Apparent speed increases at low luminance

Abstract

To investigate the effect of luminance on apparent speed, subjects adjusted the speed of a low-luminance rotating grating (0.31 cd/m(2)) to match that of a high-luminance one (1260 cd/m(2)). Above 4 Hz, subjects overestimated the speed of the low-luminance grating. This overestimation increased as a function of temporal rate and reached 30% around 10 Hz temporal rates. The speed overestimation became significant once the lower luminance was 2.4 log units lower than the high luminance comparison. Next the role of motion smear in speed overestimation was examined. First it was shown that the length of the perceived motion smear increased at low luminances. Second, the length of the visible smear was manipulated by changing the presentation time of the stimuli. Speed overestimation was reduced at shorter presentation times. Third the speed of a blurred stimulus was compared to a stimulus with sharp edges and the blurred stimulus was judged to move faster. These results indicate that the length of motion smear following a target contributes to its perceived speed and that this leads to speed overestimation at low luminance where motion traces lengthen because of increased persistence.

Figure 1

(a) A schematic top view of the experimental setup. Subjects viewed the screen from 57 cm distance. The stimuli were back projected on the screen using a video projector. To make low luminance stimuli, half of the screen was covered with neutral density filters. (b) A schematic display of the one frame of the moving gratings. Stimuli were radial gratings with black and white spokes presented on a black background and rotated around their center.

Figure 2

(a)–(c) Results of each subject plotted separately for target temporal rate versus matched temporal rate. Data for the control condition are plotted in blue (both match and target gratings had high luminance) and data for the test condition are plotted in red (the target grating had high and the match grating had low luminance). Lower match temporal rates for test compared to control condition shows the speed overestimation effect. The gray line shows the veridical match. Note that 10 Hz equals to 2 rps as the stimuli have 5 spokes. (d) Pooled data of all three subjects now showing temporal rate difference (target – match) as a function of the target grating temporal rate. Zero error indicates the perfect match to the target. Standard errors of the mean (±1 SEM) are shown as vertical lines.

Figure 3

Pooled data from all subjects are plotted. The x-axis shows the luminance reduction [log₁₀(target luminance / match luminance)] in logarithmic scale and the y-axis shows the difference between the temporal rate of the target and match (target temporal rate – match temporal rate). Zero error shows the perfect match to the target. Error bars show standard errors of the mean (±1 SEM).

Figure 4

The speed at which the rotating dot appears as a continuous circle is plotted for the two luminance conditions (3 subjects). Higher speed was required at high luminances indicating that there was less persistence. Error bars show standard errors of the mean (±1 SEM).

Figure 5

The difference between the target speed at low and high luminances is plotted as a function of target speed for (a) long and (b) short presentation times. The results show that the over-estimation effect is stronger in the long presentation condition. Error bars show standard errors of the mean (±1 SEM).

Figure 6

Temporal rate error (target temporal rate – match temporal rate) is plotted for both test and control conditions. In the test condition the target is a square-wave grating and the match is a sine-wave grating, and in the control condition both target and match are square-wave gratings. Results show that a sine-wave grating is perceived to move faster than a square-wave grating. Error bars show standard errors of the mean (±1 SEM).

Figure 7

Temporal rate error (target temporal rate – match temporal rate) is plotted for two experimental conditions. In the test condition the target is a square-wave grating and the match is a phase-scrambled grating, and in the control condition both target and match are square-wave gratings. Results show that phase-scrambled stimulus appears to move faster than the square-wave stimulus. Error bars show standard errors of the mean (±1 SEM).

Figure 8

Temporal rate error (target temporal rate – match temporal rate) is plotted for two experimental conditions. In the test condition the target had a Michelson contrast of 0.4 and the match had a Michelson contrast of 0.1, and in the control condition both target and match had a Michelson contrast of 0.4. Results show that there is a decrease in the matched speed of a sine-wave grating as its contrast decreases. Error bars show standard errors of the mean (±1 SEM).