Illusory reversal of extrafoveally perceived displacement

It has long been known that the temporal characteristics of human peripheral vision differ markedly from those of foveal vision. Slowly moving peripheral stimuli, for example, can give rise to dramatically exaggerated estimates of their displacement. More recently, Thorson et al. have shown that where two spots are flashed in sequence to peripheral vision with an interflash interval of 50 ms, a sensation of movement can be induced even when the spatial separation of the spots is below the static acuity threshold. These observations fit with several others, suggesting that the visual system uses separate channels for signalling 'motion' as distinct from 'change of location', and that in some circumstances the integration of motion signals may make a dominant contribution to the perception of displacement. I now report a striking illusion which seems to reinforce and extend this conclusion.