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Flash-Lag Effect: Differential Latency, Not Postdiction

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


  • 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).

  • 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.

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