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A universal signature of developmental dyslexia is literacy acquisition impairments. Besides, dyslexia may be related to deficits in selective spatial attention, in the sensitivity to global visual motion, speed processing, oculomotor coordination, and integration of auditory and visual information. Whether motion-sensitive brain areas of children with dyslexia can recognize different speeds of expanded optic flow and segregate the slow-speed from high-speed contrast of motion was a main question of the study. A combined event-related EEG experiment with optic flow visual stimulation and functional frequency-based graph approach (small-world propensity ϕ) were applied to research the responsiveness of areas, which are sensitive to motion, and also distinguish slow/fast -motion conditions on three groups of children: controls, untrained (pre-D) and trained dyslexics (post-D) with visual intervention programs. Lower ϕ at θ, α, γ1-frequencies (low-speed contrast) for controls than other groups represent that the networks rewire, expressed at β frequencies (both speed contrasts) in the post-D, whose network was most segregated. Functional connectivity nodes have not existed in pre-D at dorsal medial temporal area MT+/V5 (middle, superior temporal gyri), left-hemispheric middle occipital gyrus/visual V2, ventral occipitotemporal (fusiform gyrus/visual V4), ventral intraparietal (supramarginal, angular gyri), derived from θ-frequency network for both conditions. After visual training, compensatory mechanisms appeared to implicate/regain these brain areas in the left hemisphere through plasticity across extended brain networks. Specifically, for high-speed contrast, the nodes were observed in pre-D (θ-frequency) and post-D (β2-frequency) relative to controls in hyperactivity of the right dorsolateral prefrontal cortex, which might account for the attentional network and oculomotor control impairments in developmental dyslexia.

The ability to judge speed is a fundamental aspect of visual motion processing. Speed judgments are generally assumed to depend on signals in motion-sensitive, directionally selective, neurons in areas such as V1 and MT. Speed comparisons might therefore be expected to be most accurate when they use information within a common set of directionally tuned neurons. However, there does not appear to be any published evidence on how well speeds can be compared for movements in different directions. We tested speed discrimination judgments between pairs of random-dot stimuli presented side-by-side in a series of four experiments (n = 65). Participants judged which appeared faster of a reference stimulus moving along the cardinal or oblique axis and a comparison stimulus moving either in the same direction or in a different direction. The bias (point of subjective equality) and sensitivity (Weber fraction) were estimated from individual psychometric functions fitted for each condition. There was considerable between-participants variability in psychophysical estimates across conditions. Nonetheless, participants generally made more acute comparisons between stimuli moving in the same direction than those moving in different directions, at least for conditions with an upwards reference (~20% difference in Weber fractions). We also showed evidence for an oblique effect in speed discrimination when comparing stimuli moving in the same direction, and a bias whereby oblique motion tended to be perceived as moving faster than cardinal motion. These results demonstrate interactions between speed and direction processing, thus informing our understanding of how they are represented in the brain.

So far, it remains largely unresolved to what extent neuronal noise affects behavioral responses. Here, we investigate, where in the human visual motion pathway noise originates that limits the performance of the entire system. In particular, we ask whether perception and eye movements are limited by a common noise source, or whether processing stages after the separation into different streams limit their performance. We use the ocular following response of human subjects and a simultaneously performed psychophysical paradigm to directly compare perceptual and oculomotor system with respect to their speed discrimination ability. Our results show that on the open-loop condition the perceptual system is superior to the oculomotor system and that the responses of both systems are not correlated. Two alternative conclusions can be drawn from these findings. Either the perceptual and oculomotor pathway are effectively separate, or the amount of post-sensory (motor) noise is not negligible in comparison to the amount of sensory noise. In view of well-established experimental findings and due to plausibility considerations, we favor the latter conclusion. (C) 2010 Elsevier Ltd. All rights reserved.

Twenty-two naı̈ve undergraduates participated in a psychophysical experiment designed to elucidate the neural events that allow us to see subtle motion differences. Half of the subjects practiced extensively on a direction-discrimination task while the other half practiced extensively on a speed-discrimination task. The stimulus conditions in the two groups were identical. The results indicated that the learning curves for direction discrimination were significantly steeper than those for speed discrimination. Additionally, the significant practice-based improvements on each motion task did not transfer to the other motion task. The different learning rates and the lack of transfer suggest that the neural events mediating speed discrimination are at least partially independent from those mediating direction discrimination, and vice versa, even under identical stimulus conditions.

During adaptation to a moving pattern, perceived speed decreases. Thus we know that the adapted visual system does not simply code the absolute speed of a stimulus. We hypothesised that adaptation to a moving stimulus serves to optimise coding of changes in speed at the expense of maintaining an accurate representation of absolute speed. In this case we would expect discrimination of speeds around the adapted level to be preserved or enhanced by motion adaptation. Speed discrimination thresholds were measured for sinusoidal gratings (1.25 cpd; 12.5 Hz; 40% contrast) with and without prior adaptation to moving, static, and flickering stimuli. After adaptation to motion in the same direction as the test, seven of eight subjects showed a reduction of perceived speed in the adapted region, and seven showed enhanced discrimination. Similar effects were found for adaptation to motion in the opposite direction to the test and to counter-phase flicker, suggesting that adaptation is driven by temporal modulation rather than by motion per se. We conclude that motion adaptation preserves or enhances differential speed sensitivity at the expense of an accurate representation of absolute speed.

Velocity matching using the method of Constant Stimuli shows that perceived velocity varies with contrast [Thompson, P. (1982). Perceived rate of movement depends upon contrast. Vision Research, 22, 377-380]. Random contrast jitter would therefore be expected to increase the slopes of psychometric functions, and thus the velocity discrimination threshold. However, McKee, S., Silverman, G., and Nakayama, K. [(1986) Precise velocity discrimination despite random variation in temporal frequency. Vision Research, 26, 609-620] found no effect of contrast jitter on thresholds, using the method of single stimuli. To determine whether this apparent discrepancy is due to the difference in methodology, or to the different ranges of temporal frequencies used in the two studies, we used the method of single stimuli to measure psychometric functions at three different velocities (0.5, 2.0 and 4.0 degrees/s). We found that contrast jitter increased thresholds at low but not at high velocities. Separate analysis of the psychometric functions at each contrast level showed that increases in contrast increased perceived velocity at low standard speeds (0.5 degree/s) but not at high. We conclude that the effect of contrast on perceived speed is real, and not a methodological artefact, but that it is found only at low temporal frequencies.

We measured Weber fractions for discriminating the speed of cyclopean gratings and Weber fractions for discriminating the speed of luminance gratings. Of our 14 observers, five were unable to see the cyclopean grating sufficiently well to discriminate its speed. One observer experienced great difficulty in discriminating the speed of cyclopean gratings, even though her threshold for detecting cyclopean gratings was low, and even though she discriminated the speed of luminance gratings on the basis of the task–relevent variable. But several observers based their speed discriminations on trial-to-trial variations of the task–relevent variable while ignoring associated trial-to-trial variations in all task–irrelevant variables (specifically: displacement; temporal frequency; spatial frequency; and presentation duration). We conclude that the visual systems of these observers contain a specialized neural mechanism for the speed of cyclopean gratings that supports acute discriminations of speed (Weber fractions were as low as 0.05–0.07).

To determine whether speed discrimination improves when the retinal image is stabilized against the effects of eye movements, thresholds were measured under stabilized and normal viewing conditions. In the normal viewing conditions, eye movements were recorded and used to estimate retinal-image speeds. Stimulus reference speed for sinusoidal gratings varied from 0.5 to 8.0 deg/sec. Results showed that speed discrimination thresholds, expressed as Weber ratios, decreased with increasing stimulus speed for both the normal and stabilized viewing conditions. Stabilized viewing thresholds were higher than normal viewing thresholds only at the slowest stimulus reference speed. However, when speed discrimination thresholds were expressed as a function of the estimated retinal speed, there was no difference in thresholds for the stabilized and normal viewing conditions. A retinal-image model, whereby speed discrimination depends on retinal-image motion, explains the results.

Three experiments were conducted to analyse the effect of contrast and adaptation state on the ability of human observers to discriminate the motion of drifting gratings. In the first experiment, subjects judged the direction of briefly presented gratings, which slowly drifted leftward or rightward. The test gratings were enveloped in space by a raised cosine function and in time by a Gaussian. The centre of the spatial envelope was either 2 deg left or right of the fixation point. An adaptive staircase procedure was used to find the velocities, at which the observer judged the motion direction in 75% of the presentations as leftwards or rightwards, respectively. In the second experiment, subjects judged the relative speed of two simultaneously presented gratings. Stimulus contrast was varied in both experiments from 0.01 to 0.32. Discrimination threshold vs contrast functions were measured before and after adaptation to a high-contrast (0.4) grating drifting at rates between 2 and 32 Hz. In a third experiment, subjects matched, before and after adaptation, the relative speed of a test stimulus, which had a constant contrast (0.04 or 0.08) and a variable speed, to that of a reference stimulus having a variable contrast but a constant speed. The results indicate that, before adaptation, direction and speed discrimination thresholds are independent of test contrast, except when test contrast approaches the detection threshold level. Adaptation to a drifting grating increases the lower threshold of motion (LTM) and the speed discrimination threshold (delta V/V) for low test contrasts. In addition, the point of subjective stationarity (PSS) shifts towards the adapted direction and this shift is more pronounced for low test contrasts. The perceived speed of a drifting grating increases with increasing contrast level. Adaptation to a drifting grating shifts the perceived speed vs log contrast function downwards and to the right (toward higher contrast levels) and this shift is greatest for adaptation frequencies between 8 and 16 Hz. We further explored the effects of adaptation contrast (0.04, 0.4 and 0.9) and adaptation drift direction (iso- or contra-directional) on the perceived speed versus contrast function. The effect of adaptation is greatest for iso-directional drift and increases with increasing adaptation contrast. The results are discussed in terms of a contrast gain control model of adaptation.

The motion of an object can be described by a single velocity vector, or equivalently, by direction and speed separately. Similarly, our ability to see subtle differences in the motion of two objects could be constrained by either a velocity-based sensory response, or separate sensory responses to direction and speed. To distinguish between these possibilities we investigated whether direction discrimination and speed discrimination were differentially affected by changes in the axis-of-motion. Psychophysical data from 12 naive observers indicated that direction discrimination depended on axis-of-motion, but speed discrimination did not. The difference suggests that a velocity-based sensory response is not the limiting factor on the two tasks. Instead, the results imply that the sensory response which constrains speed discrimination is at least partially independent from the sensory response which constrains direction discrimination.

The minimum information necessary to specify motion requires a change in position across time. Previous studies have shown that human motion measurements improve with more than two frames of motion. This study clarifies how motion information is integrated to produce the best speed and direction discrimination. Using random-dot kinematograms, fine-direction discrimination thresholds and speed discrimination thresholds are assessed as a function of dot lifetime. Specifically, we ask if performance on both tasks depends on dot lifetime in the same manner. If both speed and direction discrimination performance improve the same way with increasing dot lifetime, this would indicate that both tasks have the same integration limit and both tasks may depend on the same underlying mechanisms. Experiment 1 shows that for both tasks a four-frame dot lifetime is necessary for observers to reach asymptotic threshold levels. The absolute level of performance improves with increasing stimulus duration or signal-to-noise ratio, but the integration limit itself does not vary. Experiment 2 examines whether this integration limit is constrained by the number of frames or by the temporal duration of the dot lifetime. The data in Experiment 2 suggest that both a minimum number of samples and a minimal temporal integration period determine the integration limit for recruitment mechanisms. The results suggest that speed and fine-direction discrimination depend upon the same underlying motion mechanisms. These results are discussed in relation to possible underlying physiological substrates and computational models of motion measurement.

We have previously shown that contrast affects speed perception, with lower-contrast, drifting gratings perceived as moving slower. In a recent study, we examined the implications of this result on motion models of speed perception that use the amplitude of the response of linear spatio-temporal filters to determine speed. In this study, we investigate whether the contrast dependence of speed can be understood within the context of models in which speed estimation is made using the temporal frequency of the response of the linear spatio-temporal filters. We measured the effect of contrast on flicker perception and found that contrast manipulations produce opposite effects on perceived drift rate and perceived flicker rate, i.e., reducing contrast increases the apparent temporal frequency of counterphase modulated gratings. This finding argues that, if a temporal frequency-based algorithm underlies speed perception, either flicker and speed perception must not be based on the output of the same mechanism or contrast effects on perceived spatial frequency reconcile the disparate effects observed for perceived temporal frequency and speed.

We used speed discrimination tasks to measure the ability of observers to combine speed information from multiple stimuli distributed across space. We compared speed discrimination thresholds in a classical discrimination paradigm to those in an uncertainty/search paradigm. Thresholds were measured using a temporal two-interval forced-choice design. In the discrimination paradigm, the n gratings in each interval all moved at the same speed and observers were asked to choose the interval with the faster gratings. Discrimination thresholds for this paradigm decreased as the number of gratings increased. This decrease was not due to increasing the effective stimulus area as a control experiment that increased the area of a single grating did not show a similar improvement in thresholds. Adding independent speed noise to each of the n gratings caused thresholds to decrease at a rate similar to the original no-noise case, consistent with observers combining an independent sample of speed from each grating in both the added- and no-noise cases. In the search paradigm, observers were asked to choose the interval in which one of the n gratings moved faster. Thresholds in this case increased with the number of gratings, behavior traditionally attributed to an input bottleneck. However, results from the discrimination paradigm showed that the increase was not due to observers' inability to process these gratings. We have also shown that the opposite trends of the data in the two paradigms can be predicted by a decision theory model that combines independent samples of speed information across space. This demonstrates that models typically used in classical detection and discrimination paradigms are also applicable to search paradigms. As our model does not distinguish between samples in space and time, it predicts that discrimination performance should be the same regardless of whether the gratings are presented in two spatial intervals or two temporal intervals. Our last experiment largely confirmed this prediction.

Although it is well known that motion-in-depth can be detected using binocular cues, it is not known whether those cues can be used to judge the speed of an object moving in depth. There are at least t two possible binocular cues that could be used by the visual system to calculate three dimensional (3-D) speed: the rate of change of binocular disparity, or a comparison of the speeds of motion in the two eyes. We tested which of these cues is used to discriminate the speed of motion-in-depth. First, speed discrimination was measured for a dot moving away from the observer in depth (along the z -axis) and for a random dot stereogram in which a central square moved away from the observer in depth. These stimuli contained both disparity and monocular motion cues. Speed discrimination thresholds were as good for 3-D motion as for monocular sideways motion. Second, a dynamic random dot stereogram (in which the random dot pattern was replaced by a new dot pattern every frame) was used to remove consistent monocular cues. 3-D speed discrimination was now very poor, suggesting that the rate of change of disparity is not a good cue for 3-D speed. Finally, we tested whether observers were able to use the monocular motion cue from one eye to perform the speed discrimination task, or whether there had to be a comparison of the two eyes' monocular cues. By adding a small x-axis velocity component (with random direction) to the z -axis motion, it was possible to disrupt the monocular motion signals without altering the speed of the motion in 3-D. This manipulation did not disrupt the observers' performance, suggesting that monocular speed cues were not being used independently but that there was a comparison of monocular motion signals from the two eyes.

Motion transparency requires that the visual system distinguish different motion vectors and selectively integrate similar motion vectors over space into the perception of multiple surfaces moving through or over each other. Using large-field (7 degrees x 7 degrees) displays containing two populations of random-dots moving in the same (horizontal) direction but at different speeds, we examined speed-based segmentation by measuring the speed difference above which observers can perceive two moving surfaces. We systematically investigated this 'speed-segmentation' threshold as a function of speed and stimulus duration, and found that it increases sharply for speeds above approximately 8 degrees/s. In addition, speed-segmentation thresholds decrease with stimulus duration out to approximately 200 ms. In contrast, under matched conditions, speed-discrimination thresholds stay low at least out to 16 degrees/s and decrease with increasing stimulus duration at a faster rate than for speed segmentation. Thus, motion segmentation and motion discrimination exhibit different speed selectivity and different temporal integration characteristics. Results are discussed in terms of the speed preferences of different neuronal populations within the primate visual cortex.

We evaluated the hypothesis that smooth pursuit eye movements affect speed discrimination thresholds of distal stimuli because they alter the retinal image speed. Subjects judged speed differences of sine-wave gratings while they simultaneously pursued a superimposed moving bar. Speed discrimination thresholds were measured, under conditions of controlled eye movements, for grating speeds of 0.5 and 2.0 deg/sec across a range of eye velocities. Thresholds were simulated using a Monte Carlo method based on the retinal speed hypothesis, and the simulation predictions were compared to the psychophysically determined thresholds. The simulation results provided a good match to the psychophysical data for conditions where the eye moved at a slower speed than the grating, regardless of whether the eye moved in the same or opposite direction. However, when the eye moved at a faster speed than the grating in the same direction, the psychophysical thresholds were significantly higher than predicted by the simulation. Control experiments and analyses rule out explanations based on relative motion cues, saccadic involvement, and attentional demands. Copyright © 1996 Elsevier Science Ltd.

Several studies have shown that the precision of smooth pursuit eye speed can match perceptual speed discrimination thresholds during the steady-state phase of pursuit [Kowler, E., & McKee, S. (1987). Sensitivity of smooth eye movement to small differences in target velocity. Vision Research, 27, 993–1015; Gegenfurtner, K., Xing, D., Scott, B., & Hawken, M. (2003). A comparison of pursuit eye movement and perceptual performance in speed discrimination. Journal of Vision, 3, 865–876]. Recently, Osborne et al. [Osborne, L. C., Lisberger, S. G., & Bialek, W. (2005). A sensory source for motor variation. Nature, 437, 412–416; Osborne, L. C., Hohl, S. S., Bialek, W., & Lisberger S. G. (2007). Time course of precision in smooth-pursuit eye movements of monkeys. Journal of Neuroscience, 27, 2987–2998] claimed that pursuit precision during the initiation phase of pursuit also matches the sensory variability, implying that there is no motor noise added during pursuit initiation. However, these results were derived from a comparison of monkey pursuit data to human perceptual data from the literature, which were obtained with different stimuli. To directly compare precision for perception and pursuit, we measured pursuit and perceptual variability in the same human observers using the same stimuli. Subjects had to pursue a Gaussian blob in a step-ramp paradigm and give speed judgments on the same or in different trials. Speed discrimination thresholds were determined for different presentation durations. The analysis of pursuit precision was performed for short intervals containing the initiation period only and also for longer intervals including steady-state pursuit. In agreement with published studies, we found that the Weber fractions for psychophysical speed discrimination were fairly constant for different presentation durations, even for the shortest presentation duration of 150ms. Pursuit variability was 3–4 times as high for the analysis interval (300ms) containing the open-loop phase only. For pursuit analysis intervals of 400–500ms, pursuit variability approached perceptual variability. Our results show that, for the stimuli we used, the motor system contributes at least 50% to the total variability of smooth pursuit eye movements during the initiation phase.