Detectability of amplitude- and frequency-modulation of suprathreshold sine-wave gratings

The detectability of amplitude- and frequency-modulation of sine-wave carrier gratings was measured for a number of carrier frequencies, modulation frequencies and carrier contrasts. The results show that for low modulation frequencies (spatial frequency components close together; separation less than 0.5 x carrier frequency) modulation thresholds are determined by both the amplitude and phase spectrum of the stimulus; for high modulation frequencies (frequency components more widely separated) they are determined by the amplitude spectrum only. The dependence of modulation thresholds on carrier contrast was found to be very similar, irrespective of the modulation frequency, carrier frequency and kind of modulation selected, and was in agreement with known results of contrast-difference threshold measurements and of masking experiments.