Thresholds for the detection of contrast differences and spatial frequency differences present in a sine-wave grating with suprathreshold contrast were measured as a function of the size of the grating and the number of cycles in the grating. For gratings with the same number of cycles, these thresholds are found to be independent of the size of the grating over the entire range of sizes tested (1.5 log unit): the thresholds do not change when the stimulus is enlarged by some scale factor. We call this phenomenon "scale invariance". The thresholds decrease with increasing number of grating cycles, at least up to the point where 16 cycles are present. This support the view that "spatial pooling" or "spatial integration" mechanisms act over a larger area than the "functional summation area" found in threshold contrast measurements (about 10 cycles high and wide).