Moving ahead through differential visual latency

Abstract

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.

An empirical explanation of the flash-lag effect

“…Given the importance of understanding how the perception of speed and position are generated by the visual system, several explanations specific to these aspects of motion perception have been offered. These are: (i) that the visual system compensates for neuronal latencies by extrapolating the expected position of a moving object from information in the stimulus (5, 6, 36, 37); (ii) that the effect occurs because stimulus processing entails shorter latencies for moving stimuli than for static flashes (7,(38)(39)(40)(41); (iii) that the flash-lag effect is a consequence of ''anticipation'' in early retinal processing (42); and (iv) that the visual system relies on ''postdiction'' or positional biasing by shifting position computations in the direction of motion signals that occur after the flash (43)(44)(45)(46).…”

Physical, Neural, and Mental Timing

“…I like this option best because it enables us to understand how unitary experiences can arise from temporally dispersed neural signals, as in the above-mentioned plunge experience. It also eliminates worries about the change of time delays with intensity (Purushothaman, Patel, Bedell, & Ogmen, 1998). This brief review on the flash-lag phenomenon illustrates how one can relatively easily set up a ''playground'' for study of the relations between physical time, neural time, and mental time.…”

The Role of Attention in Motion Extrapolation: Are Moving Objects ‘Corrected’ or Flashed Objects Attentionally Delayed?

“…Recently, Purushothaman et al (1998) manipulated the luminance of the moving and the flashed items relative to the background and found that the flash-lag effect decreased with increased contrast of the flash, and decreased contrast of the moving item relative to the background. Taking a different approach, Whitney and Murakami (1998) showed that a moving object does not appear to overshoot the point where it abruptly reverses direction at the moment of a flash (Whitney et al 2000).…”

“…3 Two accounts of the flash-lag effect Multiple accounts of the flash-lag effect have been forwarded, such as visible persistence (Mach 1897;Nijhawan 1992;Krekelberg and Lappe 2000), informational content (MacKay 1958), motion extrapolation and delay (Nijhawan 1994(Nijhawan , 1997Khurana and Nijhawan 1995;Nijhawan and Khurana, in press), attentional delay (Baldo and Klein 1995), differential visual latencies (Purushothaman et al 1998;Whitney and Murakami 1998;Whitney et al 2000), and most recently, postdiction (Eagleman and Sejnowski 2000). We shall outline the extrapolation-and-delay account and the attentional account in detail to provide the motivation behind the present experiments and reserve discussion of the other accounts until after the presentation of the results.…”