Effect of adaptation direction on the motion VEP and perceived speed of drifting gratings

The N200 amplitude of the motion-onset VEP evoked by a parafoveal grating of variable contrast (0.5-64%), constant speed (2 degrees/s), direction (horizontally rightward), and spatial frequency (2 cpd) was studied before and after adaptation to a stationary or drifting grating (1, 2, or 4 degrees/s rightward or leftward). These results are compared to those for the pattern-appearance VEP. Psychophysical measurements were made simultaneously of the perceived speed. While iso-directional (rightward) adaptation leads to a mean amplitude reduction of 39%, the decrease after counter-directional adaptation has a size of 20%. The post-adaptation matches of perceived speed differ in dependence on the iso-directional adapting speed and decrease on average to 98%, 85%, and 69% of the pre-adapt perceived speed after 1, 2, and 4 degrees/s adapting speeds, respectively. The perceived speed is moderately reduced (83% of the pre-adapt value) after counter-directional adaptation nearly independently of the adapting speed. A model of velocity processing is presented, which enables us to predict the trends of the experimental motion VEP and perceived speed data.     

Spatial-frequency discrimination of drifting gratings

Spatial-frequency discrimination thresholds were measured for briefly (300 msec) presented sinewave gratings having a contrast one logarithmic unit above detection threshold. The gratings were drifted at rates varying from 1.1 to 40 Hz. In a two-interval forced-choice paradigm thresholds were determined for vertically and obliquely oriented gratings. Three reference spatial frequencies (1, 4, 12 c/deg) were tested. For the 1 c/deg reference spatial frequency, spatial-frequency discrimination thresholds were constant over the wide range of drift rates used. For 4 and 12 c/deg reference gratings, discrimination thresholds were constant for drift frequencies up to 14 Hz. For drift frequencies beyond 14 Hz, spatial-frequency discrimination thresholds increased abruptly, rising from approx. 6% at 14 Hz to 25% at 40 Hz drift rate. Measurements with obliquely oriented gratings yielded comparable results. The increase in the spatial-frequency discrimination threshold for medium-high spatial frequencies and high temporal frequencies might reflect an increase in the spatial frequency bandwidth of the mechanisms sensitive to these stimulus frequencies.     

Contrast detection and direction discrimination of drifting gratings

Observers performed simple detection and left right discrimination of drifting sinusoidal gratings. Ratio of detection to discrimination sensitivities was measured under variations in several experimental parameters. In the first experiment, it was found that combinations of spatial and temporal frequency which resulted in the same velocity produced similar detection discrimination ratios. At an exposure duration of 800 msec, the relationship between the ratio and velocity described a power function with the intercept at 0.6 sec^?1. Decreasing duration shifted the curve to higher velocities. I examined the effect of grating orientation in a second experiment. Visual sensitivity was poorer for oblique than for vertical gratings with detection and discrimination exhibiting similar size anisotropies. In a third experiment, observers viewed gratings presented to different retinal loci, Visual performance in both detection and discrimination fell with greater eccentricity. However, motion discrimination fell more steeply resulting in an increase in the ratio. The results demonstrate that form and motion analyzing mechanisms cannot be distinguished by their response to changes of spatial frequency, orientation or retinal locus.     

Phase discrimination in chromatic compound gratings

Phase discrimination thresholds were measured for yellow/green isoluminant and non-isoluminant compound gratings in which the amplitude of the two components (f and 3f) was twice the detection threshold. The phase discrimination threshold at isoluminance was worse than in all the other conditions, which gave broadly similar threshold data. It is suggested that this is due to a positional uncertainty in the neural representation of the isoluminant stimuli.     

Brightness shift in drifting ramp gratings isolates a transient mechanism

A brightness shift is demonstrated for moving ramp stimuli. The data suggest that the transient visual response to changing luminance is combined with the sustained response to produce an overall impression of brightness. We attribute the brightness shift to the saturation of the transient response: a limitation on the maximum transient when responding to rapid brightening or dimming. The brightness shift provides a new technique for isolating the responses of the transient mechanisms.     

Relationship between motion VEP and perceived velocity of gratings: effects of stimulus speed and motion adaptation

The N200 amplitude of the motion-onset VEP evoked by a parafoveal grating of variable speed (0.25–13.5°/s), constant spatial frequency (2 cpd), contrast (4%), and direction (horizontally rightward) was studied before and after adaptation to a stationary or drifting grating (1 or 4°/s). Psychophysical measurements were made simultaneously of the perceived speed. In the unadapted condition the slope of the N200 amplitude versus speed function is positive, but lower for high compared to low speeds. The N200 amplitude increases slightly after stationary adaptation. An increase in perceived speed is also evident after stationary adaptation. This increase is more pronounced for low compared to high speeds. Motion adaptation reduces N200 amplitudes over the entire speed range, whereas perceived speeds change from under-estimation to over-estimation when the speed exceeds 1.8°/s after 1°/s adaptation and 4.5°/s after 4°/s adaptation. The simultaneous evaluation of motion VEP and psychophysical results supports the view that the neurons generating the N200 component are also involved in speed perception. The data suggest the existence of a limited number (three or more) speed channels.     

The influence of spatial frequency on perceived temporal frequency and perceived speed

Speed matching experiments were conducted using drifting gratings of different spatial frequencies in order to assess the influence of spatial frequency on perceived speed. It was found that gratings of high spatial frequency appear to drift more slowly than low spatial frequency gratings of the same actual velocity. The perceived temporal frequency of a counterphase grating similarly declines as spatial frequency increases. The previously reported effect of temporal frequency on perceived spatial frequency probably does not contribute to these phenomena. Our results suggest that the motion sensors thought to operate within different spatial frequency ranges have different velocity transfer functions, a fact not incorporated in existing computational models of motion perception.     

Contrast matching across spatial frequencies for isoluminant chromatic gratings

Contrast matching was performed with isoluminant red-green and s-cone gratings at spatial frequencies ranging from 0.5 to 8 c/deg. Contrast threshold curves were low-pass in shape, in agreement with previous findings. Contrast matching functions resembled threshold curves at low contrast levels, but became flat and independent of spatial frequency at high contrasts. Thus, isoluminant chromatic gratings exhibited contrast constancy at suprathreshold contrast levels in a similar manner as has been demonstrated for achromatic gratings.     

The effects of color-specific adaptation on the perceived duration of gratings

The perceived duration of briefly presented chromatic gratings was found to change with prior chromatic grating adaptation. Perceived durations of red and green, vertical and horizontal square wave gratings were measured by having subjects judge when the intermittently presented stimuli appeared continuously present.With monocular viewing, the apparent duration of a grating decreased only after adaptation to gratings of the same color and orientation. When viewed with the other eye, there were no significant changes in apparent duration, no matter what the grating color. These results extend the finding that duration judgements are sensitive to adaptation based on cortical receptive field parameters. The lack of interocular transfer is consistent with most visual aftereffects generated with colored gratings.     

Textured backgrounds alter perceived speed

Both the luminance contrast of an object, and the nature of the background texture over which it moves, are known to influence its perceived speed. In this study the effect of object contrast upon perceived speed was investigated for targets moving across textured patterns of various contrasts. Experiment 1 showed a strong effect of contrast for objects moving over homogenous backgrounds, that was reduced or abolished if the object moved over a textured background. A further experiment suggested that this reduction may be the result of an increase in target visibility, perhaps as a result of additional 'second order' motion signals produced by motion over texture backgrounds. A final experiment suggested that two processes were occurring: (1) higher contrast backgrounds appeared to increase the perceived speeds of all objects; and (2) that higher contrast backgrounds eliminated the contrast induced changes in perceived speed.     

Luminance texture increases perceived speed

Previous psychophysical experiments have demonstrated that various factors can exert a considerable influence on the apparent velocity of visual stimuli. Here, we investigated the effects of superimposing static luminance texture on the apparent speed of a drifting grating. In Experiment 1, we demonstrate that superimposing static luminance texture on a drifting luminance modulated grating can produce an increase in perceived speed. This supports the hypothesis that texture changes perceived speed by providing landmarks to assess relative motion. In Experiment 2, we showed that contrary to static luminance texture, dynamic luminance texture did not increase perceived speed. This demonstrates that texture must provide reliable spatial landmarks in order to generate an increase in perceived speed. The results of Experiment 3 demonstrate that perceived speed depends on the size of the area covered by texture. This suggests that luminance texture and the motion stimulus interacted with each other over a limited spatial scale and that these local responses are then pooled to determine the speed of the motion stimulus. In Experiment 4, we showed that static texture contrast could produce a greater effect than motion stimulus contrast on perceived speed and that these effects could still be observed at brief presentation times. We discuss these findings in the context of models proposed to account for phenomena in the perception of speed.     

Center-surround effects on perceived speed

We investigated whether center-surround interactions affect perceived speed in a manner similar to their effects on direction discrimination thresholds [e.g. Tadin, D., Lappin, J. S., Gilroy, L. A., & Blake, R. (2003). Perceptual consequences of center-surround antagonism in visual motion processing. Nature, 424, 312-315]. Observers were asked to match the speed of a test stimulus (a grating, with fixed contrast and no surround) to that of a reference stimulus of variable contrast and with a variably sized surround, moving at one of two possible velocities (1 and 12 cps). At 1-cps, both lowering contrast and increasing surround-size resulted in a decrease in perceived speed, except for very low contrast stimuli, where a larger surround resulted in an increase in perceived speed. Although the effect of surround-size was comparable in the two velocity conditions, the effect of contrast was different at 12-cps. That is, in the 12 cps condition, a decrease in perceived speed was observed only for the lowest contrast used. Our results suggest that, at least for the lower velocity used, center-surround interactions affect perceived speed in a manner analogous to their effect on direction discrimination.     

Latency for the perceived offset of brief target gratings

Observers adjusted a brief auditory probe to coincide with the phenomenal offset of a 50-msec target grating of varying spatial frequency. Increasing spatial frequency increased the chosen latency for the probe, while increasing grating luminance decreased the chosen latency for the probe. These results were interpreted in terms of the sensitivity of off-responses in the visual system to these stimulus variables. These latencies for off-responses were specifically distinguished from visual persistence which is generally longer lasting and exhibits opposite relationships with the same stimulus variables.     

Misperceptions of speed for chromatic and luminance grating stimuli

Errors in the perception of speed of moving visual stimuli can occur when presented stimuli are of unequal contrast and when they appear alongside additional modifier stimuli that move at different speeds. We have examined these misperceptions for chromatic and luminance grating stimuli in order to assess to what extent these different kinds of motion cue might be utilised in the analysis of speed of moving objects. We show that the dependence on contrast of speed matching for luminance and chromatic stimuli is similar over a range of stimulus speeds greater than 4 deg/s. Differences between the contrast dependencies of speed perception for chromatic and luminance stimuli are only evident at slow speeds (< 4 deg/s) and low contrasts. The presence of modifier stimuli can directly influence the perceived speed at both high and low velocities and contrasts. This influence was found to be independent of the modifiers' chromaticity and was greatest when the modifiers were adjacent to and presented simultaneously with the test and reference stimuli. However, the modifiers were still able to induce measurable changes in perceived speed for increased separations over space and time. Taken together these results indicate that whilst differences do exist in the contrast dependencies of speed perception for chromatic and luminance stimuli, they are evident only for a narrow range of stimulus parameters (i.e. low speed and low contrast). There appears to be ample scope for interactions between chromatic and luminance contrast in speed perception where there is the capacity to pool this information over a relatively broad spatio-temporal extent.     

Anisotropies of perceived contrast and detection speed

Gratings of different orientations were compared in terms of both apparent contrast and detection speed. Magnitude estimates demonstrated that oblique gratings appear perceptually to have lower contrasts than horizontal or vertical gratings of the same physical contrast. This anisotropy of perceived contrast holds across a wide supratheshold range of physical contrasts. Even when gratings of the different orientations are equated in terms of perceived contrast, an oblique effect of detection time remains. The magnitude of this residual anisotropy of detection speed decreases as contrast is increased from threshold, such that this second anisotropy is observed only at a restricted range of lower contrasts.     

Effect of signal intensity on perceived speed

The effect of signal intensity (proportion of dots moving in the same direction compared to noise dots that move in random directions) on perceived speed was investigated. It was found that increasing signal level decreased the perceived speed of the stimulus. This finding indicates that global-motion pooling processes play a role in the extraction of speed information. It is suggested that the amount of relative motion in the stimulus influences perceived speed, with perceived speed increasing with increasing relative motion. The results are discussed in relation to the notion that speed and direction are processed, at least in part, differently.     

Testing the Bayesian model of perceived speed

In a recent Bayesian model by Weiss, Simoncelli, and Adelson, motion perception is biased by a prior favoring slow speeds. This model predicts qualitatively an impressive variety of phenomena, including the dependence of perceived speed on contrast. We show that the model can also generate quantitative predictions: for a drifting grating with contrast c, perceived speed is proportional to c(q)/(k(q) + c(q)), with k, q constants. We tested this expression on measurements of perceived speed as a function of contrast. Observers indicated the slower of two drifting gratings, a test and a standard. For each test contrast we found the test speed that appeared to match the standard speed. The model fits the data, but only if q is less than 2, the value it would have if the internal representation of contrast were linear. The Bayesian model can make correct quantitative predictions, but needs to be extended to incorporate a more realistic, nonlinear representation of contrast.