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Haci H. Kafaligonul, Saumil S. Patel, Haluk Ogmen, Harold E. Bedell, Gopathy Purushothaman; Simultaneous flash-lag effects in two directions reveal a slow stage of multi-directional motion integration. Journal of Vision 2006;6(6):641. doi: 10.1167/6.6.641.
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Purpose. We used an illusion first discovered by Cai et al. (Cai's Line-length Effect: CLE) to investigate flash-lag effects simultaneously in two directions of motion. In the CLE, both position and length at the time of a flash are misperceived for a line that moves horizontally and simultaneously expands vertically.
Methods. In 1-D motion conditions, subjects (N=2) indicated (a) the perceived position of a horizontally moving line when its vertical length remained constant and (b) the vertical length of a line when its horizontal position remained constant. In 2-D motion conditions, perceived horizontal position and vertical length were measured in the CLE paradigm. A flashed bar (1.2 LU above detection threshold) signaled the subjects when to judge perceived position or length. The moving lines were either 1.8 or 3.2 LU above their detection threshold and changed position and/or length at 1.9 deg/sec.
Results. The average perceived lead of both position (equivalent to the classical flash-lag) and length increases virtually identically as the detectability of the moving/changing line increases in the 1-D and 2-D conditions (range = 42 to 47 ms/LU). However, the perceived lead of position and length is approximately 50 ms less at both levels of line detectability in the 2-D compared to the 1-D conditions.
Conclusion. Within the framework of the dual-channel differential latency hypothesis, the additional processing delay for the 2-D motion condition is attributed to an increase in the complexity in motion integration when horizontal motion is combined with vertical expansion or contraction.
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