According to Campbell and Robson (1968), the human contrast sensitivity function shows a typical band-pass filter shape peaking at around 4 cycles per degree, with sensitivity dropping off either side of the peak. That finding has led many to claim that the human visual system is most sensitive in detecting contrast differences occurring at 4 cycles per degree the spatial frequency at which humans can detect lower contrast differences than at any other angular frequency. However, the claim of frequency sensitivity is problematic given, for example, that changes of distance do not seem to affect the relevant perceptual patterns (as noted, for example, in the figure caption to Solomon and Pelli (1994) While the latter authors are referring specifically to letters, they make no objective distinction between these and other shapes. The relative insensitivity of contrast effects to distance (and thus spatial frequency) may also be observed by casual inspection of a paradigmantic sweep grating, as may be observed here