Integration of signals across space and time by the visual system is well studied in behavioural studies and, within specific limits, is described by Ricco's area and Bloch's law respectively. What is unclear is whether the area of integration changes as a function of time. There are two reasons to hypothesize that it might. First, even during fixation, the eyes continually move. The brain could compensate for small eye movement by increasing the size of the area of integration across time. Second, the receptive fields of some individual neurons shift over time. When neurons are combined in populations, the area of integration might be expected to resemble concentric travelling waves. In our experiment, stimuli consisted of two probes presented with varying spatial (0 to .9dva) and temporal (0 to 100ms) separation, randomly appearing at one of four known-locations. A 4-AFC target detection task was used to identify the luminance thresholds where probes of varying spatial and temporal properties were detected with 75% accuracy. As spatial separation increased, the temporal separation at which the probes are most easily detected also increased. That is, probes with moderate spatial separation are more easily detected when they also have temporal separation than when the two probes are presented simultaneously. These findings indicate an interaction between space and time during integration of signals. The pattern of results is compatible with drifting receptive fields.