensitivity for all dot motion directions without a velocity component in the same direction of the object movement is enhanced when the object and the dots have a common velocity component. Thus primary motion in any direction can be exploited by the secondary stage, and primary and secondary system both feed into the object motion percept. Furthermore it is suggested from the shape of the secondary tuning curve that the outputs from the two layers of motion processing do not superimpose linearly, but are combined by some sort of veto-like mechanism which increases the directional sensitivity when the two processing layers experience movement along the same direction.

Adult↗
PubMed1993

Theta motion: a paradoxical stimulus to explore higher order motion extraction.

Apparent motion stimuli of increasing complexity have been applied to analyse the mechanisms underlying visual motion perception. In the present paper it is investigated how motion detectors respond to three classes of stimuli which are realized as random-dot kinematograms. (i) In the most conventional stimuli, Fourier motion, a group of dots is displaced coherently in a random-dot pattern. (ii) In drift-balanced motion stimuli a bar made of static random dots is shifted in front of another random-dot pattern. (iii) In the novel class of stimuli, theta motion, an object which is exclusively defined by dot motion into one direction, is moving itself into the opposite direction. It is shown in psychophysical experiments that human observers perceive the direction of object motion in all three classes of stimuli. Simple motion detectors, however, only extract the motion direction of the object in the case of Fourier stimuli, and in the case of drift-balanced stimuli, if a nonlinear preprocessing is assumed. Any of the model alternatives discussed so far just detects the moving dots but not the object in a theta-stimulus, as is illustrated by a combinatorial analysis using a simplified version of a motion detector of the correlation type, which operates on a discrete time scale and takes only discrete values. In order to account for the detection of theta-motion, a model consisting of two hierarchical layers of motion detectors is developed, and simulated for conditions as used in the psychophysical experiments. The perception of theta-motion and the two-layer model is discussed in relation to psychophysical data and theoretical considerations from the literature, to try to incorporate the proposed two-layer model into a general scheme of visual motion processing.

Computer Simulation↗
PubMed1993

Cortical potentials in humans reflecting the direction of object motion.

In the monkey, cortical areas can be localized which are specific for the processing of form, colour, or motion, and it is expected that the human visual cortex is organized in a similar way. The recording of scalp potentials generated by neural activity of underlying cortical areas is a non-invasive method which can be used to study the functional organization of the visual cortex with a high temporal resolution. In the present study we recorded slow cortical potentials from normal subjects to investigate how motion stimuli of variable complexity are processed in human visual cortex. The results show that the pattern of cortical activation is dependent on the type of stimulus. When random dots were moving within the entire stimulus field, or during counterphase flicker, maximal activation occurred over occipital electrode sites. During object motion a pronounced activation is recorded at parietal locations, with the direction of object motion being reflected by the time course of this activation.

Evoked Potentials, Visual↗
PubMedDate not supplied

Cortical potentials reflecting motion processing in humans.

Motion processing is a fundamental task of visual systems, and in the monkey cortical areas can be identified which appear to be functionally specialized for motion processing. The human visual system is expected to be organized in a similar way. A noninvasive method to study the functional organization of the visual cortex is the recording of scalp potentials generated by neural activity of the underlying cortical areas. In the present study, we recorded slow cortical potentials from normal subjects in order to investigate how motion stimuli are processed. Three classes of object motion were realized as random dot kinematograms, namely Fourier motion, drift-balanced motion, and theta motion, because they require mechanisms of increasing complexity in order to be extracted. Large-field motion and counterphase flicker were used as control stimuli. Three basic results were obtained: (1) The responses evoked by the three classes of object motion do not differ significantly in their course and distribution of activation. (2) The distributions of cortical activation evoked by object motion, and the control stimuli are different. During object motion the maximum activation occurs over the superior parietal cortex. Large-field motion activates occipital and parietal locations to the same extent, and during counterphase flicker the activity is maximum over the occipital lobe. Thus, the parietal slow potentials are interpreted to specifically reflect the cortical processing of object motion. (3) The time course of the activation reflects changes in the spatial position of the object: the amplitude of a transient negative component (TNC) which occurs 240 ms after motion onset decreases with increasing eccentricity of motion onset. The consecutive sustained negative component (SNC), which persists until the movement stops, decreases during centrifugal and increases during centripetal object motion. These results can be understood on the basis of physiological and anatomical knowledge about the mapping of the visual field on the cortex.

Adult↗