FM phase and the continuity illusion

Abstract

In experiment 1 subjects discriminated between a ‘‘regular’’ 1-kHz FM tone and one where the FM phase reversed midway through, at a zero crossing of the modulator waveform. Zero-peak FM depth was 100 Hz. Performance was near-perfect when the modulator frequency was 2.5 Hz, but declined sharply as it increased above 10 Hz. This is consistent with instantaneous frequency being smoothed over a finite time window, and with the phase reversal producing a ‘‘bump’’ in the window output, which decreases for faster modulations. In experiment 2 listeners discriminated between an FM tone containing a phase reversal and one where the FM depth of one-half modulator cycle was increased by various amounts (‘‘df’’)—thereby also producing a bump in the window output. Performance reached a minimum (at df= approx 5%), consistent with the theory. Finally, a 200-ms gap was inserted at the phase reversal point in a 5-Hz FM stimulus, removing subjects’ ability to detect a phase reversal. Filling the gap with noise introduced a sensation of continuity, but subjects could not detect a phase reversal, even though this would have been easy in a physically continuous stimulus. Results suggest that FM phase is not encoded explicitly in the auditory system.