Absence of a chromatic linear motion mechanism in human vision

We have investigated motion mechanisms in central and perifoveal vision using two-frame random Gabor kinematograms with isoluminant red-green or luminance stimuli. In keeping with previous results, we find that performance dominated by a linear motion mechanism is obtained using high densities of micropatterns and small temporal intervals between frames, while nonlinear performance is found with low densities and longer temporal intervals [Boulton, J. C., & Baker, C. L. (1994) Proceedings of SPIE, computational vision based on neurobiology, 2054, 124-133]. We compare direction discrimination and detection thresholds in the presence of variable luminance and chromatic noise. Our results show that the linear motion response obtained from chromatic stimuli is selectively masked by luminance noise; the effect is selective for motion since luminance noise masks direction discrimination thresholds but not stimulus detection. Furthermore, we find that chromatic noise has the reverse effect to luminance noise: detection thresholds for the linear chromatic stimulus are masked by chromatic noise but direction discrimination is relatively unaffected. We thus reveal a linear 'chromatic' mechanism that is susceptible to luminance noise but relatively unaffected by color noise. The nonlinear chromatic mechanism behaves differently since both detection and direction discrimination are unaffected by luminance noise but masked by chromatic noise. The double dissociation between the effects of chromatic and luminance noise on linear and nonlinear motion mechanisms is not based on stimulus speed or differences in the temporal presentations of the stimuli. We conclude that: (1) 'chromatic' linear motion is solely based on a luminance signal, probably arising from cone-based temporal phase shifts; (2) the nonlinear chromatic motion mechanism is purely chromatic; and (3) we find the same results for both perifoveal and foveal presentations.     

Impaired spatial and binocular summation for motion direction discrimination in strabismic amblyopia

Amblyopia is characterised by visual deficits in both spatial vision and motion perception. While the spatial deficits are thought to result from deficient processing at both low and higher level stages of visual processing, the deficits in motion perception appear to result primarily from deficits involving higher level processing. Specifically, it has been argued that the motion deficit in amblyopia occurs when local motion information is pooled spatially and that this process is abnormally susceptible to the presence of noise elements in the stimulus. Here we investigated motion direction discrimination for abruptly presented two-frame Gabor stimuli in a group of five strabismic amblyopes and five control observers. Motion direction discrimination for this stimulus is inherently noisy and relies on the signal/noise processing of motion detectors. We varied viewing condition (monocular vs. binocular), stimulus size (5.3-18.5°) and stimulus contrast (high vs. low) in order to assess the effects of binocular summation, spatial summation and contrast on task performance. No differences were found for the high contrast stimuli; however the low contrast stimuli revealed differences between the control and amblyopic groups and between fellow fixing and amblyopic eyes. Control participants exhibited pronounced binocular summation for this task (on average a factor of 3.7), whereas amblyopes showed no such effect. In addition, the spatial summation that occurred for control eyes and the fellow eye of amblyopes was significantly attenuated for the amblyopic eyes relative to fellow eyes. Our results support the hypothesis that pooling of local motion information from amblyopic eyes is abnormal and highly sensitive to noise.     

Perceiving motion transparency in the absence of component direction differences

The simultaneous perception of multiple motion components within the same region in the visual field is a difficult processing task, which can be solved by human observers for a range of transparently moving stimuli. We use transparently moving gratings to study this phenomenon psychophysically, focussing on configurations in which individual components move in the same direction and can only be discriminated by speed differences. We first demonstrate that the stimuli are perceived as transparent and then proceed to quantify how the strength of motion transparency changes while component grating parameters such as fundamental spatial frequency, speed and luminance are varied. The results were consistent with perception resolving a signal detection task of separating two superimposed global motion signals corresponding to each of the components. We also identify the importance of broadband stimuli containing edges, both for perceiving transparency with the same direction stimulus configuration, and for static transparency. The local density of edges has a direct influence on the strength of perceived transparency, suggesting that local motion detection at the edges of the stimuli, which is sensitive to speed differences, may be critical to solve the task. The work suggests that there may be a simultaneous retinotopic representation of the two speeds of motion analogous to that accomplished by the motion direction tuned neurons found across regions of visual cortex.     

Limited featured-based attention to multiple features

Attending to a feature (e.g., color or motion direction) can enhance the early visual processing of that feature. However, it is not known whether one can simultaneously enhance multiple features. We examined people’s ability to attend to multiple features in a feature cueing paradigm. Each trial contained two intervals consisting of a random dot motion stimulus. One interval (noise) had 0% coherence (no net motion), while the other interval (signal) moved in a particular direction with varying levels of coherence. Participants reported which interval contained the signal in one of three cueing conditions. In the one-cue condition, a line segment preceded the stimuli indicating the direction of the signal with 100% validity. In the two-cue condition, two lines preceded the stimuli, indicating the signal would move in one of the two cued directions. In the no-cue condition, no line segment appeared before the dot stimuli. In several experiments, we consistently observed a lower detection threshold in the one-cue condition than the no-cue condition, showing that participants can enhance processing of a single feature. However, detection threshold was consistently higher for the two-cue than one-cue condition, indicating that participants could not simultaneously enhance two motion directions as effectively as one direction. This finding revealed a severe capacity limit in our ability to enhance early visual processing for multiple features.     

Amblyopic perception of biological motion

Although a number of low-level visual deficits in amblyopia have been identified, it is still unclear to what extent these deficits extend throughout the visual processing hierarchy. Biological motion perception can be a useful measure of local and global visual processing since the point-light stimuli that are often used to study this ability carry both local motion and global form information. To investigate the integrity of the biological motion processing system in amblyopia, we employed both detection and discrimination tasks with coherent or scrambled point-light walkers either alone or embedded in different types of point-light masks. These manipulations allowed for control over the amount of form and/or motion information available to the observers that could be used for task performance. We found that amblyopic eyes could process both the global form and local motion components of point-light walkers, indicating intact processing for these stimuli. However, amblyopic eyes did show an increased susceptibility to the addition of masking dots suggesting that segregation of signal from noise is deficient in amblyopia.     

Motion perceptual learning: when only task-relevant information is learned

The classic view that perceptual learning is information selective and goal directed has been challenged by recent findings showing that subthreshold and task-irrelevant information can induce perceptual learning. This study demonstrates a limit on task-irrelevant learning as exposure to suprathreshold task-irrelevant signals failed to induce perceptual learning. In each trial, two random-dot motion stimuli were presented in a two-alternative forced-choice task. Observers either decided which of the two contained a coherent motion signal (detection task), or whether the coherent motion direction was clockwise or counterclockwise relative to a reference direction (discrimination task). Whereas the exact direction of the coherent motion signal was irrelevant to the detection task, detection of the coherent motion signal was necessary for the discrimination task. We found that the detection trainees improved only their detection but not discrimination sensitivity, whereas the discrimination trainees improved both. Therefore, the importance of task relevance was demonstrated in both detection and discrimination learning. Furthermore, both detection and discrimination training along a single pedestal direction transferred to a broad range of pedestal directions. The profile of the discrimination transfer (as a function of pedestal direction) narrowed for the discrimination trainees.     

Low-level sensory plasticity during task-irrelevant perceptual learning: Evidence from conventional and double training procedures

Studies of perceptual learning have focused on aspects of learning that are related to early stages of sensory processing. However, conclusions that perceptual learning results in low-level sensory plasticity are controversial, since such learning may also be attributed to plasticity in later stages of sensory processing or in readout from sensory to decision stages, or to changes in high-level central processing. To address this controversy, we developed a novel random dot motion (RDM) stimulus to target motion cells selective to contrast polarity by ensuring the motion direction information arises only from signal dot onsets and not their offsets, and used these stimuli in the paradigm of task-irrelevant perceptual learning (TIPL). In TIPL, learning is achieved in response to a stimulus by subliminally pairing that stimulus with the targets of an unrelated training task. In this manner, we are able to probe learning for an aspect of motion processing thought to be a function of directional V1 simple cells with a learning procedure that dissociates the learned stimulus from the decision processes relevant to the training task. Our results show direction-selective learning for the designated contrast polarity that does not transfer to the opposite contrast polarity. This polarity specificity was replicated in a double training procedure in which subjects were additionally exposed to the opposite polarity. Taken together, these results suggest that TIPL for motion stimuli may occur at the stage of directional V1 simple cells. Finally, a theoretical explanation is provided to understand the data.     

A method for generating a "purely first-order" dichoptic motion stimulus

In the present technical article, we describe a method for generating a new dichoptic motion stimulus, the monocular components of which are dynamic random noise without constant figural cues for feature-tracking-based motion. Our dichoptic motion stimulus adds a new line of evidence, which supports the original conclusion of M. Shadlen and T. Carney (1986) that motion detection can be solely derived from early binocular motion processing. Further, we describe novel motion displays in which monocular motion and binocular motion are in opposite directions with variable intensity ratios. Our dichoptic stimuli will serve as a useful tool to investigate the interaction between low-level binocular motion detectors and monocular motion detectors without requiring feature extraction before motion detection.     

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.     

Human velocity and direction discrimination measured with random dot patterns

In the present experiments three different motion discrimination tasks were studied using a random dot pattern as stimulus: velocity discrimination, direction discrimination and discrimination of opposite directions. The analysis of the motion of random dot patterns is based on motion sensitive mechanisms without the confounding interference of position sensitive mechanisms (Nakayama and Tyler, 1981). Furthermore, since isotropic random dot patterns contain no dominant orientation, a change in the direction of motion does not parallel a change in orientation. Hence the use of a random dot pattern as stimulus allows velocity and direction discrimination to be compared. Human velocity discrimination displays a U-shaped dependence on the stimulus velocity: the JNDs, expressed as Weber-fractions, are minimal for velocities ranging from 4 to 64 deg.sec-1. The Weber-fractions in velocity, determined with a staircase procedure tracking a 84% correct response level, were about 7% at the optimal speeds. The velocity discrimination curve obtained with the random dot pattern is similar to that obtained with light bars. Human direction discrimination, defined as the smallest difference in direction which can be resolved, also displays a U-shaped dependence on the stimulus velocity. Direction discrimination thresholds decrease up to a velocity of 4 deg.sec-1, they then stay at a constant level up to 128 deg.sec-1. Beyond this velocity the thresholds increase again. The mean direction discrimination threshold was 1.8 deg at optimal speeds. Discrimination of opposite directions, determined for the same conditions as those for which velocity and direction discrimination thresholds were determined, was better than the 90% response level at all speeds. However at low contrast, opposite directions are reliably discriminated only at intermediate speeds. Perceiving a coherent moving random dot pattern is supposed to be based on a cooperation between a large number of local motion detectors. In order to evaluate the importance of detector output pooling, the influence of the size of the pattern and of the presentation time on the three discrimination tasks was measured. The results indicate that the pooling requirements are task dependent. A somewhat larger pooling is required for velocity discrimination than for direction discrimination, whereas for discrimination of opposite directions only a few local motion detectors are involved.     

Theoretical predictions and experimental findings of visual deficits in glaucoma

Purpose: To investigate the motion and colour processing in normal subjects and in patients with primary open angle glaucoma, and to develop theoretical models in order to explain present findings and to understand the processes involved. Methods: Movement processing was investigated using an optical motion projection system, capable of generating continuous motion of a single dot target with controlled speed, displacement and motion direction. Displacement and direction discrimination thresholds (i.e. target displacement at a constant speed needed for 75% probability of detection of motion or correct discrimination of its direction) were measured for both foveal and peripheral stimulus presentations and for a range of speeds. Colour vision was tested using a novel method which employs a random luminance modulation technique to mask out achromatic contrast changes and allowing detection of a spatially structured object based on the processing of chromatic signals only (Barbur et al. 1993). Results: Changes in the parameters of the motion detection model (e.g., delay time and receptor regularity) that was developed to predict the experimental data in normal subjects also predicts an increase in displacement threshold and poorer sensitivity for discrimination of motion direction in glaucoma patients. The results obtained show that patients may report stimulus motion in 80% of presentations whilst performing at chance when required to discriminate the direction of motion. Chromatic discrimination ellipses measured for foveal stimulus presentations in glaucoma patients were often similar to those measured in normal subjects. In the periphery, however, much larger chromatic displacement thresholds were measured in all directions. Conclusions: The technique used to measure the chromatic discrimination ellipses is sensitive to glaucomatous nerve damage and the results suggest a larger threshold elevation in the periphery than at the fovea. The motion model developed to explain performance in normal vision also predicts the experimental findings in glaucoma patients and may be used to predict degraded motion discrimination in patient groups with similar pathologies.     

Global motion processing in human color vision: a deficit for second-order stimuli

The investigation of the mechanism of global motion in color vision has been limited because the processing of the first-order chromatic RDK elements, based on low-level linear motion detectors, is impaired. Here we return to this problem by using second-order elements in a global motion stimulus. Second-order RDK elements were circular contrast-modulated (CM) envelopes of a low-pass filtered noise carrier. The stimuli were achromatic or isolated L/M- or S-cone opponent mechanisms. We measured simultaneously detection and motion direction identification thresholds at 100% motion coherence and at different RDK speeds with a 2-AFC paradigm. We found that direction identification thresholds were higher than detection thresholds for both chromatic and achromatic stimuli. The gap between these thresholds was greater for the chromatic than the achromatic stimuli and motion direction thresholds for the chromatic RDK were very high or impossible to obtain. We also measured global motion performance (RDK speed of 4 deg/s) by varying the coherence of limited lifetime RDK stimuli. Global motion thresholds could only be obtained for achromatic stimuli and not for chromatic ones. Within the limits of the present stimulus conditions, we found no global motion processing of second-order chromatic stimuli.     

Perceptual learning of motion discrimination by mental imagery

Perceptual learning can occur when stimuli are only imagined, i.e., without proper stimulus presentation. For example, perceptual learning improved bisection discrimination when only the two outer lines of the bisection stimulus were presented and the central line had to be imagined. Performance improved also with other static stimuli. In non-learning imagery experiments, imagining static stimuli is different from imagining motion stimuli. We hypothesized that those differences also affect imagery perceptual learning. Here, we show that imagery training also improves motion direction discrimination. Learning occurs when no stimulus at all is presented during training, whereas no learning occurs when only noise is presented. The interference between noise and mental imagery possibly hinders learning. For static bisection stimuli, the pattern is just the opposite. Learning occurs when presented with the two outer lines of the bisection stimulus, i.e., with only a part of the visual stimulus, while no learning occurs when no stimulus at all is presented.     

Global speed processing: evidence for local averaging within,but not across two speed ranges

A primary task of the visual system is to extract the direction and speed of animate objects from the retinal image. We examined global speed processing by determining how local speeds are integrated and whether integration occurs across all speeds or within fixed speed ranges. The first experiment addressed how local motion signals are combined to determine the speed of an object in motion. Observers judged the speed of a moving cloud of dots that took a random walk in direction while the dots inside the cloud moved somewhat independently of the cloud itself. The apparent speed of the cloud of dots is found to change in proportion with the dot speed and is well predicted by calculating the average speed resulting from nearest neighbour matches across stimulus frames. The second experiment addressed whether local speeds are combined across all speeds or within fixed speed ranges for the detection of global motion. Global dot motion (GDM) stimuli that moved in a radial or rotational directions moving at a low speed of 1.2 degrees /s or a high speed of 9.6 degrees /s were used to measure the thresholds for detecting structured motion as a function of the speed of noise dots (0 degrees /s-10.8 degrees /s) added to the stimulus. With low-speed targets, only additional noise dots moving at low speeds interfered with signal detection. High-speed targets were only interfered with by dots moving at high speeds. This finding established the existence of at least two independent speed tuned systems in the range of speeds tested. Experiment 3 investigated how speed signals are combined within a system to determine the global speed. Using sectored radial GDM stimuli the perceived speed of the fastest dots was measured as a function of whether the speed of the dots in alternate sectors either activated the high or low-speed systems. Averaging only occurred when dots were all within the sensitivity range of the high-speed system, however, if alternate sectors activated separate speed systems, averaging did not occur. Thus local speeds are averaged, independent of direction, to derive a global speed estimate, but averaging only occurs within, and not across, speed tuned mechanisms.     

Contrast dependence of short-range apparent motion

The apparent motion of band-pass filtered random dot kinematograms was assessed by measurements of two alternative direction discrimination performance. In the case of two-dimensional (isotropically filtered) stimuli, the results were largely independent of contrast: at Michelson contrasts of 5 and 50%, near-perfect direction discrimination was obtainable for a limited range of displacements. However for one-dimensional (grating) stimuli, apparent motion seems to be highly dependent on contrast. At 5% contrast, performance was comparable to that obtained with the two-dimensional stimuli. At 50% contrast the motion percept broke down to a large extent, with consistently poor direction discrimination being obtained. The breakdown of apparent motion is interpreted in terms of a decreased signal-to-noise ratio in the pooled response of motion detectors that are tending to contrast saturation. Here, "noise" refers to the sampling components present in any apparent motion sequence. Evidence relating to the sampling frequency of the motion sequence is presented to support the hypothesis. It is argued that the discrepancy between the results obtained with one- and two-dimensional stimuli support the idea that motion is initially encoded by orientationally tuned mechanisms.     

Feature-based attentional integration of color and visual motion

In four variants of a speeded target detection task, we investigated the processing of color and motion signals in the human visual system. Participants were required to attend to both a particular color and direction of motion in moving random dot patterns (RDPs) and to report the appearance of the designated targets. Throughout, reaction times (RTs) to simultaneous presentations of color and direction targets were too fast to be reconciled with models proposing separate and independent processing of such stimulus dimensions. Thus, the data provide behavioral evidence for an integration of color and motion signals. This integration occurred even across superimposed surfaces in a transparent motion stimulus and also across spatial locations, arguing against object- and location-based accounts of attentional selection in such a task. Overall, the pattern of results can be best explained by feature-based mechanisms of visual attention.     

Learning motion discrimination with suppressed MT

We studied perceptual learning in motion discrimination when the brain's middle temporal area (MT/V5) was functionally suppressed. This was achieved by using the "paired-dots" motion stimulus where the two dots in a pair always move in counter-phase over a short distance [J. Neurosci. 14 (1994) 7357]. The motion directional signal of the stimulus is therefore always 0 on average. As a result, this stimulus activates MT in Rhesus monkeys no more than flicker noise does [J. Neurosci. 14 (1994) 7367]. We added a new manipulation to eliminate the Glass pattern in the original stimulus that would have otherwise provided a static orientation cue. Two such new motion stimuli were presented sequentially, in a 2AFC task. Subjects decided if the global motion-axis of the stimuli changed clockwise or counter-clockwise. When the task difficulty was set at 60% correct, none of the subjects could learn with feedback, even though their performance was well above chance. However, when the task difficulty was set instead at 70% correct, a new group of subjects was able to learn. Hence, learning motion discrimination was possible when MT was presumably eliminated.     

Orientation discrimination across the visual field: matching perceived contrast near threshold

Performance can often be made equal across the visual field by scaling peripherally presented stimuli according to F=1+E/E2 where E2 is the eccentricity at which stimulus size must double to maintain foveal performance levels. Previous studies suggest that E2 for orientation discrimination is in the range of 1.5 degrees -2 degrees when stimuli are presented at contrasts well above detection threshold. Recent psychophysical and physiological evidence suggests spatial reorganization of receptive fields at near-threshold contrasts. Such contrast-dependent changes in receptive field structure might alter the amount of size scaling necessary to equate task performance across the visual field. To examine this question we measured orientation discrimination thresholds for a range of stimulus sizes and eccentricities (0 degrees -15 degrees ). We used the same procedure previously employed except that stimuli were presented at near-threshold contrasts. We controlled for the effects of perceptual contrast on thresholds through a matching procedure. A standard line of 3 degrees in length presented at fixation was set to 2 just noticeable differences above detection threshold. The perceived contrast of all other stimuli was adjusted by the subject to match this one. Orientation discrimination thresholds were then obtained at these matching contrasts for all stimulus sizes and eccentricities. E2 values of 3.42 degrees and 3.50 degrees were recovered for two subjects; these values were about a factor of two larger than E2 values previously found for this task when stimuli were presented at higher physical contrasts.     

Common-fate motion processing: Interaction of the On and Off pathways

The processing of common-fate (motion-defined-form) signals was investigated using a modified version of the global dot-motion stimulus. The primary aim of the studies was to determine whether such stimuli could be processed by a form-specific motion system. This was achieved by investigating the interaction of the On and Off pathways in the processing of these stimuli, given that they have been shown to be pooled differently by the standard-motion and form systems (pooled versus independent, respectively). The number of signal dots was fixed at four and the number of noise dots was varied to establish the threshold signal-to-noise ratio required to determine the direction of the signal dots (up/down). The same dots remained signal dots over the three-frame motion sequence. The effect of different spatial patterns (square, horizontal line, vertical line, T-shape, random shape and no local grouping) and the interaction of contrast-polarity information, i.e. the interaction of the On and Off pathways, were investigated. To minimise the possibility of attentional tracking of the signal dots, the first motion frame contained 12 distracter patterns, i.e. noise dots arranged into the same pattern as the signal dots. Results indicate that performance is better for certain spatial arrangements than others and that contrast-polarity information appears to be pooled independently in the processing of the fixed-geometric shapes but not variable-shapes. These results are not due to differences in the spatial-frequency content of the stimuli and the use of a two-frame sequence ruled out attention-based tracking. This difference in the pooling of the On and Off pathways indicates that the different stimuli are processed by different systems, with the geometric conditions possibly being processed by a form-specific system.     

Ideal observer analysis of signal quality in retinal circuits

The function of the retina is crucial, for it must encode visual signals so the brain can detect objects in the visual world. However, the biological mechanisms of the retina add noise to the visual signal and therefore reduce its quality and capacity to inform about the world. Because an organism's survival depends on its ability to unambiguously detect visual stimuli in the presence of noise, its retinal circuits must have evolved to maximize signal quality, suggesting that each retinal circuit has a specific functional role. Here we explain how an ideal observer can measure signal quality to determine the functional roles of retinal circuits. In a visual discrimination task the ideal observer can measure from a neural response the increment threshold, the number of distinguishable response levels, and the neural code, which are fundamental measures of signal quality relevant to behavior. It can compare the signal quality in stimulus and response to determine the optimal stimulus, and can measure the specific loss of signal quality by a neuron's receptive field for non-optimal stimuli. Taking into account noise correlations, the ideal observer can track the signal-to-noise ratio available from one stage to the next, allowing one to determine each stage's role in preserving signal quality. A comparison between the ideal performance of the photon flux absorbed from the stimulus and actual performance of a retinal ganglion cell shows that in daylight a ganglion cell and its presynaptic circuit loses a factor of ～10-fold in contrast sensitivity, suggesting specific signal-processing roles for synaptic connections and other neural circuit elements. The ideal observer is a powerful tool for characterizing signal processing in single neurons and arrays along a neural pathway.