Second-order Motion Research Articles

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.

Perception of Fourier and non-Fourier motion by larval zebrafish.

A moving grating elicits innate optomotor behavior in zebrafish larvae; they swim in the direction of perceived motion. We took advantage of this behavior, using computer-animated displays, to determine what attributes of motion are extracted by the fish visual system. As in humans, first-order (luminance-defined or Fourier) signals dominated motion perception in fish; edges or other features had little or no effect when presented with these signals. Humans can see complex movements that lack first-order cues, an ability that is usually ascribed to higher-level processing in the visual cortex. Here we show that second-order (non-Fourier) motion displays induced optomotor behavior in zebrafish larvae, which do not have a cortex. We suggest that second-order motion is extracted early in the lower vertebrate visual pathway.

Heading judgement from second-order motion

We examined human heading judgement from second-order motion which was generated by random-dots with the contrast polarity determined randomly on each frame. It was found that human observers can judge heading fairly accurately from second-order motion when pure translation is simulated or when self-motion toward a ground plane with gaze rotation is simulated but they cannot when self-motion toward cloud-like random dots with gaze rotations is simulated. It is suggested that the human visual system cannot decompose the flow fields into rotational and translational components by using second-order motion information alone, but it can do in some ways from the flow field of the ground plane.

A transient deficit of motion perception in human

We studied the motion perception abilities in a young adult, SF, who had her right occipito-temporal cortices resected to treat epilepsy. Following resection, SF showed transient deficits of both first- and second-order motion perception that recovered to normal within weeks. Previous human studies have shown either first- or second n order motion deficits that have lasted months or years after cerebral damage. SF also showed a transient defect in processing of shape-from-motion with normal perception of shape from non-motion cues. Furthermore, she showed greatly increased reaction times for a mental rotation task, but not for a lexical decision task. The nature and quick recovery of the deficits in SF resembles the transient motion perception deficit observed in monkey following ibotenic acid lesions, and provides additional evidence that humans possess specialized cortical areas subserving similar motion perception functions.

Initiation of smooth-pursuit eye movements to first-order and second-order motion stimuli.

Since normal human subjects can perform smooth-pursuit eye movements only in the presence of a moving target, the occurrence of these eye movements represents an ideal behavioural probe to monitor the successful processing of visual motion. It has been shown previously that subjects can execute smooth-pursuit eye movements to targets defined by luminance and colour, the first-order stimulus attributes, as well as to targets defined by derived, second-order stimulus attributes such as contrast, flicker or motion. In contrast to these earlier experiments focusing on steady-state pursuit, the present study addressed the course of pre-saccadic pursuit initiation (less than 100 ms), as this early time period is thought to represent open-loop pursuit, i.e. the eye movements are exclusively driven by visual inputs proceeding the onset of the eye movement itself. Eye movements of five human subjects tracking first- and second-order motion stimuli had been measured. The analysis of the obtained eye traces revealed that smooth-pursuit eye movements could be initiated to first-order as well as second-order motion stimuli, even before the execution of the first initial saccade. In contrast to steady-state pursuit, the initiation of pursuit was not exclusively determined by the movement of the target, but rather due to an interaction between dominant first-order and less-weighted second-order motion components. Based on our results, two conclusions may be drawn: first and specific for initiation of smooth-pursuit eye movements, we present evidence supporting the notion that initiation of pursuit reflects integration of all available visual motion information. Second and more general, our results further support the hypothesis that the visual system consists of more than one mechanism for the extraction of first-order and second-order motion.

What limits the contribution of second-order motion to the perception of surface shape?

Both motion and stereopsis can be derived from contrast as well as luminance defined stimuli. It is currently assumed that these two different sources of information about objects feed into one common stage. Thus it would not be expected that their role in visual perception would be different. Here we show that although motion can be carried by contrast-defined elements, such motion is not used to define three-dimensional (3D) surfaces. A similar effect has been reported in stereopsis; although such contrast-defined elements can give signed disparity signals they nevertheless do not contribute to the percept of shape. We show that the reason for this lies in the inability of the second order signals to cohere or bind across space/spatial scales rather than a characteristic of the elementary motion signals per se.

A lesion of cortical area V2 selectively impairs the perception of the direction of first-order visual motion.

Lesions of area MT/V5 in monkeys and its presumed homologue, the motion area, in humans impair motion perception, including the discrimination of the direction of global motion in random dot kinematograms. Here we report the results of similar tests on patient TF, who has a discrete and very small, unilateral infarct in the medial superior part of the right occipital cortex. Structural MRI, co-registered in software with a standardized human brain atlas, reveals that the lesion involves area V2. The patient was impaired in his retinotopically corresponding left lower quadrant on several motion tasks including discrimination in random dot kinematograms of direction, speed and motion-defined discontinuity. He was also impaired on tasks selectively involving first-order motion based on luminance contrast but not on second-order motion based on texture contrast. The results show that even though area MT/V5 is intact, motion perception is abnormal and, in particular, his perception of first-order motion is impaired.

Computational modelling of interleaved first- and second-order motion sequences and translating 3 f+4 f beat patterns

Despite detailed psychophysical, neurophysiological and electrophysiological investigation, the number and nature of independent and parallel motion processing mechanisms in the visual cortex remains controversial. Here we use computational modelling to evaluate evidence from two psychophysical studies collectively thought to demonstrate the existence of three separate and independent motion processing channels. We show that the pattern of psychophysical results can largely be accounted for by a single mechanism. The results demonstrate that a low-level luminance based approach can potentially provide a wider account of human motion processing than generally thought possible.

Larger effect of aging on the perception of higher-order stimuli

Widespread deficits are known to accompany normal aging. Contrast thresholds of older and younger observers were measured for static and drifting gratings defined by luminance (first-order) or by contrast (second-order), and for a temporally segmented second-order motion stimulus. Results showed that older individuals had a larger threshold elevation for the perception of second-order stimuli than for the perception of first-order stimuli. This suggests a dissociation between the mechanisms underlying the perception of first and second-order stimuli, and demonstrates that aging may affect the more numerous processing steps required for the analysis of higher level stimuli.

Blindness to form from motion despite intact static form perception and motion detection

We studied the motion perception, including form and meaning generated by motion, in a hemianopic patient who also had visual perceptual impairments in her seeing hemifield as a result of a lesion in ventral extrastriate cortex. She was unable to recognise 2- or 3-dimensional forms, and even borders, generated by motion alone, failed to recognise mimed actions or the Johannson ‘biological motion’ display, and ceased to recognise people well-known to her when they moved. Her performance with static displays, although impaired, could not explain her inability to perceive shape or derive meaning from moving displays. Unlike a motion-blind patient, she can still see and describe the motion, with the exception of second-order motion, but not what it creates or represents.

Interactions between first- and second-order motion revealed by optokinetic nystagmus.

A previous study has suggested that second-order motion is ineffective at driving optokinetic nystagmus (OKN) when presented alone. First- and second-order motion cues interact in creating the perception of motion. Is there an interaction between first- and second-order cues in the control of eye movements? We presented combinations of first- and second-order cues moving in the same or opposite directions and measured the eye movements evoked, to look for a modification of the oculomotor response to first-order motion by simultaneously presented second-order cues. Dynamic random noise was used as a carrier for first- and second-order drifting gratings (13.4 degrees/s; 0.25 cycles/degree; 64 x 48 degrees screen viewed at 28.5 cm). Second-order gratings were defined by spatial modulation of the luminance flicker frequency of noise pixels of constant contrast (50%). A first-order, luminance-defined grating (13.4 degrees/s; 0.25 cycles/degree; variable contrast from 4-50%) was moved in either the same or the opposite direction. Eye movements were recorded by video-oculography from six subjects as they looked straight ahead. The gain (eye velocity/stimulus velocity) of first-order-evoked OKN increased with contrast. The presence of flicker-defined second-order motion in the opposite direction attenuated this OKN below a first-order contrast of 15%, although it had little effect at higher contrasts. When first- and second-order motion were in the same direction, there was an enhancement of the OKN response. We conclude that second-order motion can modify the optokinetic response to simultaneously presented first-order motion.

No local cancellation between directionally opposed first-order and second-order motion signals

Despite strong converging evidence that there are separate mechanisms for the processing of first-order and second-order motion, the issue remains controversial. Qian, Andersen and Adelson (J. Neurosci., 14 (1994), 7357–7366) have shown that first-order motion signals cancel if locally balanced. Here we show that this is also the case for second-order motion signals, but not for a mixture of first-order and second-order motion even when the visibility of the two types of stimulus is equated. Our motion sequence consisted of a dynamic binary noise carrier divided into horizontal strips of equal height, each of which was spatially modulated in either contrast or luminance by a 1.0 c/deg sinusoid. The modulation moved leftward or rightward (3.75 Hz) in alternate strips. The single-interval task was to identify the direction of motion of the central strip. Three conditions were tested: all second-order strips, all first-order strips, and spatially alternated first-order and second-order strips. In the first condition, a threshold strip height for the second-order strips was obtained at a contrast modulation depth of 100%. In the second condition, this height was used for the first-order strips, and a threshold was obtained in terms of luminance contrast. These two previously-obtained threshold values were used to equate visibility of the first-order and second-order components in the third condition. Direction identification, instead of being at threshold, was near-perfect for all observers. We argue that the first two conditions demonstrate local cancellation of motion signals, whereas in the third condition this does not occur. We attribute this non-cancellation to separate processing of first-order and second-order motion inputs.

Perceptual motion standstill in rapidly moving chromatic displays.

In motion standstill, a quickly moving object appears to stand still, and its details are clearly visible. It is proposed that motion standstill can occur when the spatiotemporal resolution of the shape and color systems exceeds that of the motion systems. For moving red-green gratings, the first- and second-order motion systems fail when the grating is isoluminant. The third-order motion system fails when the green/red saturation ratio produces isosalience (equal distinctiveness of red and green). When a variety of high-contrast red-green gratings, with different spatial frequencies and speeds, were made isoluminant and isosalient, the perception of motion standstill was so complete that motion direction judgments were at chance levels. Speed ratings also indicated that, within a narrow range of luminance contrasts and green/red saturation ratios, moving stimuli were perceived as absolutely motionless. The results provide further evidence that isoluminant color motion is perceived only by the third-order motion system, and they have profound implications for the nature of shape and color perception.

Attention mechanisms for multi-location first- and second-order motion perception

We applied the external noise plus attention paradigm to study attention mechanisms involved in concurrent first-order and second-order motion perception at two spatial locations. Cued to attend to one of the locations, the observer was instructed to independently judge direction of motion of either first-order (Experiment 1) or second-order (Experiment 2) motion stimuli at both locations in every trial. Across trials, systematically controlled amounts of external noise were added to the motion displays. We measured motion threshold at three performance criteria in every attention×external noise condition. We find that observers could, without any loss, simultaneously compute first-order motion direction at two widely separated spatial locations across a broad range of external noise conditions. However, considerable loss occurred at the unattended location in processing second-order motion direction at two separated spatial locations. We conclude that, under the conditions investigated in the current study, (1) in first-order motion perception, the visual system could simultaneously process motion direction at two widely separated locations without any capacity limitation; (2) in second-order motion perception, attending to a spatial location enhances stimulus contrast at that location by a factor of about 1.37 (or equivalently, reduces the internal additive noise by a factor of about 0.73).

Induced motion at texture-defined motion boundaries.

When a static textured background is covered and uncovered by a moving bar of the same mean luminance we can clearly see the motion of the bar. Texture-defined motion provides an example of a naturally occurring second-order motion. Second-order motion sequences defeat standard spatio-temporal energy models of motion perception. It has been proposed that second-order stimuli are analysed by separate systems, operating in parallel with luminance-defined motion processing, which incorporate identifiable pre-processing stages that make second-order patterns visible to standard techniques. However, the proposal of multiple paths to motion analysis remains controversial. Here we describe the behaviour of a model that recovers both luminance-defined and an important class of texture-defined motion. The model also accounts for the induced motion that is seen in some texture-defined motion sequences. We measured the perceived direction and speed of both the contrast envelope and induced motion in the case of a contrast modulation of static noise textures. Significantly, the model predicts the perceived speed of the induced motion seen at second-order texture boundaries. The induced motion investigated here appears distinct from classical induced effects resulting from motion contrast or the movement of a reference frame.

Stages in motion processing revealed by the ocular following response.

Motion perception and associated involuntary eye movements depend on factors such as the physical attributes of the stimulus and visual attention. Cues from spatial changes in luminance (first-order motion in the Fourier domain) or more complicated transitions involving two-dimensional patterns (second-order, non-Fourier) require rather different kinds of analyses to detect their net motion. During a fixation task we monitored eye movements induced by the onset of motion to examine the functional properties of the monkey cortical motion processing system. Eye movement velocity was indistinguishable to first- and second-order motion; concomitant response latency confirmed an additional calculation is required to detect the direction and velocity of second-order motion.

Motion aftereffect of combined first-order and second-order motion.

When, after prolonged viewing of a moving stimulus, a stationary (test) pattern is presented to an observer, this results in an illusory movement in the direction opposite to the adapting motion. Typically, this motion aftereffect (MAE) does not occur after adaptation to a second-order motion stimulus (i.e. an equiluminous stimulus where the movement is defined by a contrast or texture border, not by a luminance border). However, a MAE of second-order motion is perceived when, instead of a static test pattern, a dynamic test pattern is used. Here, we investigate whether a second-order motion stimulus does affect the MAE on a static test pattern (sMAE), when second-order motion is presented in combination with first-order motion during adaptation. The results show that this is indeed the case. Although the second-order motion stimulus is too weak to produce a convincing sMAE on its own, its influence on the sMAE is of equal strength to that of the first-order motion component, when they are adapted to simultaneously. The results suggest that the perceptual appearance of the sMAE originates from the site where first-order and second-order motion are integrated.

The role of neural mechanisms of attention in solving the binding problem.

We thank L. Chelazzi, V. Ferrera, P. Fries, S. Kastner, and A. Rossi for helpful comments on the manuscript.

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 (∼4°) when the range of dot directions was less than 90° 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.

Second-order reversed phi.

In a first-order reversed-phi motion stimulus (Anstis, 1970), the black-white contrast of successive frames is reversed, and the direction of apparent motion may, under some conditions, appear to be reversed. It is demonstrated here that, for many classes of stimuli, this reversal is a mathematical property of the stimuli themselves, and the real problem is in perceiving forward motion, which involves the second- or third-order motion systems or both. Three classes of novel second-order reversed-phi stimuli (contrast, spatial frequency, and flicker modulation) that are invisible to first-order motion analysis were constructed. In these stimuli, the salient stimulus features move in the forward (feature displacement) direction, but the second-order motion energy model predicts motion in the reversed direction. In peripheral vision, for all stimulus types and all temporal frequencies, all the observers saw only the reversed-phi direction of motion. In central vision, the observers also perceived reversed motion at temporal frequencies above about 4 Hz, but they perceived movement in the forward direction at lower temporal frequencies. Since all of these stimuli are invisible to first-order motion, these results indicate that the second-order reversed-phi stimuli activate two subsequent competing motion mechanisms, both of which involve an initial stage of texture grabbing (spatiotemporal filtering, followed by fullwave rectification). The second-order motion system then applies a Reichardt detector (or equivalently, motion energy analysis) directly to this signal and arrives at the reversed-phi direction. The third-order system marks the location of features that differ from the background (the figure) in a salience map and computes motion in the forward direction from the changes in the spatiotemporal location of these marks. The second-order system's report of reversed movement dominates in peripheral vision and in central vision at higher temporal frequencies, because it has better spatial and temporal resolution than the third-order system, which has a cutoff frequency of 3-4 Hz (Lu & Sperling, 1995b). In central vision, below 3-4 Hz, the third-order system's report of resolvable forward movement of something salient (the figure) dominates the second-order system's report of texture contrast movement.