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PubMed1994

Ensemble models of the movement aftereffect and the influence of eccentricity.

Moving random-pixel arrays (RPAs) were used to study the movement aftereffect (MAE) for translational texture motion and to quantify the contribution of RPA-sensitive motion sensors to the MAE as a function of eccentricity. Size-scaled patterns were used to make a fair comparison across eccentricities. At the upper end of the velocity range it was found, for all eccentricities, that motion sensors tuned to velocities exceeding about 10-20 deg s-1 do not contribute to the translational MAE, even though they do contribute to motion perception. As a consequence the subpopulation of local motion sensors that contributes to the MAE shrinks with eccentricity, because there are fewer low-velocity-tuned and more high-velocity-tuned motion sensors for increasing eccentricity. Thus there is a quantitative, but not a qualitative, difference between the MAEs generated at different eccentricities.

Adult↗
PubMed2001

Recognition of point-light biological motion displays by young children.

We tested the ability of children 3-5 years of age to recognise biological motion displays. Children and adults were presented with moving point-light configurations depicting a walking person, four-legged animals (dogs), and a bird. Participants were able to reliably recognise displays with biological motion, but failed in the identification of a static (four consecutive frames taken from each sequence) version. The results indicate that, irrespective of the highly reduced and unusual structural information available in point-light displays, biological motion is sufficient for reliable recognition of human and non-human forms at an age as early as 3 years. Moreover, 5-year-olds exhibit the ceiling level of recognition. The findings are discussed in the context of the neuropsychological and brain mechanisms involved in biological motion perception.

Adolescent↗
PubMed2001

A new approach to analysing texture-defined motion.

It has been widely accepted that standard low-level computational approaches to motion processing cannot extract texture-defined motion without applying some pre-processing nonlinearity. This has motivated accounts of motion perception in which luminance- and texture-defined motion are processed by separate mechanisms. Here, we introduce a novel method of image description where motion sequences may be described in terms of their local spatial and temporal gradients. This allows us to assess the local velocity information available to standard low-level motion mechanisms. Our analysis of several texture-motion stimuli shows that the information indicating correct texture-motion velocity and/or direction is present in the raw luminance measures. This raises the possibility that luminance-motion and texture-motion may be processed by the same cortical mechanisms. Our analysis offers a way of looking at texture-motion processing that is, to our knowledge, new and original.

Electrophysiology↗
PubMed1988

Pattern and motion vision without Laplacian zero crossings.

The zero crossings of band-limited signals are known to be rich in information. Recent models of information processing in biological visual systems have proposed that image structure is represented initially by such zero crossings, after the image has been bandpass filtered at multiple scales by neural receptive fields whose two-dimensional profiles resemble the Laplacian of a Gaussian. Because the resulting zero crossings generally correspond to physically meaningful structures such as edges and occlusion boundaries, and indeed under some conditions can exhaustively specify the original image, such representations have proven useful and efficient in a variety of machine vision problems. However, some simple information-processing operations that are apparent in human pattern and motion vision can be shown to be impossible in such representations, because the zero crossings in the bandpassed signals do not capture the necessary information (at any scale of analysis), because the information that they provide is misleading, or because there are no such crossings in the signals after delta 2 G sigma filtering at any scale. Examples are provided of simple visual signal-processing tasks (texture discrimination, motion perception, pattern detection) that human beings can perform effortlessly but that cannot be performed in the proposed multiscale delta 2 G sigma zero-crossings schemes. These perceptual capabilities offer evidence against this model of early image representation in human vision.

Animals↗
PubMedDate not supplied

Inappropriateness of cataract extraction: an analysis in two Israeli hospital settings.

OBJECTIVE: To assess and compare the appropriateness of cataract extraction in two Israeli regional hospitals. SETTINGS: Two Israeli hospitals located in different geographic areas. DESIGN: A randomized sample of 150 patients was drawn from a list of all patients who underwent cataract surgery at the two study hospitals during 1995. Detailed extraction of hospital medical records was performed. The appropriateness of cataract surgery was assessed using the Medical Review System, an interactive expert system that assesses the appropriateness of selected medical and surgical procedures. RESULTS: The rates of cataract surgery in the two hospitals were 0.54 and 0.59 operations per 1,000 population, respectively, and the age-adjusted rates per 1,000 population were 5.7 and 6.2, respectively. The percentage of patients with only light perception or hand-motion perception in the operated eye before the operation was 62.2%, with no difference in the two hospitals. There was not a significant difference in the distribution of visual acuity before the operation; however, there was a significant difference in the distribution of visual acuity after the surgery. Rates of inappropriate surgeries in the two hospitals were found to be similar to the inappropriate rate in the United States (1.3%). The preoperative visual acuity of patients undergoing cataract surgery in Israel was inferior to the visual acuity of patients undergoing cataract surgery in the United States. CONCLUSION: To increase quality and cost-effectiveness in the Israeli medical system, future studies of this type are warranted in connection with surgical procedures.

Cataract Extraction↗
PubMed1999

Organization of disparity-selective neurons in macaque area MT.

Neurons selective for binocular disparity are found in a number of visual cortical areas in primates, but there is little evidence that any of these areas are specialized for disparity processing. We have examined the organization of disparity-selective neurons in the middle temporal visual area (MT), an area shown previously to contain an abundance of disparity-sensitive neurons. We recorded extracellularly from MT neurons at regularly spaced intervals along electrode penetrations that passed through MT either normal to the cortical surface or at a shallow oblique angle. Comparison of multiunit and single-unit recordings shows that neurons are clustered in MT according to their disparity selectivity. Across the surface of MT, disparity-selective neurons are found in discrete patches that are separated by regions of MT that exhibit poor disparity tuning. Within disparity-selective patches of MT, we typically observe a smooth progression of preferred disparities (e.g. , near to far) as our electrode travels parallel to the cortical surface. In electrode penetrations normal to the cortical surface, on the other hand, MT neurons generally have similar disparity tuning, with little variation from one recording site to the next. Thus disparity-tuned neurons are organized into cortical columns by preferred disparity, and preferred disparity is mapped systematically within larger, disparity-tuned patches of MT. Combined with other recent findings, the data suggest that MT plays an important role in stereoscopic depth perception in addition to its well known role in motion perception.

Animals↗
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↗
PubMed2004

Multiplicative nonlinearity in the perception of apparent motion.

Evidence is reported indicating that the perception of apparent motion is better predicted by the multiplicative combination of luminance changes at two element locations than by the sum or squared-sum of the luminance changes, or by the motion energy in the stimulus. Because the results were obtained with a stimulus for which motion was specified by simultaneous luminance changes, they support a Reichardt-style motion detector model, but without the asymmetrical delay specified by current versions. Motion direction in the modified model relies on asymmetrical stimulus information rather than asymmetrical delay. That is, one subunit of the detector responds to changes in luminance toward the background luminance (the start of the motion path), and the other to changes in luminance away from the background luminance (the end of the motion path).

Fourier Analysis↗
PubMed2006

Global motion integration in the postero-medial part of the lateral suprasylvian cortex in the cat.

In cats, the postero-medial part of lateral suprasylvian cortex (PMLS) is generally considered a key area for motion processing. While behavioral studies have indeed supported the role of PMLS cortex in higher order motion integration (Cereb Cortex 6:814-822, 1996), there is no evidence that individual PMLS cells can perform such analysis (Vis Neurosci 5:463-468, 1990; J Neurophysiol 63:1529-1543, 1990). Given the fundamental importance of understanding the neural substrate subtending higher order motion processing, we investigated whether PMLS neurons can signal the direction of motion of complex random dot kinematograms (RDKs) wherein comprising elements do not provide any local coherent motion cues. Results indicated that most PMLS cells (82%) can integrate the displacement of individual elements into a global motion percept. Their large receptive fields allowed the integration of motion for elements separated by large spatial intervals (up to 4 degrees ). In most cases, the analysis of complex RDK motion necessitated the contribution of the area of the visual field beyond the classical receptive field. None of the complex RDK-sensitive cells were found to be pattern-motion selective when tested with plaid patterns. Our results provide the first evidence that receptive fields of PMLS neurons can perform global motion analysis and support the behavioral evidence that this area is implicated in complex motion processing (Cereb Cortex 6:814-822, 1996). It also further corroborates the findings that PMLS neurons cannot signal the true direction of a plaid pattern (Vis Neurosci 5:463-468, 1990; J Neurophysiol 63:1529-1543, 1990). Providing that these same neurons can signal the direction of complex RDKs, there may be distinct cortical mechanisms for processing different types of complex motion.

Action Potentials↗
PubMed2001

The integration of orientation information in the motion correspondence problem.

We examine how differently oriented components contribute to the discrimination of motion direction along a horizontal axis. Stimuli were two-frame random-dot kinematograms that were narrowband filtered in spatial frequency. On each trial, subjects had to state whether motion was to the left or the right. For each stimulus condition, Dmax (the largest displacement supporting 80% correct direction discrimination performance) was measured. In experiment 1, Dmax was measured for orientationally narrowband stimuli as a function of their mean orientation. Dmax was found to increase as the orientation of the stimuli became closer to the axis of motion. Experiment 2 used isotropic stimuli in which some orientation bands contained a coherent motion signal, and some contained only noise. When the noise band started at vertical orientations and increased until only horizontal orientations contained a coherent motion signal, Dmax increased slightly. This suggests that near-vertical orientations interfere with motion perception at large displacements when they contain a coherent motion signal. When the noise band started at horizontal and increased until only vertical orientations contained the motion signal, Dmax decreased steadily. This implies that Dmax depends at least partly on the most horizontal motion signal in the stimulus. These results were contrasted with two models. In the first, the visual system utilises the most informative orientations (nearest horizontal). In the second, all available orientations are used equally. Results supported an intermediate interpretation, in which all orientations are used but more informative ones are weighted more heavily.

Discrimination, Psychological↗
PubMed1997

Can illusory motion disrupt tracking real motion?

When rotating stripes or other periodic stimuli cross the retina at a critical rate, a reversal in the direction of motion of the stimuli is often seen. This illusion of motion perception was used to explore the roles of retinal and perceived motion in the generation of optokinetic nystagmus. Here we show that optokinetic nystagmus is disrupted during the perception of this illusion. Thus, when perceived and actual motion are in conflict, subjects fail to track the veridical movement. This observation suggests that the perception of motion can directly influence optokinetic nystagmus, even in the presence of a moving retinal image. A conflict in the neural representation of motion in different brain areas may explain these findings.

Humans↗
PubMed2001

Motion detection in human vision: a unifying approach based on energy and features.

Most studies of human motion perception have been based on the implicit assumption that the brain has only one motion-detection system, or at least that only one is operational in any given instance. We show, in the context of direction perception in spatially filtered two-frame random-dot kinematograms, that two quite different mechanisms operate simultaneously in the detection of such patterns. One mechanism causes reversal of the perceived direction (reversed-phi motion) when the image contrast is reversed between frames, and is highly dependent on the spatial-frequency content of the image. These characteristics are both signatures of detection based on motion energy. The other mechanism does not produce reversed-phi motion and is unaffected by spatial filtering. This appears to involve the tracking of unsigned complex spatial features. The perceived direction of a filtered dot pattern typically reflects a mixture of the two types of behaviour in any given instance. Although both types of mechanism have previously been invoked to explain the perception of motion of different types of image, the simultaneous involvement of two mechanisms in the detection of the same simple rigid motion of a pattern suggests that motion perception in general results from a combination of mechanisms working simultaneously on different principles in the same circumstances.

Energy Metabolism↗
PubMed1990

How to unconfound the directional and orientational information in visual neuron's response.

When drifting bars or gratings are used as visual stimuli, information about orientation specificity (which has a period of 180 degrees) and direction specificity (which has a period of 360 degrees) is inherently confounded in the response of visual cortical neurons, which have long been known to be selective for both the orientation of the stimulus and the direction of its movement. It is essential to "unconfound" or separate these two components of the response as they may respectively contribute to form and motion perception, two of the main streams of information processing in the mammalian brain. Wörgötter and Eysel (1987) recently proposed the Fourier transform technique as a method of unconfounding the two components, but their analysis was incomplete. Here we formally develop the mathematical tools for this method to calculate the peak angles, bandwidths, and relative strengths, the three most important elements of a tuning curve, of both the orientational and the directional components, based on the experimentally-recorded neuron's response polar-plot. It will be shown that, in the 1-D Fourier decomposition of the polar-plot along its angular dimension, 1) the odd harmonics contain only the directional component, while the even harmonics are contributed to by both the orientational and the directional components; 2) the phases and the amplitudes of all the harmonics are related, respectively, to the peak angle and the bandwidth of the individual component. The basic assumption used here is that the two components are linearly additive; this in turn is immediately testable by the method itself.

Animals↗
PubMed2004

A visual motion sensor based on the properties of V1 and MT neurons.

The motion response properties of neurons increase in complexity as one moves from primary visual cortex (V1), up to higher cortical areas such as the middle temporal (MT) and the medial superior temporal area (MST). Many of the features of V1 neurons can now be replicated using computational models based on spatiotemporal filters. However until recently, relatively little was known about how the motion analysing properties of MT neurons could originate from the V1 neurons that provide their inputs. This has constrained the development of models of the MT-MST stages which have been linked to higher level motion processing tasks such as self-motion perception and depth estimation. I describe the construction of a motion sensor built up in stages from two spatiotemporal filters with properties based on V1 neurons. The resulting composite sensor is shown to have spatiotemporal frequency response profiles, speed and direction tuning responses that are comparable to MT neurons. The sensor is designed to work with digital images and can therefore be used as a realistic front-end to models of MT and MST neuron processing; it can be probed with the same two-dimensional motion stimuli used to test the neurons and has the potential to act as a building block for more complex models of motion processing.

Humans↗
PubMed2004

Neuronal representation of occluded objects in the human brain.

Occluding surfaces frequently obstruct the object of interest yet are easily dealt with by the visual system. Here, we test whether neural areas known to participate in motion perception and eye movements are regions that also process occluded motion. Functional magnetic resonance imaging (fMRI) was used to assess brain activation while subjects watched a moving ball become occluded. Areas activated during occluded motion included the intraparietal sulcus (IPS) as well as middle temporal (MT) regions analogous to monkey MT/MST. A second experiment showed that these results were not due to motor activity. These findings suggest that human cortical regions involved in perceiving occluded motion are similar to regions that process real motion and regions responsible for eye movements. The intraparietal sulcus may be involved in predicting the location of an unseen target for future hand or eye movements.

Adolescent↗
PubMed2003

Failure of signed chromatic apparent motion with luminance masking.

It has been suggested that there are two types of chromatic motion mechanisms: signed chromatic motion, in which correspondence across successive frames is based on chromatic content of image regions, and unsigned chromatic motion based on movement of chromatically-defined borders. We investigate whether signed and unsigned red-green chromatic motion are mediated by a genuinely chromatic mechanism. Direction discrimination of signed and unsigned red-green chromatic motion were measured in the presence of a dynamic luminance masking noise. Increasing the luminance noise contrast systematically impaired signed motion, regardless of contrast and speed. This result suggests that signed red-green chromatic motion is derived from a luminance-based signal, rather than a genuinely chromatic motion mechanism. In the case of unsigned chromatic motion, there is no effect of luminance masking noise, indicating there exists a genuine chromatic mechanism for second-order motion perception.

Color Perception↗