Dissociation between spatial and temporal integration mechanisms in Vernier fusion

The visual system constructs a percept of the world across multiple spatial and temporal scales. This raises the questions of whether different scales involve separate integration mechanisms and whether spatial and temporal factors are linked via spatio-temporal reference frames. We investigated this using Vernier fusion, a phenomenon in which the features of two Vernier stimuli presented in close spatio-tem- poral proximity are fused into a single percept. With increasing spatial offset, perception changes dramat- ically from a single percept into apparent motion and later, at larger offsets, into two separately perceived stimuli. We tested the link between spatial and temporal integration by presenting two successive Ver- nier stimuli presented at varying spatial and temporal offsets. The second Vernier either had the same or the opposite offset as the first. We found that the type of percept depended not only on spatial offset, as reported previously, but interacted with the temporal parameter as well. At temporal separations around 30–40 ms the majority of trials were perceived as motion, while above 70 ms predominantly two sepa- rate stimuli were reported. The dominance of the second Vernier varied systematically with temporal off- set, peaking around 40ms ISI. Same-offset conditions showed increasing amounts of perceived separation at large ISIs, but little dependence on spatial offset. As subjects did not always completely fuse stimuli, we separated trials by reported percept (single/fusion, motion, double/segregation). We found systematic indications of spatial fusion even on trials in which subjects perceived temporal segregation. These findings imply that spatial integration/fusion may occur even when the stimuli are perceived as temporally separate entities, suggesting that the mechanisms responsible for temporal segregation and spatial integration may not be mutually exclusive.