IOC, vector sum, and squaring: three different motion effects or one?

Bowns (Vision Research, 36(22) (1996), 3685) argued that there are distinct features in two-component moving patterns (plaids) that if tracked move in the same direction as (1) the intersection of constraints direction (IOC) Adelson and Movshon (Nature, 300 (1992), 523); and (2) the vector sum direction (VS) Yo and Wilson (Vision Research, 32(1) (1992), 135). The IOC and VS are hypotheses of how the motion of single components is combined to give pattern motion. This paper shows that there are also features that provide an explanation for a reversed motion described by Derrington, Badcock, and Holroyd (Vision Research, 32(4), (1992), 699), and investigates why reversals only occur under specific conditions. Section 3 replicates the original study by Derrington et al. (1992) and confirms that the reversals are limited to low temporal frequencies. Section 4 varies the spatial displacement of features that also predict reversals and shows that the temporal frequency at which reversals occur varies and is linearly dependent on the displacement of these specified features. Derrington et al. (1992) showed that reversals only occur when components have oblique angles, and suggested an explanation in terms of speed differences. Section 5 was not consistent with this hypothesis. An alternative explanation for why reversals only occur at oblique angles, and at low spatial frequencies is provided in terms of feature properties. Results supporting the IOC, vector sum, and squaring have previously been interpreted in terms of three disparate mechanisms. This may not be necessary.