Figure and ground in space and time: 2. Frequency, velocity, and perceptual organization

The figure-ground organization of an ambiguous bipartite pattern in which the two regions of the pattern contained sine-wave gratings which differed in spatial frequency was examined for two pairs of spatial frequencies: 1 and 4 cycles deg-1, and 1 and 8 cycles deg-1. The region of higher spatial frequency underwent contrast reversal at one of four rates: 0, 3.75, 7.5, or 15 Hz. The region of lower spatial frequency was equated with either the temporal frequency or the velocity of the grating of higher spatial frequency in three sets of conditions: one stationary condition, three in which temporal frequency was equated, and three in which velocity was equated. For the 1 and 4 cycles deg-1 pair, the region of lower spatial frequency tended to be seen as the background a higher percentage of the time. There were significant linear trends for the appearance as background of the region of lower spatial frequency with respect to the magnitude of the velocity difference between the two regions of the pattern. The faster the 1 cycle deg-1 grating moved with respect to the 4 cycles deg-1 grating, the higher the percentage of the time it was seen as the ground. The results for the 1 and 8 cycles deg-1 pair were in some cases unexpected in that the 8 cycles deg-1 grating was seen as the ground behind the 1 cycle deg-1 grating even though it was of a higher spatial frequency and moved at a slower velocity. The spatiotemporal tuning of the visual system is discussed.