Campbell and Maffei (1970) reported a linear relationship between the logarithm of the grating pattern contrast and the amplitude of the visual evoked response (VEP). This enabled them to predict the pattern visibility by extrapolating to zero amplitude. By using a new digital fast sweep technique for acquiring and analysing steady state evoked potentials we show that, at clearly suprathreshold contrast levels, such a linear relationship only exists for certain spatial frequencies. In general, the VEP saturates with increasing contrast in a way that critically depends on stimulus spatial frequency. Such a dependency on spatial frequency is not obtained for suprathreshold contrast perception which is characterized by a remarkable contrast constancy. Thus the amplitude of the pattern VEP does not convey information about visual perception other than contrast detection thresholds.