Integration across a linear trajectory: An examination of contributions to the flash-lag effect
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The flash-lag effect (FLE) refers to the perceived misalignment of two stimuli that are perfectly temporally and spatially aligned. In a typical FLE display, one stimulus (the "target") is moving while the other stimulus (the "probe") is flashed alongside the target. At the time the probe is perceived, the moving target is erroneously perceived not proximate to the flashed probe, but rather displaced forward into its trajectory. Thus perception of the flashed probe seems to "lag" behind perception of the target. Eight theories of the FLE are discussed in detail. They rely, to different extents, on notions of motion extrapolation, positional sampling, differential neural transmission speeds, neural facilitation, subthreshold neural activity, spatial attention, and salient perceptual elements of the stimuli. The experiments described herein were used to test varying subsets of those theories. In Experiments 1 and 2 the plausibility of a theory introduced in this work was tested and partially validated. In Experiments 3-6, the contribution to the FLE of target motion following and preceding the probe-target judgment was measured. It was found that relatively slow targets moving for a relatively long period of time generate the largest FLE. Those results were problematic for some theories and demanded clarification from others. In Experiments 7a-7d potential subthreshold neural activity around the moving stimulus, which is key to several theories of the FLE, was measured using a stimulus embedded in the trajectory of the moving stimulus. The results suggested activity surrounding the moving stimulus served to reduce the time it took for objects within the moving stimulus' trajectory to reach threshold. The activity enabling that reduction did not appear to be inhibitory in nature. After a discussion of the adequacy of the explanations in light of the evidence, it was concluded that several of the explanations failed to fully account for the FLE. Finally, it was concluded that a neural facilitation-based theory of the FLE with a mechanism that allows motion of the target before and after the probe to influence the magnitude of the FLE would be most suitable to explain the data.