Flash-lag effect: differential latency, not postdiction.

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 flashlag 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 flashinitiated 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 s9 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). 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 s9 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 (L9f in Fig. 1) and the latency of a low-luminance moving line should be relatively long. The differentiallatency 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,