Effects of stimulus speed on direction discriminations

Real-Time Avatar-Based Feedback to Enhance the Symmetry of Spatiotemporal Parameters After Stroke: Instantaneous Effects of Different Avatar Views

Gait asymmetry, one of the hallmarks of post stroke locomotion, often persists despite gait rehabilitation interventions, impacting negatively on functional mobility. Real-time feedback and biological cues have been studied extensively in recent years, but their applicability to post-stroke gait symmetry remain questionable. This proof-of-concept study examined the feasibility and instantaneous effects of real-time visual feedback provided in the form of an avatar in twelve participants with stroke on gait symmetry and other gait-related outcomes. The visual avatar was presented via three different views from the back, front and paretic side. Avatar feedback from the paretic side view showed significant increase in bilateral step length, paretic swing time and treadmill walking speed, but no significant differences were found in symmetry measures in any of the three views. Those who had changes in symmetry ratio >0 were grouped as responders to spatial symmetry improvement in the side view. The responders had a significantly higher Chedoke-McMaster Stroke Assessment foot score and presented with a larger initial step length on the paretic side. Furthermore, all participants provided positive feedback and no adverse effects were observed during the experiment. Overall, these findings suggest that real-time avatar-based feedback can be used as an intervention to improve post-stroke gait asymmetry.

Age, eye movement and motion discrimination

Age is known to affect sensitivity to retinal motion. However, little is known about how age might affect sensitivity to motion during pursuit. We therefore investigated direction discrimination and speed discrimination when moving stimuli were either fixated or pursued. Our experiments showed: (1) age influences direction discrimination at slow speeds but has little affect on speed discrimination; (2) the faster eye movements made in the pursuit conditions produced poorer direction discrimination at slower speeds, and poorer speed discrimination at all speeds; (3) regardless of eye-movement condition, observers always combined retinal and extra-retinal motion signals to make their judgements. Our results support the idea that performance in these tasks is limited by the internal noise associated with retinal and extra-retinal motion signals, both of which feed into a stage responsible for estimating head-centred motion. Imprecise eye movement, or later noise introduced at the combination stage, could not explain the results.

Global motion detection is impaired in cats deprived early of pattern vision

We investigated global motion detection in binocularly deprived cats (BD cats) and control cats (C cats). The cats were trained in the two-choice free running apparatus for a food reward. The positive stimulus was a moving random-dot pattern with all dots moving in one direction, the negative stimulus was the same random-dot pattern but stationary. The BD cats were severely impaired in detection of global motion stimulus as compared with the C cats. In contrast, their level of performance in a simple relative motion detection task (one square) did not differ from that in the C cats. However, in more complex relative motion detection task (two squares) the performance of the BD cats was impaired. The deficit in the detection of global motion in BD cats may be due to impairments of their Y-pathway.

Pursuit affects precision of perceived heading for small viewing apertures

We investigated the interaction between extra-retinal rotation signals and retinal motion signals in heading perception during pursuit eye movement. For limited viewing aperture, the variability in perceived heading strongly depends on the pattern of motion directions. Heading towards a point outside the aperture generates nearly parallel aperture flow. This results in lower precision of perceived heading than heading that renders the radial pattern of flow visible. We ask if the precision is limited by the pattern of flow visible on the retina or that on the screen. During fixation, the two patterns are identical. They are decoupled during pursuit, since pursuit changes radial flow within the aperture on the screen into nearly parallel flow on the retina, and vice versa. The extra-retinal signal is known to reduce systematic errors in the direction of pursuit, thus compensating for the rotational flow during pursuit. We now ask if the extra-retinal signal also affects the precision of heading percepts. It might if at the spatial integration stage the rotational flow has been subtracted out already. A compensation beyond the integration stage, however, cannot undo the change in retinal motion directions so that an effect of pursuit on precision cannot be avoided. We measured the variable and systematic errors in perceived heading during fixation and pursuit for a frontal plane approach, while varying duration, dot lifetime and aperture size. We found precision is effected by pursuit as much as predicted from the pattern of retinal flow, while compensation is significantly greater than zero. This means that the interaction between the extra-retinal signal and visual motion signals takes place after spatial integration of local motion signals. Furthermore, compensation increased significantly with longer duration (0.5-3.0 s), but not with larger aperture size (10-50 degrees ). A larger aperture size did increase the eccentricity of perceived heading.

Temporal facilitation for moving stimuli is independent of changes in direction

A flash that is presented aligned with a moving stimulus appears to lag behind the position of the moving stimulus. This flash-lag phenomenon reflects a processing advantage for moving stimuli (Metzger, W. (1932) Psychologische Forschung 16, 176-200; MacKay, D. M. (1958) Nature 181, 507-508; Nijhawan, R. (1994) Nature 370, 256-257; Purushothaman, G., Patel, S.S., Bedell, H.E., & Ogmen, H. (1998) Nature 396, 424; Whitney, D. & Murakami, I. (1998) Nature Neuroscience 1, 656-657). The present study measures the sensitivity of the illusion to unpredictable changes in the direction of motion. A moving stimulus translated upwards and then made a 90 degrees turn leftward or rightward. The flash-lag illusion was measured and it was found that, although the change in direction was unpredictable, the flash was still perceived to lag behind the moving stimulus at all points along the trajectory, a finding that is at odds with the extrapolation hypothesis (Nijhawan, R. (1994) Nature 370, 256-257). The results suggest that there is a shorter latency of the neural response to motion even during unpredictable changes in direction. The latency facilitation therefore appears to be omnidirectional rather than specific to a predictable path of motion (Grzywacz, N. M. & Amthor, F. R. (1993) Journal of Neurophysiology 69, 2188-2199).

Direction discrimination of cyclopean (stereoscopic) and luminance motion

This study compared direction discrimination of cyclopean (stereoscopic) and luminance motion involving stimuli equated for effective strength. The stimuli were random-walk cinematogram (RWC) displays whose signal and noise discs were created from binocular disparity differences embedded in a dynamic random-dot stereogram or from luminance differences. Experiment 1 measured global motion detection thresholds for cyclopean and luminance stimuli by manipulating the proportion of signal to noise discs. Detection thresholds for cyclopean motion were about 25% whereas detection thresholds for luminance motion were 5%, thus five times more cyclopean motion events than luminance events were necessary to elicit threshold responding. Experiment 2 measured thresholds for discriminating the direction of cyclopean and luminance motion under conditions of equal stimulus strength by presenting the motion displays at equal multiples of detection threshold. Direction discrimination thresholds (ranging from about 5-30 deg, depending upon conditions) were similar for cyclopean and luminance motion, thus the precision with which the pooling of local motion events in one direction can be discriminated from the pooling of events in a slightly different direction is the same for cyclopean and luminance stimuli. The finding that cyclopean motion information is pooled is consistent with the idea that the direction of cyclopean motion is coded in the responses of a population of directionally selective mechanisms.

A reduction in the number of directionally selective neurons extends the spatial limit for global motion perception

Dynamic random-dot targets were used to study neural mechanisms underlying motion perception. Performance of cats with severely reduced numbers of cortical directionally selective neurons (reduced DS) was compared to that of normal animals. We assessed the spatial properties of the residual motion mechanism by measuring direction discriminations at various dot displacements. At small displacements, reduced DS cats' motion integration thresholds for opposite direction discrimination were nearly normal. At larger displacements, their thresholds surpassed those of normal cats and their upper displacement limit (dmax) was increased by 0.35 deg. The accuracy of direction discrimination was reduced at small displacements, but at larger displacements direction difference thresholds of reduced DS cats approached or surpassed those of normals. These data were compared to the performance of humans who showed an extension of dmax for peripherally viewed targets. The data support the hypothesis that expansion in spatial scale of the motion mechanism may contribute to extension of dmax. Additional support for this hypothesis is provided by a modified direction discriminating line-element model. The model also suggests that changes in sampling of motion mechanisms in the reduced DS system may play a role.

Dependence of speed and direction perception on cinematogram dot density

In the present experiments, we find that with abrupt decreases in dot density of random-dot cinematograms, perceived speed decreases, while with abrupt increases in dot density, perceived speed increases. Further, in steady-state conditions, perceived speed is also affected in the same way, but to a lesser degree, by the dot density of cinematograms. Direction discrimination of random-dot cinematograms is enhanced when dot density increases abruptly from one stimulus to the next, but is degraded when dot density decreases abruptly. Finally, speed discrimination remains constant even when density changes abruptly. The perceived-speed and direction-discrimination data are consistent with the Motion Coherence theory which motivated this study, and with models that include a smoothing stage similar to this theory. Of the other models that we consider, most predict that increasing dot density reduces perceived speed. The speed-discrimination data could not distinguish between the different theories.

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.

Discrimination of differences in speed and flicker rate depends on directionally selective mechanisms

The present study compared discriminations of differences in speed to differences in temporal frequency and examined the role of directionally selective mechanisms in such discriminations. In measuring the contrast dependence of speed and temporal frequency discriminations two different techniques were used to reduce the role of directionally selective mechanisms. The first was the virtual elimination of directional selectivity in the visual cortex of cats by stroboscopic rearing. The second was the reduction of directional sensitivity in normal humans and cats by testing with gratings of high spatial and low temporal frequency. Discrimination of the temporal frequency of sinusoidal gratings flickered in counterphase was worse than discrimination of speeds of moving gratings. Under conditions that maximize the sensitivity of directional mechanisms (low spatial, moderate temporal frequency) Weber fractions for speed and flicker in all normal observers (cats and humans) were constant at higher contrast and increased only as contrast began to approach threshold. In strobe-reared cats sensitivity for direction was 10 times lower than sensitivity for detection. They were able to discriminate speeds and temporal frequencies only at contrasts that exceeded contrast threshold for direction. This was also true for a normal cat whose sensitivity for direction was reduced by increasing the spatial frequency of the grating. In all cases Weber fractions for flicker as a function of contrast were greater than but paralleled those for speed.

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.

Velocity discrimination in the cat

After considerable training (over 2 years) we measured the just noticeable differences (JNDs) in velocity as a function of reference velocity in three cats. The velocity discrimination curve plotting JNDs in velocity, expressed as Weber fractions as a function of reference velocity is U-shaped with optimal performance at reference speeds between 25 and 60 degrees/sec. The discrimination curve changed little with a tenfold change in slit width. Compared to the human velocity discrimination curve determined with the same test apparatus, the feline curve is narrower and shifted towards faster velocities and larger Weber fractions. These results support our specific linking hypothesis between velocity tuned cells as observed in cortical areas 17 and 18 of the cat, and velocity discrimination.

Colliding targets: evidence for spatial localization within the motion system

The ability to judge the relative location of moving targets is seriously degraded if the targets move in different directions. The vernier acuity for a target in which the two components move in the same direction is not impaired until target velocity exceeds about 4 deg/sec. If the components are moving along trajectories which differ in direction by more than 15 deg, vernier thresholds rise significantly at target speeds greater than 1 deg/sec. The conditions which affect stationary vernier acuity, i.e. separation of target components, duration, and orientation, do not account for the loss in acuity. Our results suggest that localization for moving targets depends on directionally-selective motion detectors.