The Flash-Lag Effect as a Motion-Based Predictive Shift
Fig 7
Dependence of the FLE with respect to contrast and duration of the stimuli.
(A) Using the same data format as in Fig 5, we show the spatial distribution of the estimated response (zoomed around its physical position at the perceived time of the flash at full contrast which is indicated by a cross) for different different relative contrast levels C indicated at each row. The different columns correspond from left to right to different conditions where the contrast of the dot is manipulated (first two columns)—respectively at the beginning of the cycle (i.e. flash-initiated) cycle, the mid-point (i.e. standard cycle)— or where the contrast of the flash is varied (right-end column). Note that in the standard FLE case (middle column), the model already responds to very low values of dot contrast in a nearly all-or-none fashion. By comparison, the responses to the dot or the flash during the initial phase of the trajectory gradually increased with contrast. In particular, the dot’s lag seems to increase more rapidly with respect to contrast. (B) These qualitative results are best illustrated by plotting in the first column the precision of the response as measured by the inverse standard deviation of the estimated position as a function of contrast of the different conditions. Coherent with the results illustrated in (A), the precision of the representation varies gradually against contrast of the flash or moving dot in the early phase whereas it changes more rapidly and abruptly as a function of the moving dot’s contrast in the standard FLE. Consequently, we estimated in the second column the spatial lag that is expected when changing the contrast of the stimuli (± one standard deviation). Coherently with psychophysical results, increasing the contrast of the moving dot gradually increases the FLE in the flash-initiated cycle but has only limited effects in the standard FLE when above a given precision as it rapidly reaches a saturating value of ≈0.2 corresponding to a full compensation of the fixed delay. Consistent with [38], these results show the role of spatial uncertainty in dynamically tuning the estimated position and, ultimately, in influencing the spatial lag in the FLE. (C) As shown by [47], flash duration modulates FLE. We show here the precision for the flash as a function of time with respect to duration. While the peak remained at t = .5 s (that is, at t = .6 s when including the delay), we tested for different durations, respectively .03, .05, .08, .13, .25 in s (as marked by colored horizontal bars). The respective measured time to reach the maximal precision are given by tmax (in s), showing that precision was high for T ≥.05 s (that is, 50 ms). Notice that this value was used for all the experiments described above.