Motion capture and phase congruency

Purpose: To study the relationship between motion capture and phase congruency of stimulus components. Methods: An abruptly contrast-reversing squarewave grating comprises two equivalent fundamentals moving in opposite directions. Altering the phase of one of them causes an apparent motion of the high-frequency edge components in the direction of the in-phase fundamental (motion capture). We measured the strength of this illusion by adjusting (by 2AFC) the relative contrast of the two fundamentals until the motion of the high frequency edges was annulled, for several phase values. The out-of-phase fundamental was held at fixed contrast, while that of the other was varied. The task was performed by several trained and naive subjects at various spatial (0.25-2 cycles/deg) and temporal (1-8 Hz) frequencies and contrasts of the high frequency components. To infer the filter characteristics of the mechanisms selective to the stimulus phase congruency we analysed the data using a Factor and Principal Component Analysis (PCA). Results: 1) There was a strong dependence of capture on phase. At 0 phase offset any small deviation from balanced contrast induced motion of the high spatial frequencies in the direction of the strongest fundamental. At 90 deg offset, the high frequencies were always captured by the in-phase fundamental. Intermediate phases induced edge motion only for a limited range of contrast of the in-phase fundamental. 2) The general trend of capture did not depend on the spatial frequency or base contrast of the pattern. 3) PCA revealed two clusters of strong correlation amongst low spatial frequencies (0.25 - 0.5 c/deg) and amongst medium spatial frequencies (1-2 c/deg), probably reflecting the activity of independent spatial channels. 4) The data were predicted by applying a spatio-temporal energy model (ARVO, 1992), using non-directional spatio-temporal filters. Conclusion: Low spatial frequencies can capture the motion of high-spatial frequencies, provided that they are similar enough in phase to produce clear maxima of local energy.