Binocular influences on global motion processing in the human visual system.

This study investigates four key issues concerning the binocular properties of the mechanisms that encode global motion in human vision: (1) the extent of any binocular advantage; (2) the possible site of this binocular summation; (3) whether or not purely monocular inputs exist for global motion perception; (4) the extent of any dichoptic interaction. Global motion coherence thresholds were measured using random-dot-kinematograms as a function of the dot modulation depth (contrast) for translational, radial and circular flow fields. We found a marked binocular advantage of approximately 1.7, comparable for all three types of motion and the performance benefit was due to a contrast rather than a global motion enhancement. In addition, we found no evidence for any purely monocular influences on global motion detection. The results suggest that the site of binocular combination for global motion perception occurs prior to the extra-striate cortex where motion integration occurs. All cells involved are binocular and exhibit dichoptic interactions, suggesting the existence of a neural mechanism that involves more than just simple summation of the two monocular inputs.

The extent of the dorsal extra-striate deficit in amblyopia.

Previously, we have shown that humans with amblyopia exhibit deficits for global motion discrimination that cannot be simply ascribed to a reduction in visibility or contrast sensitivity. Deficits exist in the processing of global motion in the fronto-parallel plane that suggest reduced extra-striate function (i.e., MT) in amblyopia. Here, we ask whether such a deficit also exists for rotation and radial components of optic flow that are first processed at higher sites along the dorsal pathway (i.e., MSTd). We show that similar motion processing deficits occur in our amblyopic group as a whole for translation, rotation, and radial components of optic flow and that none of these can be solely accounted for by the reduced visibility of the stimuli. Furthermore, on a subject-by-subject basis there is no significant correlation between the motion deficits for radial and rotational motion and those for translation, consistent with independent deficits in dorsal pathway function up to and including MSTd.

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.

Adaptation to second-order motion results in a motion aftereffect for directionally-ambiguous test stimuli.

The magnitude of the motion aftereffect (MAE) obtained following adaptation to first- or second-order motion was measured in two experiments using a nulling method. The second-order motion adaptation stimulus was composed of contrast-modulated noise produced by multiplying two-dimensional random noise by a drifting, 1 c/deg, vertical sine grating. The first-order motion adaptation stimulus was composed of luminance-modulated noise produced by adding, rather than multiplying, the sine grating and noise field. The test stimuli were directionally-ambiguous first- or second-order motion patterns composed of either two oppositely drifting sine gratings added to static noise or its contrast-modulated equivalent. The amplitudes of the two drifting components were manipulated such that as one increased in amplitude the other decreased in amplitude by the same degree. This technique was employed to estimate the null point at which the test no longer appeared to drift in the direction opposite the adaptation direction. In the first experiment all stimuli were equated for visibility by presenting them at the same multiple of threshold and all possible combinations of first- and second-order motion adaptation and test stimuli were examined. The results were similar for all conditions: following adaptation the amplitude of the test component drifting in the same direction as adaptation needed to be approximately twice that of the oppositely drifting component in order to null the perception of unidirectional motion of the test. In a second experiment, the effects of manipulating the amplitude (visibility) of the first- and second-order motion adaptation stimuli on MAE magnitude were investigated. This revealed an approximately linear relationship between MAE magnitude and the amplitudes of the adaptation stimuli. The results demonstrate that, contrary to the findings of several previous studies, adaptation to second-order motion does produce a substantial movement aftereffect. Cross-adaptation between first- and second-order motion stimuli also occurs under appropriate conditions and produces aftereffects that are comparable in magnitude when the stimuli are equated for visibility.

How similar must the Fourier spectra of the frames of a random-dot kinematogram be to support motion perception?

Direction-discrimination performance was measured for two-frame random-dot kinematograms in which one or both frames were spatial frequency filtered with a one octave band-pass filter and the centre frequency of this filter was varied in the range 0.75-9 c/deg independently for each frame. When both frames were filtered so that they contained common (overlapping) spatial frequencies direction discrimination was extremely good but it deteriorated rapidly as the degree of spectral overlap between the two frames decreased. These results are consistent with previous findings that suggest that the mechanisms that mediate the initial stages of motion detection are narrowly tuned for spatial frequency and cannot combine information conveyed at disparate frequencies in order to compute an unambiguous estimate of the direction of local motion. However, when only one of the frames was band-pass filtered and the other was unfiltered (broadband), the correct direction of stimulus motion could be discriminated reliably for a broad range of filter centre frequencies. Performance was best when the centre frequency of the filtered frame was at medium spatial frequencies and tended to deteriorate as the centre frequency approached either extreme of the spatial frequency range examined. This basic pattern of results may be attributed to the visual system's differential sensitivity to the Fourier components present in the unfiltered frame.

Discrimination of the speed and direction of global second-order motion in stochastic displays.

The ability to integrate local second-order motion signals over space and time was examined using random-dot-kinematograms (RDKs) in which the dots were defined by spatial variation in the contrast, rather than luminance, of a random noise field. When either the speeds or the directions of the individual dots were selected at random from a range of possible values, globally the stimulus appeared to drift either in a single direction or at a single speed in a manner analogous to that reported previously for first-order (luminance-defined) RDKs. To quantify the precision with which observers could extract the global stimulus motion, speed- and direction-discrimination thresholds were measured using pairs of RDKs, one of which (the comparison) comprised dots whose speeds or directions were assigned stochastically and the other (the standard) comprised dots that all had the same drift direction and speed. Speed-discrimination thresholds were of the order of 8% and changed little as the range of dot speeds (bandwidth) of the comparison increased, in that performance was almost as good when the individual dot speeds were selected at random from a range spanning 3.84 deg/s as when all the dots moved at the same speed. There was a tendency for the perceived global speed of the comparison RDK to decrease as the speed bandwidth was increased and perceived speed tended to coincide with the geometric mean speed of the dots rather than the arithmetic mean speed. Direction-discrimination thresholds were lowest (approximately 4 degrees) when the range of dot directions was less than 90 degrees but increased markedly thereafter. Observers were able to perform both discrimination tasks when the lifetimes of the dots comprising the RDKs was reduced from 25 to 2 frames, a manipulation that prevented observers from determining the overall speed or direction of image motion from the extended trajectories of individual dots within the display. Thresholds under these conditions were somewhat higher but were otherwise comparable to those obtained with a dot lifetime of 25 frames. The similarities between the present results and those of previous studies that have employed first-order RDKs suggest that the extraction of the global speed and direction of each type of motion is likely to be based on computationally similar principles.

The properties of the motion-detecting mechanisms mediating perceived direction in stochastic displays.

Previous studies [e.g. Baker & Hess, 1998. Vision Research, 38, 1211-1222] have shown that perceived direction in displays composed of multiple, limited-lifetime, Gabor micropatterns (G) is influenced by movement both at the fine spatial scale of the internal luminance modulation (first-order motion) and the coarse spatial scale of the Gaussian, contrast window (second-order motion). However it is presently indeterminate as to whether this pattern of results is indicative of the processes by which first-order and second-order motion signals interact within the visual system per se or those by which motion information, irrespective of how it is defined, is utilised across different spatial scales. To address this issue, and more generally the properties of the mechanisms that analyse motion in such displays, we employed stochastic motion sequences composed of either G, G added to a static carrier (G + C) or G multiplied with a carrier (G*C). Crucially G*C, unlike both G and G + C, micropatterns contain no net first-order motion and second-order motion only at the scale of the internal contrast modulation. For small displacements perceived direction in all cases showed a dependence on the internal sinusoidal spatial structure of the micropatterns and characteristic oscillations were typically observed, consistent with models in which first-order motion and second-order motion are encoded on the basis of similar low-level mechanisms. Importantly for larger displacements, and also when the internal spatial structure was randomised on successive exposures (so that motion at this spatial scale was unreliable), performance tended to be veridical for all types of micropattern, even though under these conditions displacements of the G*C micropatterns should have been invisible to current, low-level, motion-detecting schemes. This suggests that both low-level motion sensors and mechanisms utilising a different motion-detecting strategy such as high-level, attentive, feature-tracking may mediate perceptual judgements in stochastic displays.

Evidence for separate motion-detecting mechanisms for first- and second-order motion in human vision.

Current theories of second-order motion perception postulate that such motion is detected by either a high-level mechanism which computes the temporal correspondences between "features" extracted from the image, or low-level motion mechanisms which operate on a nonlinear, neural transformation of the luminance profile of the image. Theories which favour the latter strategy either suggest that first- and second-order motion are detected by a common mechanism or else that distinct mechanisms exist for the two types of motion, both operating on similar principles. The aim of this study was to differentiate between these possibilities. Observers were required to judge the direction of multiframe motion sequences in which the frames alternated between sinusoidal variations in luminance (first order) and similar variations in contrast (second order). On each frame the modulation signal was displaced by some fraction of its spatial period. The motion sequences were designed such that integration of both types of frame (first and second order) would lead to unambiguous motion in a particular direction whilst separate analysis of first- or second-order frames alone would yield ambiguous motion. The results show clearly that observers were unable to integrate the first- and second-order frames of such motion sequences. However, when observers were presented with motion sequences in which the frames alternated between two, different types of second-order image (variations in the contrast or size of the elements constituting a random noise field) perceived direction was always consistent with integration of both image types. This is taken as support for models that suggest that first- and second-order motion are processed by distinct mechanisms in the visual system and that each mechanism is only sensitive to one type of motion. It is suggested that several varieties of second-order motion stimuli may be regarded as equivalent to contrast-modulated images when considered in terms of the effects of local spatiotemporal filtering operations carried out by the human visual system. In this respect, our results are consistent with the "texture grabber" concept of Werkhoven, Sperling and Chubb [(1993) Vision Research, 33, 463-485].

The perceived speed of second-order motion and its dependence on stimulus contrast.

Speed matches were obtained, using a spatial two-alternative forced-choice task, between a second-order motion stimulus and a first-order motion stimulus. The second-order motion stimulus was composed of contrast-modulated noise [produced by multiplying two-dimensional (2-d), static noise by a drifting, one-dimensional (1-d) sinusoid]. The first-order motion stimulus was composed of luminance-modulated noise (produced by summing, rather than multiplying, 2-d noise and a drifting sine grafting). In Expt 1, the relationship between the perceived speed of first- and second-order motion was examined. The motion stimuli had the same spatial frequency (1 or 3 c/deg) and were equated for visibility by presenting them at the same multiple of direction-identification threshold. Over a range of physical speeds, the perceived speeds of the first-order and second-order motion stimuli were identical when their physical speeds were the same. In Expt 2, the effect of varying stimulus "contrast" (contrast modulation depth) on the perceived speed of second-order motion was examined. The contrast of the first-order motion stimulus was fixed and speed matches were obtained for second-order motion stimuli at several contrast modulation depths. The motion stimuli had the same spatial (1 or 4 c/deg) and temporal (5 or 20 Hz) frequencies. It was found that the perceived speed of second-order motion was approximately linearly related to log modulation depth. In agreement with previous studies we also confirmed that the perceived speed of first-order motion is similarly dependent on stimulus contrast (luminance modulation depth).(ABSTRACT TRUNCATED AT 250 WORDS)

Dual multiple-scale processing for motion in the human visual system.

A number of psychophysical and physiological studies have suggested that first- and second-order motion signals are processed, at least initially, by independent pathways, and that the two pathways both consist of multiple motion-detecting channels that are each narrowly tuned to a different spatial scale (spatial frequency). However, the precise number and nature of the mechanisms that subserve first- and second-order motion perception in human vision remain both controversial and speculative. We sought to clarify this issue by conducting selective adaptation experiments, in which modulation-depth thresholds for identifying the direction of stimulus motion of first-order (luminance-defined) and second-order (contrast-defined) drifting gratings were measured both prior to and following adaptation to motion. The drift direction, spatial frequency and stimulus type (either first- or second-order) of the adaptation and test stimuli were systematically manipulated. When the adaptation and test stimuli were either both first-order gratings or both second-order gratings, robust elevations of direction-identification thresholds were found and, importantly, these aftereffects exhibited both direction-selectivity and spatial-frequency selectivity. Cross-over-adaptation effects between first- and second-order gratings were also sometimes observed, but were very weak and not spatial-frequency selective. These findings give direct support for the existence of multiple-scale processing for first- and second-order motion in the human visual system and provide additional evidence that the two varieties of motion are initially processed by independent pathways.

Sensitivity to second-order motion as a function of temporal frequency and eccentricity.

There is considerable evidence that second-order motion, such as motion consisting of a drifting contrast modulation, is detected separately from first-order motion. Some previous studies have shown that the rate at which sensitivity declines as either drift speed or eccentricity increases is the same for both types of motion. However, these studies have used second-order motion stimuli based on static noise carriers, which we have shown (Smith & Ledgeway, 1997) may be inappropriate because they can give rise to local first-order artifacts. By using dynamic noise carriers, we isolate the second-order motion mechanism and show that its temporal response is much worse than that of the first-order system but that its rate of sensitivity loss with increasing stimulus eccentricity is indeed similar to that of the first-order motion system.

Changes in perceived speed following adaptation to first-order and second-order motion.

To investigate whether or not adaptation to second-order motion can cause changes in perceived speed, measurements of perceived speed were obtained for two varieties of motion: (i) contrast-modulated two-dimensional static noise (second-order motion); and (ii) luminance-modulated noise (first-order motion). The test stimulus (either first-order or second-order) was presented to one side of a central fixation spot and a comparison stimulus (always first-order) was simultaneously presented on the opposite side. The observer's task was to indicate which of the two motion stimuli appeared to drift faster. The perceived speed of the test stimulus was measured with and without prior adaptation to motion on one side of the fixation spot only (that of the test stimulus). The modulation depth of the adaptation stimulus was always half that of the test stimulus and all test patterns were equated for visibility. The pattern of results for second-order motion was similar to that for first-order motion. Typically, adaptation reduced perceived speed, particularly when the adaptation speed was faster than the test speed. However, when the adaptation speed was low relative to the test speed, increases in perceived speed were found. Cross-over adaptation effects between first-order and second-order motion were also observed. Robust velocity aftereffects were found for second-order motion when the noise was dynamic or was high-pass filtered, suggesting that first-order (luminance) artifacts were not responsible for the velocity aftereffects observed. We conclude that the perceived speeds of first-order and second-order motion appear to be encoded in human vision using similar computational principles (but not necessarily utilizing the same mechanism), since the same pattern of results was found for the two varieties of motion.

Separate detection of moving luminance and contrast modulations: fact or artifact?

We have investigated first-order artifacts in second-order motion perception. Subjects were required to identify the orientation and direction of a drifting sinusoidal contrast modulation. When the carrier consisted of static two-dimensional noise, performance often reflected the use of first-order artifacts that arise from stochastic local biases in the noise, rather than the detection of the contrast modulation per se. This stimulus, which has been used widely for studying second-order motion, therefore appears to be inappropriate for that purpose. In contrast, global distortion products arising from luminance non-linearities do not appear to provide usable artifacts. Two manipulations were employed to eliminate local first-order artifacts: the use of dynamic noise and the use of high-pass filtered static noise. These two manipulations gave similar results, which were quite different from those obtained with broadband static noise. We argue that performance with both of these image types reflects the activity of a true second-order motion mechanism. A characteristic property of this mechanism is that it cannot specify direction at the threshold for detecting orientation. Direction thresholds are around 50% higher than orientation thresholds when first-order artifacts are eliminated.

Transparent motion from feature- and luminance-based processes.

The apparent motion of a discretely displaced complex waveform with a periodic contrast modulation or "beat" of frequency f and sinusoidal components of frequencies 3f and 4f was examined at various interstimulus intervals (ISIs). At short ISIs perception of motion of both the beat and an aliased component of the waveform results in transparent motion. At longer ISIs motion is perceived only in the direction of the features of the waveform. The transparent motion observed at short ISIs indicates that, under certain conditions, "short-range" motion sensors do not constrain "long-range" feature processing and both may be active simultaneously.

Impoverished second-order input to global linking in human vision.

Recent evidence points to the importance of global operations across spatial regions larger than individual cortical receptive fields. Studies of contour integration and motion trajectory detection suggest that network operations between local detectors underlie the encoding of extended contours in space and extended trajectories in motion. Here we ask whether such network operations also occur between second-order-detectors known to exist in visual cortex. We compared performance for stimuli composed of either first-order or second-order elements equated for visibility, and we show that unlike the first-order case, there is little or no linking interaction between local second-order detectors. Near chance performance was found for elements defined by second-order attributes when observers had to identify either an elongated spatial contour or an extended motion trajectory embedded in noise elements. This implies that the network operations thought to underlie these two global tasks receive, at best, an impoverished input from local detectors that encode second-order image attributes.

Putting order into the development of sensitivity to global motion.

We studied differences in the development of sensitivity to first-versus second-order global motion by comparing the motion coherence thresholds of 5-year-olds and adults tested at three speeds (1.5, 6, and 9 degrees s(-1)). We used Random Gabor Kinematograms (RGKs) formed with luminance-modulated (first-order) or contrast-modulated (second-order) concentric Gabor patterns with a sinusoidal spatial frequency of 3c deg(-1). To achieve equal visibility, modulation depth was set at 30% for first-order Gabors and at 100%, for second-order Gabors. Subjects were 24 adults and 24 5-year-olds. For both first- and second-order global motion, the motion coherence threshold of 5-year-olds was less mature for the slowest speed (1.5 degrees s(-1)) than for the two faster speeds (6 and 9 degrees s(-1)). In addition, at the slowest speed, the immaturity was greater for second-order than for first-order global motion. The findings suggest that the extrastriate mechanisms underlying the perception of global motion are different, at least in part, for first- versus second-order signals and for slower versus faster speeds. They also suggest that those separate mechanisms mature at different rates during middle childhood.

The effects of spatial offset, temporal offset and image speed on sensitivity to global motion in human amblyopia.

The presence of a general global motion processing deficit in amblyopia is now well established, although its severity may depend on image speed and amblyopia type, but its underlying cause(s) is still largely indeterminate. To address this issue and to characterize further the nature of the global motion perception deficit in human amblyopia, the effects of varying spatial offset (jump size-Δs) and temporal offset (delay between positional updates-Δt) in discriminating global motion for a range of speeds (1.5, 3 and 9°/s) in both amblyopic and normal vision were evaluated. For normal adult observers (NE) and the non-amblyopic eye (FE) motion coherence thresholds measured when Δt was varied were significantly higher than those when Δs was varied. Furthermore when Δt was varied, thresholds rose significantly as the speed of image motion decreased for both NEs and FEs. AE thresholds were higher overall than the other eyes and appeared independent of both the method used to create movement and speed. These results suggest that the spatial and temporal limits underlying the perception of global motion are different. In addition degrading the smoothness of motion has comparatively little effect on the motion mechanisms driven by the AE, suggesting that the internal noise associated with encoding motion direction is relatively high.

Visual deficits in amblyopia constrain normal models of second-order motion processing.

It is well established that amblyopes exhibit deficits in processing first-order (luminance-defined) patterns. This is readily manifest by measuring spatiotemporal sensitivity (i.e. the "window of visibility") to moving luminance gratings. However the window of visibility to moving second-order (texture-defined) patterns has not been systematically studied in amblyopia. To address this issue monocular modulation sensitivity (1/threshold) to first-order motion and four different varieties of second-order motion (modulations of either the contrast, flicker, size or orientation of visual noise) was measured over a five-octave range of spatial and temporal frequencies. Compared to normals amblyopes are not only impaired in the processing of first-order motion, but overall they exhibit both higher thresholds and a much narrower window of visibility to second-order images. However amblyopia can differentially impair the perception of some types of second-order motion much more than others and crucially the precise pattern of deficits varies markedly between individuals (even for those with the same conventional visual acuity measures). For the most severely impaired amblyopes certain second-order (texture) cues to movement in the environment are effectively invisible. These results place important constraints on the possible architecture of models of second-order motion perception in human vision.

The effect of displacement on sensitivity to first- and second-order global motion in 5-year-olds and adults.

We compared the development of sensitivity to first- versus second-order global motion in 5-year-olds (n=24) and adults (n=24) tested at three displacements (0.1, 0.5 and 1.0 degrees). Sensitivity was measured with Random-Gabor Kinematograms (RGKs) formed with luminance-modulated (first-order) or contrast-modulated (second-order) concentric Gabor patterns. Five-year-olds were less sensitive than adults to the direction of both first- and second-order global motion at every displacement tested. In addition, the immaturity was smallest at the smallest displacement, which required the least spatial integration, and smaller for first-order than for second-order global motion at the middle displacement. The findings suggest that the development of sensitivity to global motion is limited by the development of spatial integration and by different rates of development of sensitivity to first- versus second-order signals.