Contrast sensitivity functions were measured in two ways: by counterphase modulation; and by in-phase modulation, in which the luminance of every point of a spatial grating was sinusoidally modulated in phase with one another. Contrast sensitivity functions measured by counterphase modulated gratings show spatiotemporal interactions that represent bandpass filtering. Contrast sensitivity functions measured by in-phase modulation are even more sharply bandpass in the spatial frequency domain, but they are spatiotemporally decoupled i.e. the effects of spatial and temporal variables are separable. This difference can be explained by the composition of the frequency components of the counterphase and the in-phase modulated gratings and by masking between components. The results suggest that the intrinsic filters are low-pass exponentials, that they are spatiotemporally separable, and that the spatiotemporal interaction of counterphase gratings is best described as masking by the zero spatiotemporal frequency component of the test gratings. © 1994.