Expansive micropsia

Elements in an optic flow field increase in angular size as the field expands with forward motion. If the element size is held constant, there is a resulting illusion of the elements shrinking in size with field expansion. Macpherson (1993 Perception 22 Supplement, 121) referred to this as 'expansive micropsia'. The origin of this phenomenon is unclear, but one possible explanation is that it is a result of size-constancy scaling. We have tested the size-constancy explanation by estimating the magnitude of the illusion for a range of stimulus configurations representing candidates for both plausible and implausible flow fields. The results indicate that the magnitude of the illusion is affected significantly by absolute size, being larger for small square elements (0.5 deg × 0.5 deg) than for larger (1.5 deg × 1.5 deg) ones. Further, there is a marked ceiling effect with the extent of the field expansion. Finally, the size-shift illusion with line targets is as large when there is an instantaneous displacement accompanied by a random rotation ('implausible') as when the temporal sequence represents a 'plausible' target expansion. We conclude that size constancy does not provide a comprehensive account of the illusory size contraction, and advance an alternative hypothesis based on the M-scaling of size-sensitive detectors with eccentricity.