Technical Comments

The Position of Moving Objects

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Science  18 Aug 2000:
Vol. 289, Issue 5482, pp. 1107
DOI: 10.1126/science.289.5482.1107a


  • Figure 1

    The flash-lag effect and temporal integration of position signals. (A) The flash-lag stimulus. Seven dots in perfect alignment rotate around the central fixation point. The outer dots (gray) are visible only intermittently; the inner dots are continuously visible. Observers perceive a spatial offset between the outer, flashed dots and the inner dots (black dots show the percept). (B) The temporal-integration hypothesis as a space-time diagram. The motion trajectory of the inner dots is represented as the solid line; the outer dots are visible only fromt 0 to t 1, fromt 2 to t 3, and fromt 4 onwards. When the outer dots are invisible, we assume that an internal position signal represents their last visible position (dashed line). The percept at timet percept is the average difference of the two position signals (solid and dashed lines) over a temporal-integration window Δ. This average is equivalent to the shaded area divided by Δ.

  • Figure 1

    Stimulus time course and data distinguishing the flash-lag effect from the Fröhlich effect. (A) Two stationary objects were presented for 2.5 sec (stationary cue), then removed for 30 ms. After that Δt, the objects were presented again and immediately began to move in opposite directions (to avoid eye movements). In a second experimental condition, no stationary cue was presented. (B) Blue symbols refer to the perceived Fröhlich effect, the degree to which the initial position of the moving objects appeared shifted in the direction of their motion. Red symbols refer to the perceived flash-lag effect, the degree to which the flashes appeared to trail behind the position of the moving objects. The flash-lag effect was constant whether the cue was presented or not; the Fröhlich effect was reduced dramatically when the cue was presented.

  • Figure 1

    Role of predictability in the flash-lag effect. Experimental conditions replicate those of (5): three central dots spin at 0.5 Hz; two pairs of flanking dots are flashed with an offset angle. Illusory displacement is quantified by a method of adjustment at five flash durations (six trials each duration). Each trial reflects of one of two randomly interleaved conditions: the flashes appear once every 2000 msec at the same location each time (predictable, squares), or they appear randomly from 1500 to 2500 ms after the last flash and, thus, at random locations (unpredictable, diamonds). Data on each subject are normalized to the magnitude of the lag angle in the unpredictable condition at 13 ms (absolute lag angles for that condition, n = 6 subjects: 9.2°, 10°, 6.7°, 8.3°, 19.2°, and 17.5°). Demonstrations of stimuli can be found∼eagleman/flashlag.

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