Direction Discrimination of Logarithmic Frequency Sweeps

Science Center, Bent Corridor

Dynamically changing pitch is ubiquitous in our auditory environment and important for many aspects of speech perception. Studies of both frequency sweeps (rapid, short-duration tone-glides) and frequency modulated (FM) stimuli suggest two complementary mechanisms responsible for the perception of dynamic frequency changes: one that operates primarily at slow rates of change and is based on neural phase-locking to the temporal fine structure of the stimulus, and one that uses spectral energy cues which are optimal especially for rapid rates of frequency change. The goal of the current study was to investigate the complementary role of these two mechanisms and to examine the perception of frequency sweeps that more closely parallel those of speech prosody and tonal languages. To do this, we tested sweep direction identification for logarithmic sweeps with low rates of frequency change and small transition spans (ranges of frequency change). In a single interval direction-identification task, listeners were presented with a unidirectional frequency sweep and asked whether it moved up or down. Sweeps were uniformly varied along the two dimensions of rate and transition span: the rate of frequency change was varied between 0.0147 and 0.1667 octaves/second, and the transition span was varied between 0.1 and 0.5 semitones. All stimuli were at least 50ms in length. As expected, direction sensitivity increased with increasing transition span, and subjects were better at identifying upward sweeps than downward ones. Furthermore, direction sensitivity decreased with faster rates of frequency change. Support for a phaselocking based mechanism of frequency-sweep perception is discussed.