An illusory transformation of optic flow fields without local motion interactions.

The focus of expansion (FOE) of a radially expanding optic flow pattern that is overlapped by unidirectional laminar flow is perceptually displaced in the direction of that laminar flow. There is continuing debate on whether this effect is due to local or global motion interactions. Here, we show psychophysically that under conditions without local motion transparency the illusion becomes weaker but can still be observed. In our experiments, the radial and laminar-flow fields were not presented with overlap but separately to the left and right halves of the visual field with a blank vertical strip of 15 degrees horizontal width in between. The illusory shift observed in this condition cannot be explained by local motion interactions because (a) no transparent motion was present in the stimulus, and (b) the receptive fields of cortical cells involved in the analysis of local motion cross the vertical midline of the visual field to a limited extent. We conclude that global motion detectors that integrate motion from both halves of the visual field play a role in shifting the perceived position of the FOE and that local motion interactions may be sufficient, but are not necessary for the optic flow illusion to occur.