Hearing melodies in continuous frequency modulations

A 1-kHz carrier was frequency modulated by the exponential of periodic time functions corresponding to the sum of a few sinusoids (e.g., sin ωt+sin 3ωt). The modulations were symmetric on the dimensions of time and log frequency [y(t)=y(k−t)=−y(k’−t)]. Each had three local maxima and three local minima per cycle. The peak-to-peak frequency excursion was typically 0.5 oct. At low rates of modulation, the stimuli were heard as complex glissandi. However, for modulation rates around 1.5 Hz, the dominant percept was a melodic motif comprising the pitches of the local maxima. Generally, the local minima were not heard as auditory ‘‘events’’ (pitch singularities). A similar perceptual asymmetry was not observed for sequences of discrete tones corresponding to the local maxima and minima of the continuous modulations. Additional experiments were performed to determine the frequency difference limens for maxima and minima of continuous frequency modulations. The carriers were modulated by the exponential of one cycle of a cosine function, starting at phase π or phase 0. For maxima and minima around 1 kHz, frequency shifts of maxima were markedly better detected than frequency shifts of minima. These results are in contrast to those predicted by consideration of the upward spread of masking.