Hyperacuity for luminance phase angle in the human visual system

A moving target with identical trajectories in the two eyes appeared shifted in depth if flickered with a phase difference between the eyes. The direction of the depth shift was the same as if the phase-lagging eye had been seeing the target with a spatial advance in the direction of target motion. Maximum acuity for this effect was in the region of 7 deg of phase angle, which may be expressed as a spatial difference between corresponding luminance points of about 13 sec arc. Acuity was approximately constant over a wide range of modulation periods from 50 to 150 msec when expressed in terms of phase angle, but fell off rapidly at higher and at lower frequencies. The results cannot be explained simply by either conventional disparity theory or by spatio-temporal interpolation, but suggest that inter-ocular correspondence is influenced by dynamic luminance information.     

General and specific perceptual learning in radial speed discrimination

The specificity of learning in speed discrimination was examined in three psychophysical experiments. In Experiment 1, half of the observers trained with inward motion direction on a speed discrimination task, whereas the other half trained on the same task but with outward motion direction. The results indicated that significant training-based improvement transferred from a trained radial direction to an untrained radial direction. Experiment 2 confirmed this transfer by showing that complete transfer was obtained even when stimuli moving in an untrained radial direction were used in the transfer task. In Experiment 3, observers were trained at a viewing distance of 114 cm. The results showed that learning transferred partly to the viewing distances of 57 cm and 228 cm. In summary, the present transfer results indicate that reliable generalizations can be obtained in perceptual learning of radial speed discrimination.     

Effects of daily training amount on visual motion perceptual learning

Perceptual learning has been widely used to study the plasticity of the visual system in adults. Owing to the belief that practice makes perfect, perceptual learning protocols usually require subjects to practice a task thousands of times over days, even weeks. However, we know very little about the relationship between training amount and behavioral improvement. Here, four groups of subjects underwent motion direction discrimination training over 8 days with 40, 120, 360, or 1080 trials per day. Surprisingly, different daily training amounts induced similar improvement across the four groups, and the similarity lasted for at least 2 weeks. Moreover, the group with 40 training trials per day showed more learning transfer from the trained direction to the untrained directions than the group with 1080 training trials per day immediately after training and 2 weeks later. These findings suggest that perceptual learning of motion direction discrimination is not always dependent on the daily training amount and less training leads to more transfer.     

Spatial structure, contrast polarity and motion integration

It has been shown that in the initial stages of motion processing, the ON and OFF pathways stay more or less separated. There is evidence that this distinction between motion signals from opposite contrast polarities remains at least partly intact in the integration stage of local motion information. At the same time, interactions between the two systems are also apparent. Here we constructed stimuli that contained a constant number of moving checks. The checks were either assigned only one contrast polarity, or contrast polarity was distributed across the checks either randomly or evenly. We investigated how the spatial configuration of the moving stimulus affected direction discrimination thresholds for the different polarity distributions. Our results provide new evidence for contrast-sign-specific integration of local motion signals within areas of limited size, and inhibitory interactions between these separate ON and OFF motion sensor pools.     

Direction-specific improvement in motion discrimination

With training, an observer's ability to discriminate similar directions of motion gradually improves. A series of studies reveals that this improvement, (1) is restricted to the trained direction and other, similar directions, (2) persists for at least several months, (4) shows appreciable, but not complete, transfer between the two eyes, and (5) is largely restricted to the stimulated region of the field. Moreover, the improvement in direction discrimination does not produce a concomitant change in detection thresholds. In all likelihood, most of the improvement in direction discrimination represents a change in visual function, rather than changes in nonsensory processes.     

Perceptual learning transfer between hemispheres and tasks for easy and hard feature search conditions

Perceptual learning involves modification of cortical processes so that transfer to new task variants depends on neuronal representation overlap. Neuron selectivity varies with cortical level, so that the degree of transfer should depend on training-induced modification level. We ask how different can stimuli be, how far apart can their representations be, and still induce training transfer. We measure transfer across both long distances within the visual field, namely across cerebral hemispheres, as well as across perceptual dimensions, i.e., between detection of odd color and orientation. In Experiment 1, subjects learned feature search using eccentric arrays randomly presented in the right or left hemifield. Odd elements differed in color or orientation, depending on the presentation hemifield. Following training and performance improvement, the dimensions were switched between hemifields. There was little cross-hemifield or cross-task transfer for difficult cases, and the greater transfer found for easier cases could be across hemispheres and/or perceptual dimensions. Testing these two elements separately, Experiment 2 confirmed considerable transfer across task dimensions, training one dimension and testing another, and Experiment 3 confirmed such transfer across hemifields, training search on one side and testing on the other. Results support Reverse Hierarchy Theory (M. Ahissar & S. Hochstein, 1997, 2004) in that, for easier perceptual tasks involving and modifying higher cortical levels, considerable transfer occurs both across perceptual dimensions and across visual field location even across hemifields.     

Evidence for multiple extra-striate mechanisms behind perception of visual motion gradients

Perceiving motion patterns in visual scenes in which speed or motion direction varies over space while average luminance remains constant presents a processing task that requires at least two separate stages of neural spatio-temporal filtering. We have previously probed the transfer of information between these stages of filtering identifying a largely scale invariant process in which narrowband initial motion sensitive filters are coupled with a broad range of spatial frequencies of secondary filters, with an optimal coupling – in terms of optimal observer visual sensitivity – at a frequency ratio of around twelve. In the current work, we used the same stimulus to investigate the possible presence of multiple secondary filtering mechanisms and their associated bandwidths. Using a forced choice psychophysical task with both a detection and an identification component, we presented experimental blocks containing stimuli with one of two different modulator frequencies in each trial to measure the frequency difference at which the detection performance matched the identification of the frequency. We found that at a frequency differences of about 2.2 octaves, performance of both tasks was similar, and the processing could therefore be inferred to occur in independent frequency channels. The same observation was confirmed for stimuli presented at a longer viewing distance. We conclude that for the motion gradient stimuli, there are secondary filtering mechanisms with a moderately broad bandwidth of over 2 octaves that underlie our sensitivity for detecting motion gradients of different modulation frequency. These are likely to be implemented at least in part within the dorsal stream of extra-striate cortex.     

Seeing motion in depth using inter-ocular velocity differences

An object moving in depth produces retinal images that change in position over time by different amounts in the two eyes. This allows stereoscopic perception of motion in depth to be based on either one or both of two different visual signals: inter-ocular velocity differences, and binocular disparity change over time. Disparity change over time can produce the perception of motion in depth. However, demonstrating the same for inter-ocular velocity differences has proved elusive because of the difficulty of isolating this cue from disparity change (the inverse can easily be done). No physiological data are available, and existing psychophysical data are inconclusive as to whether inter-ocular velocity differences are used in primate vision. Here, we use motion adaptation to assess the contribution of inter-ocular velocity differences to the perception of motion in depth. If inter-ocular velocity differences contribute to motion in depth, we would expect that discriminability of direction of motion in depth should be improved after adaptation to frontoparallel motion. This is because an inter-ocular velocity difference is a comparison between two monocular frontoparallel motion signals, and because frontoparallel speed discrimination improves after motion adaptation. We show that adapting to frontoparallel motion does improve both frontoparallel speed discrimination and motion-in-depth direction discrimination. No improvement would be expected if only disparity change over time contributes to motion in depth. Furthermore, we found that frontoparallel motion adaptation diminishes discrimination of both speed and direction of motion in depth in dynamic random dot stereograms, in which changing disparity is the only cue available. The results provide strong evidence that inter-ocular velocity differences contribute to the perception of motion in depth and thus that the human visual system contains mechanisms for detecting differences in velocity between the two eyes' retinal images.     

Perception of motion trajectory of object from the moving cast shadow in infants

A moving cast shadow of the object affects the perception of the object's trajectory in adults [Kersten, D., Mamassian, P., & Knill, D. C. (1997). Moving cast shadow induce apparent motion in depth. Perception, 26, 171-192]. In the present study, we investigated by using a habituation-dishabituation procedure whether infants at 4- to 7-months old discriminate the motion trajectory of a ball from the moving shadow it casts. In Experiment 1, 4- to 5-month-old and 6- to 7-month-old were tested for ability to discriminate between a "depth" display containing a ball and a cast shadow with a diagonal trajectory and an "up" display containing a ball with a diagonal trajectory and a cast shadow with a horizontal trajectory. Six- and 7-month-old, but not 4- and 5-month-old, infants looked significantly longer at the "up" display than at the "depth" display. In Experiment 2, we tested whether 4- to 5-month-old and 6- to 7-month-old infants would perceive "up" motion as categorically different from "depth" depending on the object's 3-D trajectory. We used displays containing a ball and a cast shadow with the same trajectories as those in Experiment 1 except that the cast shadows appeared above the ball. These displays did not produce 3-D impressions in adults. Neither age group of infants exhibited significant differences between "up" and "depth" displays. When the results from the two experiments are considered, 6- and 7-month-old infants discriminated the motion trajectory of the ball from the moving cast shadows. This developmental emergence of depth perception from a moving cast shadow at 6 months of age is consistent with that of other pictorial depth cues.     

Interpolation and extrapolation on the path of apparent motion

An object moving in discrete steps can appear to move continuously even along sections of the path in which no stimulus is presented. We investigated whether the internal representation of such an object is constructed by extrapolation, along the expected trajectory of the object, or by interpolation, after the subsequent reappearance of the object. Observers viewed two discs moving in an unambiguous apparent motion display, which either occasionally reversed direction or continued moving along the predicted path. Observers carried out a speeded 2AFC task on probes presented between the possible disc locations. In the continuous condition, observers' reaction times to detect and identify a probe were longer when it occurred ahead of the disc than when it occurred elsewhere on the motion path. Conversely, when the disc reversed direction, significantly less interference was observed ahead of the disc (along the predicted motion path), and significantly more interference was observed behind the disc (along the updated motion path). We conclude that the representation of a moving object in an apparent motion display is constructed by interpolation as well as extrapolation. We demonstrate that this representation is maintained and updated even outside the locus of focused attention, and that it is possible to dissociate the contributions of interpolation and extrapolation mechanisms to an object's representation.     

Dependence of stereomotion on the orientation of spatial-frequency components

It is known that sensitivity to stereoscopic motion in depth is not based upon the fine analysis of static disparities but instead is based on the binocular combination of motion-sensitive mechanisms. We show in this paper that an 'aperture problem' arises for the analysis of motion in depth, just as it does for monocular motion sensitivity. We extend Adelson and Movshon's solution to the aperture problem by intersection of perpendicular constraints to the three-dimensional case, and show that it predicts velocity matches for oblique gratings moving in depth, for orientations close to vertical. We show that binocular plaids give rise to motion in depth when the component orientations match in each eye, and the monocular motions are horizontal. The match velocities are consistent with intersection of perpendicular constraints. In three dimensions intersection of perpendicular constraints may be necessary, but is not a sufficient condition for the perception of coherent stereoscopic motion in depth.     

View combination in moving objects: The role of motion in discriminating between novel views of similar and distinctive objects by humans and pigeons

Humans and pigeons were trained to discriminate between views of similar and distinctive objects that rotated in depth coherently or non-coherently. We tested novel views that were either moving or static and were either between the training viewpoints or beyond them. With both types of motion, both species recognized views between the training viewpoints better than views beyond this range. Additionally, for humans, and to some extent for pigeons, when similar objects were learned via coherent motion, dynamic cues facilitated recognition of viewpoints predictable from the direction of motion. Overall, the results suggest that dynamic information may be added to object representations for both species.     

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.     

The dependence of motion repulsion and rivalry on the distance between moving elements

We investigated the extent to which motion repulsion and binocular motion rivalry depend on the distance between moving elements. The stimuli consisted of two sets of spatially intermingled, finite-life random dots that moved across each other. The distance between the dots moving in different directions was manipulated by spatially pairing the dot trajectories with various precisions. Data from experiment 1 indicated that motion repulsion occurred reliably only when the average distance between orthogonally moving elements was at least 21.0 arc min. When the dots were precisely paired, a single global direction intermediate to the two actual directions was perceived. This result suggests that, at a relatively small spatial scale, interaction between different directions favors motion attraction or coherence, while interaction at a somewhat larger scale generates motion repulsion. Similarly, data from experiment 2 indicated that binocular motion rivalry was significantly diminished by spatially pairing the dots, which moved in opposite directions in the two eyes. This supports the recent proposal that rivalry occurs at or after the stage of binocular convergence, since monocular cells could not have directly responded to our interocular pairing manipulation. Together, these findings suggest that the neural mechanisms underlying motion perception are highly sensitive to the fine spatial relationship between moving elements.     

On the perceived location of global motion

We measured the effects of coherent motion of one set of dots on the perceived location of Gaussian envelopes formed by luminance modulation of a second set of dots. Perceived shifts in envelope location in the direction of coherent motion were obtained even when the dots forming the envelopes did not physically move in the direction of coherent motion. In such cases, perceived shifts coincided with stimulus configurations that permitted motion integration of the envelope dots with the coherently moving dots, for example, when envelope dots moved in random directions as opposed to being static. In subsequent experiments we explored the type of motion integration underlying the positional shifts obtained. We discounted the possibility that the visual system incorrectly attributes motion signals associated with coherently moving dots to envelope dots by demonstrating that positional shifts could be obtained even when the coherent dots were laterally displaced to either side of the envelope dots such that the regions occupied by the dots did not overlap. We also discounted spatio-temporal summation within the receptive fields of low-spatial-frequency motion-sensitive mechanisms by demonstrating that positional shifts persisted even when the dot displays were high-pass filtered. These results, coupled with the observation that the proportion of coherently moving dots required to produce positional shifts correlated well with global motion thresholds measured for the same dot configurations, suggests that visual processes which underlie motion-dependent positional shifts are based at least in part on cooperative interactions of the type implicated in global motion.     

Dichoptic Perceptual Training and Sensory Eye Dominance Plasticity in Normal Vision

PurposeWe introduce a set of dichoptic training tasks that differ in terms of (1) the presence of external noise and (2) the visual feature implicated (motion, orientation), examining the generality of training effects between the different training and test cues and their capacity for driving changes in sensory eye dominance and stereoscopic depth perception.MethodsWe randomly assigned 116 normal-sighted observers to five groups (four training groups and one no training group). All groups completed both pre- and posttests, during which they were tested on dichoptic motion and orientation tasks under noisy and noise-free conditions, as well as a binocular phase combination task and two depth tasks to index sensory eye dominance and binocular function. Training groups received visual training on one of the four dichoptic tasks over 3 consecutive days.ResultsTraining under noise-free conditions supported generalization of learning to noise-free tasks involving an untrained feature. By contrast, there was a symmetric learning transfer between the signal-noise and no-noise tasks within the same visual feature. Further, training on all tasks reduced sensory eye dominance but did not improve depth perception.ConclusionsTraining-driven changes in sensory eye balance do not depend on the stimulus feature or whether the training entails the presence of external noise. We conjecture that dichoptic visual training acts to balance interocular suppression before or at the site of binocular combination.     

Motion distorts perceived depth

Two important tasks that the visual system has to perform are determining the direction of motion and the spatial location of objects. It has recently been shown that the perceived location of an object moving in the frontal-plane is displaced along the direction of motion (e.g. Nature 397 (1999) 610; Vision Research 31 (1991) 1619). The aim of the present study is to examine the extent of this interaction between motion and perceived location. The observers' task was to indicate which of two vertically separated moving stimuli was closer. The two stimuli were presented at various relative disparity offsets. The stimuli consisted of moving dot patterns (optic-flow) that simulated either fronto-parallel motion (all the dots moved one direction) or motion in depth. Motion of the dots towards the centre of the stimulus simulated object motion away from the observer and motion of the dots away from the centre of the stimulus simulated object motion towards the observer. Results indicate that motion-in-depth information can bias perceived stereoscopic-based depth. Simulated motion towards the observer made the object appear closer to the observer than the depth signalled by the disparity information and simulated motion away from the observer made it seem further away. The results of this study, when combined with those of previous studies, show that motion can distort our entire three dimensional representation of space.     

Discrimination of an orientation difference in dynamic textures

We investigated whether the response of a motion sensor was related to the specificity of sensory information (orientation and direction of motion) used to compute motion energy. This was done in two ways. First, we assessed whether orientation discrimination of a target line, which segregated by an orientation difference from a textured background, was improved with two-frame apparent motion stimulation (as compared with static presentation). Second, we investigated whether the amount of improvement (in either orientation or direction of motion discrimination) depends on a particular combination of target orientation and direction of motion (either orthogonal or parallel). We found that the percentage of correct responses in the discrimination task (a) was higher for a moving target than for a static one; (b) was higher when the target was oriented more orthogonally to motion direction than background elements; (c) was little affected by background motion and (d) decreased with frame duration in the direction of motion task whereas it was largely unaffected by frame duration in the discrimination of orientation task. These results suggest that discrimination of moving texture boundaries is based on a motion sensor tuned to a particular combination of orientation and direction of motion, which is capable of signalling the orientation of a moving target more accurately than a static sensor.     

Using the kinetic Zollner illusion to quantify the interaction between form and motion information in depth

In the kinetic Zollner illusion a stimulus moving over a background of oriented lines appears tilted away from the line orientation. This “motion-tilt” illusion is a powerful demonstration of how form information can influence the computation of motion, particularly in signaling motion direction. In the present study, using a random dot stereogram of the kinetic Zollner illusion, we examined whether and how the degree of motion tilt is affected when form and motion components of the illusion are separated in depth. In Experiment 1 we showed that increasing the depth separation (by increasing binocular disparity) between the moving stimulus and oriented lines attenuated the motion-tilt effect. Motion tilt induction was observed for depth separations of −18 to 18 arcmin in uncrossed and crossed directions, but not at larger separations. In Experiment 2 we showed that motion tilt induction in the kinetic Zollner illusion was also observed when multiple oriented planes were presented in conjunction with a moving stimulus. However, the direction and extent of the illusory motion tilt was determined by the nearest oriented plane. Collectively, these findings show that the interaction of form and motion is dependent on depth and is optimally tuned for a small range of separations.     

Mechanisms of generalization in perceptual learning

Learning in many visual perceptual tasks has been shown to be specific to practiced stimuli, while new stimuli have to be learned from scratch. Here we demonstrate generalization using a novel paradigm in motion discrimination where learning has been previously shown to be specific. We trained subjects to discriminate directions of moving dots, and verified the previous results that learning does not transfer from a trained direction to a new one. However, by tracking the subjects' performance across time in the new direction, we found that their speed of learning doubled. Therefore, we found generalization in a task previously considered too difficult to generalize. We also replicated, in a second experiment, transfer following training with 'easy' stimuli, when the difference between motion directions is enlarged. In a third experiment we found a new mode of generalization: after mastering the task with an easy stimulus, subjects who have practiced briefly to discriminate the easy stimulus in a new direction generalize to a difficult stimulus in that direction. This generalization depends on both the mastering and the brief practice. The specificity of perceptual learning and the dichotomy between learning of 'easy' versus 'difficult' tasks have been assumed to involve different learning processes at different cortical areas. Here we show how to interpret these results in terms of signal detection theory. With the assumption of limited computational capacity, we obtain the observed phenomena--direct transfer and acceleration of learning--for increasing levels of task difficulty. Human perceptual learning and generalization, therefore, concur with a generic discrimination system.