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PubMed2004

Engagement of the prefrontal cortex in representational momentum: an fMRI study.

Behavioral studies have identified a robust phenomenon that an observer's memory of the final position of a moving target is shifted a little further in its motion direction, which is usually called representational momentum (RM). However, the neural substrates underlying RM are poorly understood. The current study measured hemodynamic responses in association with RM using functional magnetic resonance imaging (fMRI). Two experiments using block and event-related designs, respectively, were conducted in which subjects compared the orientation of a probe rectangle with the remembered orientation of the final inducing figures in a set of rotating rectangles. Both experiments showed that, relative to the control task in which behavioral data did not show RM effects, RM task induced stronger activation in the prefrontal cortex. However, no activation was found in MT/MST complex in association with RM. The fMRI results suggest that RM may not simply reflect implicit motion perception and high level cognitive mechanisms underpinned by the prefrontal cortex may be involved in the RM effect.

Adult↗

PubMed1987

Functional properties of models for direction selectivity in the retina.

Poggio and Reichardt (Kybernetik, 13:223-227, 1973) showed that if the average response of a visual system to a moving stimulus is directionally selective, then this sensitivity must be mediated by a nonlinear operation. In particular, it has been proposed that at the behavioral level, motion-sensitive biological systems are implemented by quadratic nonlinearities (Hassenstein and Reichardt: Z. Naturforsch., 11b:513-524, 1956; van Santen and Sperling: J. Opt. Soc. Am. [A] 1:451-473, 1984; Adelson and Bergen: J. Opt. Soc. Am. [A], 2:284-299, 1985). This paper analyzes theoretically two nonlinear neural mechanisms that possibly underlie retinal direction selectivity and explores the conditions under which they behave as a quadratic nonlinearity. The first mechanism is shunting inhibition (Torre and Poggio: Proc. R. Soc. Lond. [Biol.], 202:409-416, 1978), and the second consists of the linear combination of the outputs of a depolarizing and a hyperpolarizing synapse, followed by a threshold operation. It was found that although sometimes possible, it is in practice hard to approximate the Shunting Inhibition and the Threshold models for direction selectivity by quadratic systems. For instance, the level of the threshold on the Threshold model must be close to the steady-state level of the cell's combined synaptic input. Furthermore, for both the Shunting and the Threshold models, the approximation by a quadratic system is only possible for a small range of low contrast stimuli and for situations where the rectifications due to the ON-OFF mechanisms, and to the ganglion cells' action potentials, can be linearized. The main question that this paper leaves open is, how do we account for the apparent quadratic properties of motion perception given that the same properties seem so fragile at the single cell level? Finally, as a result of this study, some system analysis experiments were proposed that can distinguish between different instances of the models.

Animals↗

PubMed2006

Learning to discriminate complex movements: biological versus artificial trajectories.

The recognition of complex body movements and actions is a fundamental visual capacity very important for social communication. It seems possible that movement recognition is based on a general capability of the visual system to learn complex visual motion patterns. Alternatively, this visual function might exploit specialized mechanisms for the analysis of biologically relevant movements, for example, of humans or animals. To investigate this question, we trained human observers to discriminate novel motion patterns that were generated, exploiting a new technique for stimulus generation by motion morphing. We tested the learning of different classes of novel movement stimuli. One group of stimuli was fully consistent with human movements. A second class of stimuli was based on artificial skeleton models that were inconsistent with human and animal bodies. A third group of stimuli specified the same local motion information as human movements but was inconsistent with an underlying articulated shape. Participants learned both classes of articulated movements very fast in an orientation-dependent manner. Learning speed and accuracy were strikingly similar and independent of the similarity of the stimuli with biologically relevant body shapes. For the class of stimuli without underlying articulated shape, however, we did not observe significant improvements of the discrimination performance after training. Our results indicate the existence of a fast visual learning process for complex articulated movement patterns, which likely is relevant for biological motion perception. This process seems to operate independently of the consistency of the patterns with biologically relevant body shapes but seems to require the compatibility of the learned movements with a global underlying shape.

Adult↗

PubMed1993

Dependence on stimulus onset asynchrony in apparent motion: evidence for two mechanisms.

The detection of the direction of motion was measured as a function of the spatial and temporal offset for a kinematogram stimulus presented in two-frame apparent motion. The stimulus was made up of Gabor function micro-patterns randomly distributed across the stimulus field. We show that for short stimulus onset asynchronies (SOA) performance can be predicted from the spatio-temporal Fourier power spectrum of the stimulus, whereas for long SOAs the pattern of performance is qualitatively different from such a prediction. The dependence of motion perception on SOA exhibits an abrupt change from one mode of behaviour to the other. These findings are suggestive of the operation of distinct mechanisms, one "quasi-linear" and one "nonlinear", which can be separated by temporal parameters.

Discrimination, Psychological↗

PubMedDate not supplied

The directions of nystagmus and apparent self-motion evoked by caloric tests and angular accelerations.

PURPOSE: To further investigate the direction of (I) nystagmus and (II) self-motion perception induced by two stimuli: (a) caloric vestibular stimulations and (b) a sudden halt during vertical axis rotation. SUBJECTS AND METHODS: Twelve normal humans received caloric stimulation at 44 degrees C, 30 degrees C, and 20 degrees C while in a supine position with the head inclined 30 degrees upwards. In a second test they were rotated around the vertical axis with the head randomly placed in two positions: tilted 30 degrees forward or tilted 60 degrees backward, at a constant velocity of 90 degrees /sec for 2 minutes and then suddenly stopped. After both tests they were asked to describe their sensations of self-motion. Eye movements were recorded with an infrared video-technique. RESULTS: Caloric stimulation evoked only horizontal nystagmus in all subjects and induced a non-uniform complex perception of angular in frontal and transverse planes (the former dominated) and linear movements along the antero-posterior axis (sinking dominated) of the subject's coordinates. The self-motion was felt with the whole body or with a part of the body. Generally the perception evoked by cold (30 degrees C) and warm (44 degrees C) calorics was similar, although there were some differences. The stronger stimulus (20 degrees C) evoked not only quantitative but also qualitative differences in perception. The abrupt halt of rotation induced self-motion perception and nystagmus only in the plane of rotation. The self-motion was felt with the whole body. CONCLUSION: There was no difference in the nystagmus evoked by caloric stimulation and a sudden halt of vertical axis rotation (in head positions to stimulate the horizontal canals); however, the two stimuli evoked different perceptions of self-motion. Calorics provoked the sensation of self-rotation in the frontal plane and linear motion, which did not correspond to the direction of nystagmus, as well as arcing and a reset phenomenon during angular and linear self-motion, caloric-induced self-motion can be felt predominantly or only with a part of the body, depending on the self-motion intensity. The findings indicate that, unlike the self-motion induced by sudden halt of vertical axis rotation, several mechanisms take part in generating caloric-induced self-motion.

Acceleration↗

PubMed1996

High-power graphic computers for visual simulation: a real-time--rendering revolution.

Advances in high-end graphics computers in the past decade have made it possible to render visual scenes of incredible complexity and realism in real time. These new capabilities make it possible to manipulate and investigate the interactions of observers with their visual world in ways once only dreamed of. This paper reviews how these developments have affected two preexisting domains of behavioral research (flight simulation and motion perception) and have created a new domain (virtual environment research) which provides tools and challenges for the perceptual psychologist. Finally, the current limitations of these technologies are considered, with an eye toward how perceptual psychologist might shape future developments.

Aircraft↗

PubMed2003

Pattern cues disambiguate perceived direction in simple moving stimuli.

Much of our knowledge about motion perception has been obtained by studying bars moving within apertures. When viewed within an ambiguously oriented aperture such as a circle, bars appear to move orthogonal to their orientation. We demonstrate that if the local orientation of the aperture edges is altered, a direction consistent with the edge orientation is seen. Indeed, the perceived direction can be strongly influenced by static lines separated from the edge of the moving stimulus. These results support recent suggestions that precise motion direction is likely to be determined by static orientation cues.

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↗

PubMed2006

Motion aftereffects specific to surface depth order: beyond binocular disparity.

Despite evidence for concurrent processing of motion and stereopsis from psychophysics and neurophysiology, the detailed relationship between depth and motion processing is not yet clear. Using the contingent aftereffect paradigm, we investigated how the order of surfaces presented across depth influenced motion perception. After having observers adapt to two superimposed populations of dots moving in opposite directions at different binocular disparities, we assessed how much of the motion aftereffect (MAE) was specific to absolute disparity and how much was specific to the depth order of the surfaces. The test contained two planes of moving dots at several different pairs of disparities and asked observers to report the MAE direction at one of the planes (the target). In addition to the disparity-contingent MAE (Verstraten, Verlinde, Fredericksen, & van de Grind, 1994), we found MAEs dependent on surface order. When the target surface was in front of another surface, observers more often reported the MAE in the direction opposite to the front adapting surface than the back. This effect was observed despite differences in absolute and relative disparity between the adapted and test surfaces. The results suggest that some motion information is represented in terms of surface depth order.

Adult↗

PubMed2000

Depth motion sensitivity functions.

Functions reliably describing perception of motion in depth have been established experimentally by using psychophysical methods of size and distance estimations and threshold measurements. The stimuli were generated with a new hybrid technique yielding an image refresh rate of 1667 Hz. In this way it was possible to generate rapid expansions and contractions of the moving checkerboard pattern constituting the stimulus for depth motion perception. The results showed that perceived size constancy as well as depth impression varied with oscillation frequency. Under the conditions of slow motions (oscillation frequencies around 2 Hz), perfect size constancy was obtained. Above that limit, size constancy systematically decreased, and with oscillation frequencies of about 5 Hz the perceived size constancy was close to zero when small-sized patterns were used. Under the conditions of wide field stimulation (when the pattern subtended 66 degrees of visual angle), the cut-off limit increased to 16 Hz. Since the perception of depth motion amplitudes as well as perceived velocities of the visual object are related to perceived size constancy, the findings have certain implications for theoretical explanations of depth motion perception.

Depth Perception↗

PubMed1992

Heterochromatic Fusion Nystagmus: its use in estimating chromatic equiluminance in humans and monkeys.

The use of chromatic patterns that are equated for luminance has become increasingly popular in psychophysical and neurophysiological studies of visual processing. The currently available techniques for equating different colors for brightness rely upon human reports of perceptual events that are reduced at some luminance ratio. We report here the results of a study using a technique we have recently developed that produces a vivid and compelling motion percept only at isoluminance. That is, unlike previous methods, this technique relies upon a perceptual event (motion) that actually becomes more salient at isoluminance. We also observed that the optokinesis generated by the moving pattern mirrors the perceptual reports at all luminance ratios. If used in this manner, the technique can provide an estimate of chromatic isoluminance in a variety of species and can be used to corroborate a human subject's perceptual experience.

Animals↗

PubMed1991

Displacement limit (dmax) of sampled directional motion: direct and indirect estimates.

The maximum displacement at which directional motion can be seen, known as dmax, has been said to define the spatial limits of the short-range motion system. Turano and Pantle (1985) used duration of motion aftereffect (MAE) to estimate the spatial limit of the short-range system, the assumption that dmax (a direct measure of motion perception) and MAE (an indirect measure) are equivalent indices of the same underlying perceptual process. In a series of four experiments, we examined this assumption by measuring dmax and duration of MAE across a range of displacements, stimulus waveforms (sine- or square-wave gratings), and spatial frequencies. We found that dmax and duration of MAE were affected differently by changes in the same variables. Therefore, we concluded that the two indices cannot be regarded as equivalent measures of the spatial limits of the short-range process. Two novel effects that separated MAE from motion detection are described, and suggestions for exploring them are outlined.

Adult↗

PubMed1999

Evaluation of the Night Vision Spectacles on patients with impaired night vision.

BACKGROUND: The Night Vision Spectacles (NiViS) were developed by a consortium of European companies to assist individuals who suffer from impaired night vision. They consist of a head-mounted video camera (input) and binocular displays (output) connected to a portable computer processor, which uses an algorithm to enhance the luminance and contrast of the video image. METHODS: Eighteen patients with impaired night vision were tested, including those with retinitis pigmentosa (7), Usher syndrome (2), fundus albipunctatus (1) and complete (4) and incomplete (4) congenital stationary night blindness. Normal trichromats (3) and typical, complete achromats (2) acted as controls. A battery of tests assessed: visual acuity at 5 m (projection unit) and 1 m (chart) and at high and low contrasts; contrast sensitivity; absolute and increment threshold; the influence of glare; contrast motion detection; and hand-eye performance. The tests were performed, with and without the NiViS, at three adaptation levels: low scotopic (10(-3) cd/m2), high scotopic (10(-2) cd/m2) and mesopic (10(-1) cd/m2). RESULTS: At the low and high scotopic levels, the majority of patients showed improved performance on the visual acuity, contrast sensitivity and motion contrast tests with the NiViS. At the mesopic level, the advantage with the NiViS was greatly reduced, but still present for contrast sensitivity. CONCLUSION: Patients with impaired night vision can benefit from the NiViS when performing tasks involving contrast and motion perception. Those with normal visual fields and retaining good photopic vision will benefit more than those with constricted visual fields and impaired cone vision. Recommendations regarding desirable improvements of the NiViS and suitability for the individual patient are given.

Adolescent↗

PubMed1987

Minimum points and views for the recovery of three-dimensional structure.

Mathematical analyses of motion perception have established minimum combinations of points and distinct views that are sufficient to recover three-dimensional (3D) structure from two-dimensional (2D) images, using such regularities as rigid motion, fixed axis of rotation, and constant angular velocity. To determine whether human subjects could recover 3D information at these theoretical levels, we presented subjects with pairs of displays and asked them to determine whether they represented the same or different 3D structures. Number of points was varied between two and five; number of views was varied between two and six; and the motion was fixed axis with constant angular velocity, fixed axis with variable velocity, or variable axis with variable velocity. Accuracy increased with views, decreased with points, and was greater with fixed-axis motion. Subjects performed above chance levels even when motion was eliminated, indicating that they exploited regularities in addition to those in the theoretical analyses.

Depth Perception↗

PubMed1995

Restricted ability to recover three-dimensional global motion from one-dimensional local signals: theoretical observations.

Recovering 3D information from a 2D time-varying image is a vital task which human observers face daily. Numerous models exist which compute global 3D structure and motion on the basis of 2D local motion measurements of point-like elements. On the other hand, both experimental and computational research of early visual motion mechanisms emphasize the role of oriented (1D) detectors. Therefore, it is important to find out whether indeed 1D motion signals can serve as primary cues for 3D global motion computation. We have addressed this question by combining mathematical results and perceptual observations. We show that given the 2D-projected 1D instantaneous velocity field, it is mathematically impossible to discriminate rigid rotations from non-rigid transformations and/or to recover the rotation parameters. We relate this fact to existing results in cases where localized (point-like) cues are present, and to our own experiments on human performance in global motion perception when only 1D cues are given. Taken together, the data suggest a necessary role for localized information in early motion mechanisms and call for further physiological and psychophysical research in that direction.

Cues↗

PubMed2004

The geometry of the occluding contour and its effect on motion interpretation.

Form information related to occlusion is needed to correctly interpret image motion. This work describes one of a series of investigations into the form constraints on motion perception. In the present study, we focus specifically on the geometry of the occluding contour, and in particular on whether its influence on motion can be accounted for merely by its effect on perceived occlusion. We used an occluded square moving in a circle, holding the T-junctions at points of occlusion constant while manipulating the occluding contour. We found evidence for two main influences of occluding contour geometry on motion interpretation and occlusion: the convexity of the occluding contour and additional static T-junctions that are formed elsewhere on the occluding contour. Our results suggest that convex occluding contours are more occlusive than concave ones, and that T-junctions along the contour increase or decrease the strength of occlusion depending on their orientation. Motion interpretation is influenced by both factors, but their effect on motion appears to be dominated by interactions occurring at an intermediate "semilocal" scale, which is larger than the scale at which junctions are defined, but smaller than the scale of the whole moving figure. We propose that these computations are related to occlusion but are not identical to the computations that mediate static occlusion judgments.

Form Perception↗

PubMed1990

Angular velocity, not temporal frequency determines circular vection.

This paper shows that the experienced speed of circular vection depends on stimulus speed, not on stimulus temporal frequency. But why would anyone think the contrary? The point is that many modelers in the field of motion perception believe that perceived speed is determined by temporal frequency. Moreover, the optokinetic behaviour of the fly is said to be dependent on the temporal frequency, not the speed, of the stimulus pattern (Reichardt, 1987). It was the aim of the present experiment to test the notion that the experienced speed of circular vection is proportional to stimulus velocity information, which is carried by the temporal and the spatial characteristics of light.

Adult↗

PubMed2003

Human V5 demonstrated by magnetoencephalography using random dot kinematograms of different coherence levels.

To investigate the cortical mechanisms for motion perception in human V5, we measured visual evoked magnetic fields in response to random dot kinematograms (RDKs) of three different coherence levels (50, 70 and 100%) using a 122-channel whole-head magnetometer. As the coherence level increased, the peak amplitude measured by the root mean square (RMS) of the local response increased significantly (7.4+/-1.0, 9.5+/-1.5 and 15.5+/-3.2 fT/cm on the right, 6.4+/-0.3, 7.8+/-0.7 and 12.5+/-0.9 fT/cm on the left; for the coherence level of 50, 70 and 100%, respectively). There was no significant difference between the hemispheres. As for the peak latency, there was no significant difference in terms of coherence levels or hemispheres. The response was localized posterior to the junction of the ascending limb of the inferior temporal and lateral occipital sulci (human V5). These findings indicate that processing of global motion in terms of the synchronized portion correlates well with the response amplitude but not with its latency. Thus, we could estimate the magnetic responses of human V5 non-invasively by presenting different coherence levels of the visual motion stimuli. Hemispheric laterality was recognized, although the dominant side varied among subjects.

Adult↗