Technical Comments

Flash-Lag Effect: Differential Latency, Not Postdiction

Science  10 Nov 2000:
Vol. 290, Issue 5494, pp. 1051
DOI: 10.1126/science.290.5494.1051a

A continuously moving object typically is perceived to lead a flashed object in space when the two retinal images are physically aligned, a phenomenon known as the flash-lag effect (1). Eagleman and Sejnowski (2) recently published data that they interpreted to disagree with a previous explanation of this phenomenon, the differential-latency hypothesis (3–7), and to support instead a postdiction hypothesis (8). Here we demonstrate that the data presented in (2) are fully consistent with the differential-latency hypothesis. We also provide evidence that rejects postdiction as an explanation for the flash-lag phenomenon.

According to Eagleman and Sejnowski (2), the differential-latency hypothesis predicts that the perceived flash-lag should change if the flash is temporally advanced. To test that prediction, they used a flash-initiated cycle (FIC) paradigm in which the onset of the moving object occurs synchronously with the flash (Fig. 1A). Observers were asked to “adjust the angle of a ‘pointer’ line . . . to point to the beginning of the trajectory of the moving ring” (emphasis added). Eagleman and Sejnowski found that the adjusted angle of the pointer did not depend on the stimulus onset asynchrony (SOA) between the flashed and moving objects. That finding, however, does not contradict the differential-latency hypothesis, which predicts that the flash misalignment will depend not only on the SOA but also on the dynamics of the process that computes the moving object's position (compare s and s′ in Fig. 1A), as long as the observer judges the spatial misalignment between the flashed and moving objects at the instant the flashed object is perceived. That instant in time provides a necessary temporal reference for comparing the position of the moving and flashed objects. If, by contrast, observers use the flashed object as a “spatial pointer” to the perceived starting locus of the moving object's trajectory—at s* rather than s in Fig. 1A, because of the Fröhlich effect [(9), cited in (10)]—the differential-latency hypothesis predicts that observers' reports of s* will not depend on the SOA (11).

Figure 1

(A) Space-time diagram illustrating the stimuli and the predictions of our differential-latency hypothesis in the FIC paradigm. Stimuli are shown in red; responses of the perceptual system are depicted in green. Initially, the flashed object is presented briefly at the starting spatial location of the moving object (red circle at the origin). The position of the moving object then changes at a constant speed (red line). The green squares and circles depict the computed perceptual positions of the flashed and the moving objects, respectively. The flashed and the moving objects become visible at different latencies, indicated by Lf and Lm, respectively, at spatial locations 0 and s*. The filled squares and circles indicate the part of the trajectory where these objects are visible. At the time the flashed object becomes visible (Lf), the perceived position of the moving object is s. Therefore, even though the flashed and the moving objects are physically presented at the same spatial location (the origin), the flashed object is perceived to spatially lag the moving object by s. If the latency of the flashed object decreases from Lf to L′f because of a change in the stimulus parameters, then the spatial misalignment between the moving and flashed objects changes from s to s′. When the position computation process for the moving object reaches steady state (indicated by the filled green squares running parallel to the dashed lines), the differential latency is given by (Lf − dm). (B) CM paradigm, in which the motion of the moving object starts long before the presentation of the flashed object, so that the position computation process for the moving object is in steady state. If the latency of the flashed object is very short (L"f), then it is perceived to spatially lead the moving object by s". (C) The perceived spatial flash misalignment (±1 SEM) between a high-luminance flashed object (76.3 cd/m2) and a low-luminance moving object (4.8 cd/m2), measured as the degrees of orientation of the rotating line, in the FIC and CM paradigms for four observers (two naı̈ve), and the average across the observers (AVG). The background luminance was 0.05 cd/m2. The speed of rotation was 8.3 rpm. The mean difference between the FIC and CM results was 4.85° ± 1.18° [F(1,3) = 44.63,p=0.007]. Three of the four observers showed a flash-lead in the CM condition, in accordance with (5).

The postdiction hypothesis states that the position of the moving object is computed de novo after the occurrence of the flash. Consequently, the flashed object is predicted never to spatially lead the moving object. We have shown (5), however, that the perceived misalignment between an object in continuous motion (CM) and a flashed object changes from a flash-lag to a flash-lead if the luminance of the flashed object is increased enough (Fig. 1B). Further, whereas the postdiction hypothesis predicts that the perceived misalignment in the FIC and CM conditions should always be equal, our experiments indicate that perceived misalignments differ significantly depending on which condition is used (Fig. 1C). Differential latency can account for that result if, in the FIC paradigm, the flashed object is perceived during the transient phase of the moving object's position computation process (compare s′ in Fig. 1, A and B). In the FIC paradigm, perception of the flashed object is expected to occur during this transient phase of processing because the latency of a high-luminance flash should be relatively short (L′f in Fig. 1) and the latency of a low-luminance moving line should be relatively long. The differential-latency hypothesis predicts that the perceived misalignment will be equal in the FIC and CM paradigms, as found in (2), if the perception of the flashed object occurs when the position computation for the moving object is in steady state (12).

Based on their interpretation of the differential-latency hypothesis, Eagleman and Sejnowski inferred from their experimental results that “the visual system only uses information from the 10 to 20 ms after the flash” (13). However, when they (2) modified the FIC paradigm so that the moving object reversed its direction after an adjustable delay, they observed a change in reversal times beyond 10 to 20 ms. Their conclusion that those data are inconsistent with the differential-latency hypothesis, however, failed to consider the dynamics of the position computation process for the moving object (6, 7). In their paradigm, the later the moving object reverses its direction, the less time the position computation process has to reach steady state after the reversal of motion occurs. Therefore, as the reversal time is increased, the flash misalignment is increasingly determined by the transient dynamics of the position computation process. The differential-latency hypothesis cannot predict the relationship between perceived flash misalignment and the motion-reversal time without additional information about the transient dynamics of the position computation process (14).

In summary, the data of Eagleman and Sejnowski are fully consistent with the differential-latency hypothesis. Further, the postdiction hypothesis is unable to account for the occurrence of a flash-lead when the luminance of the flashed object is sufficiently high, or for data reported here that show the effect of the initial motion trajectory on perceived misalignment.

*Also College of Optometry, University of Houston.

REFERENCES AND NOTES

Response: Patel et al.report conditions in which the flash-lag effect becomes a flash-lead effect (1) and question whether this is consistent with our postdictive model (2). We show here that their data are indeed consistent with postdiction and provide evidence that rejects differential latency as an explanation.

The fundamental assumption of the differential-latency model is that a flash takes longer to reach awareness than a continuously moving object. A necessary consequence is that flashed and moving objects that are simultaneous in the world will be perceived with an illusory temporal order (Fig 1A). To assess the differential-latency model, we asked participants to fixate a bar in rotary motion on a computer monitor. After 500 ms of rotation, end segments were flashed for 14 ms (at a random orientation within ±20° of the bar). At some time before or after the video frame with the flash, the spinning bar halted movement, and it remained stopped for the rest of the trial.

Figure 1

Comparing differential latency with postdiction. (A) Space-time diagram, after Patel et al., illustrating the differential-latency framework. Red represents events in the world; green represents perception of those events. As prescribed by the differential-latency model, flashed objects are assumed to have a delay before reaching awareness (df) that is longer than the delay for moving objects (dm). As a result, differential latency predicts that a flash that occurs at the same time as a change in movement (in this case, a halt) will be perceived to follow the change. For perceived simultaneity, the flash would have to appear well before the halt. (B) Participants compare the temporal order of a flash and the halting of a rotating bar (inset shows schematic drawing of stimulus used). Bar subtends 5° visual angle and rotates at 60 rpm; the luminance of the flash cd/m2. SOA between the flash and halt are varied over 250 ms. Results show that participants do not display an illusory misalignment of temporal order. Symbols show averages from three participants in two conditions: high-luminance bar (42.3 cd/m2; triangles fit with solid line) or low-luminance bar (1.9 cd/m2, squares fit with dashed line). The dotted line shows the psychometric curve predicted by the differential-latency model for a differential latency of 80 ms. (C) In the postdiction framework, the temporal window of integration can have different positions, sizes, or both, depending on the parameters of the stimuli. Rectangles represent the window of time from which positional information is weighted most heavily. A perceptual decision regarding the position of the moving object when the flash occurred is determined only after positional data from the window of integration has been collected.

Instead of reporting on the alignment of the flashes, as in traditional flash-lag experiments, participants were asked to report which event occurred first—the flash or the halting of the bar. Participants reported the temporal order without misperception (Fig. 1B), which indicates that it does not take longer to perceive a flash than a moving object. The differential-latency framework, by contrast, predicts a systematic shift in the data (dotted curve in Fig. 1B). The same result was obtained with both high- and low-luminance moving bars (Fig. 1B), as well as with a direction reversal or disappearance of the moving bar instead of a halt (data not shown). These results are consistent with evidence that the brain keeps excellent track of the temporal order of events (3, 4).

The results of Patel et al. are consistent with an expanded postdiction framework we have recently presented (5) for understanding the flash-lag effect under more general conditions. Our framework is summarized by three assumptions: (i) The visual system compares dynamic internal models to stimuli in the external world. These internal models are developed, in part, from information integrated in a recent window of time (6,7). (ii) As the consequence of an unpredicted event (such as a flash), the visual system devalues its internal model and relies more heavily on newly collected measurements—a strategy that reflects its imperfect prediction of the outside world (5). In the conditions used in our original report (and reflected in note 12 of Patel et al.), internal models can be devalued completely (i.e., reset) by the flash, and the fresh collection of information leaves the system in the same condition as de novo movement. In that case, the FIC and CM conditions will be expected to yield the same perceived displacement (2). (iii) The devaluation of previously collected information does not have to be all-or-none. In different experimental conditions, information before the flash will be retained to greater or lesser degrees. This will depend not only on the salience of the flash, as demonstrated in (5), but also on the salience of the moving object. Specifically, the degree to which the internal model is relied upon depends in part on the confidence of the external measurements (detectability) of the moving object.

In our framework, the low-luminance moving object used by Patelet al. engenders a low signal-to-noise ratio in the measurements. In that situation, the visual system depends more heavily on its internal model than on external measurements (7). When reliance on the internal model is stronger, a smaller amount of information that was collected before the flash is discarded. Within this framework, it is clear how a flash-lead is possible: The internal model is more resistant to devaluation, such that more pre-flash information is carried over into the interpolated (postdictive) position estimation. In this case, the CM condition can yield a flash-lead.

The postdictive framework is illustrated in Fig 1C. Positional information about the moving object is integrated from a window of time around the flash, and this positional information is interpolated to yield a position estimate. By modifying the saliences of the flash and the moving target, one can change the size or position of the window of spatiotemporal integration, such that the interpolated answer will yield flash-lag or flash-lead illusions. Such an interpolation implies that the perceptual decision is not reached until further positional data, including information after the flash, has entered the visual system. Thus the final answer is postdictive: The visual system can employ positional data that happened after the flash when making its perceptual decision about what happened at the moment of the flash.

The data presented by Patel et al. are consistent with postdiction. In contrast, a differential-latency framework is inconsistent with a test of its key assumptions (Fig. 1B). Our results suggest that the flash-lag effect is a spatial illusion, not a temporal one (8).

REFERENCES AND NOTES

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