NATURE | VOL 396 | 3 DECEMBER 1998 | www.nature.com that of the strobed segment (d s ) remains constant. The latency-difference hypothesis therefore predicts that the observed spatial lead of the moving central segment should increase.To test this prediction, we measured the spatial lead of the moving central segment as a function of the detectability of the central segment while keeping the detectability of the strobed segments constant. Here we use detectability to refer to the number of log units of luminance (Lu) above the detection threshold; detectability of the strobed segments was 0.3 Lu for subjects S.S.P. and G.P., and 0.5 Lu for T.L.N. The temporal lead of the moving central segment averaged across subjects increases systematically from 20 to 70 ms when its detectability increases by 1.0 Lu (Fig. 1b).Increasing the luminance of the strobed segments while keeping that of the moving central segment constant should decrease d s , while d m remains constant. The latencydifference hypothesis predicts that the observed spatial lead of the moving central segment should decrease and, if the luminance of the strobed segments is high enough, the moving central segment should be perceived to lag behind spatially. We tested this prediction by measuring spatial lead as a function of the detectability of the strobed segments, while keeping the detectability of the moving central segment constant (1.5 Lu above the detection threshold for subjects G.P. and T.L.N., and 0.8 Lu for S.S.P.). The observed temporal lead of the moving central segment averaged across subjects decreases systematically from 80 to ǁ30 ms as the detectability of the strobed segments increases by 1.5 to 2.0 Lu (Fig. 1c).These results support predictions of the latency-difference hypothesis and show that the motion-extrapolation mechanism does not compensate for stimulus-dependent variations in latency. Indeed, theoretical calculations show that the putative motionextrapolation mechanism must be undercompensating by at least 120 ms to account for the data in Fig. 1. But a motion-extrapolation mechanism that does not adequately compensate for variations in visual latency would not appreciably improve the accuracy of real-time visually guided behaviour.

Recent psychophysical studies have been interpreted to indicate that the perception of motion temporally either lags or is synchronous with the perception of color. These results appear to be at odds with neurophysiological data, which show that the average response-onset latency is shorter in the cortical areas responsible for motion (e.g., MT and MST) than for color processing (e.g., V4). The purpose of this study was to compare the perceptual asynchrony between motion and color on two psychophysical tasks. In the color correspondence task, observers indicated the predominant color of an 18 degrees x 18 degrees field of colored dots when they moved in a specific direction. On each trial, the dots periodically changed color from red to green and moved cyclically at 15, 30 or 60 deg/s in two directions separated by 180 degrees, 135 degrees, 90 degrees or 45 degrees. In the temporal order judgment task, observers indicated whether a change in color occurred before or after a change in motion, within a single cycle of the moving-dot stimulus. In the color correspondence task, we found that the perceptual asynchrony between color and motion depends on the difference in directions within the motion cycle, but does not depend on the dot velocity. In the temporal order judgment task, the perceptual asynchrony is substantially shorter than for the color correspondence task, and does not depend on the change in motion direction or the dot velocity. These findings suggest that it is inappropriate to interpret previous psychophysical results as evidence that motion perception generally lags color perception. We discuss our data in the context of a "two-stage sustained-transient" functional model for the processing of various perceptual attributes.

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The perceived position of a stationary Gaussian window of a Gabor target shifts in the direction of motion of the Gabor's carrier stimulus, implying the presence of interactions between the specialized visual areas that encode form, position, and motion. The purpose of this study was to examine the temporal and spatial properties of this illusory motion-induced position shift (MIPS). We measured the magnitude of the MIPS for a pair of horizontally separated (2 or 8deg) truncated-Gabor stimuli (carrier=1 or 4cpd sinusoidal grating, Gaussian envelope SD=18arc min, 50% contrast) or a pair of Gaussian-windowed random-texture patterns that drifted vertically in opposite directions. The magnitude of the MIPS was measured for drift speeds up to 16deg/s and for stimulus durations up to 453ms. The temporal properties of the MIPS depended on the drift speed. At low velocities, the magnitude of the MIPS increased monotonically with the stimulus duration. At higher velocities, the magnitude of the MIPS increased with duration initially, then decreased between approximately 45 and 75ms before rising to reach a steady-state value at longer durations. In general, the magnitude of the MIPS was larger when the truncated-Gabor or random-texture stimuli were more spatially separated, but was similar for the different types of carrier stimuli. Our results are consistent with a framework that suggests that perceived form is modulated dynamically during stimulus motion.

Previous investigators reported the impairment of foveal visual acuity by nearby flanking targets (contour interaction) is reduced or eliminated when acuity is measured using low contrast targets. Unlike earlier studies, we compared contour interaction for high and low contrast acuity targets using flankers at fixed angular separations, rather than at specific multiples of the acuity target's stroke width. Percent correct letter identification was determined in 4 adult observers for computer generated, high and low contrast dark Sloan letters surrounded by 4 equal contrast flanking bars. Two low contrast targets were selected to reduce each observer's visual acuity by 0.2 and 0.4 logMAR. The contour interaction functions measured for high and low contrast letters are very similar when percent correct letter identification is plotted against the flanker separation in min arc. These results indicate that contour interaction of foveal acuity targets occurs within a fixed angular zone of a few min arc, regardless of the size or contrast of the acuity target.