Inequality in identification of direction of frequency change (up vs. down) for rapid frequency modulated sweeps

The abilities of human subjects to identify direction (up vs. down) of frequency modulation (FM) of individual tone sweeps at various rates of FM are examined, in particular, how fast FM sweeps can be without impairing a subject's ability to accurately identify them as upward or downward. This ability may be relevant to the auditory encoding of rapid formant transitions, important perceptual cues in speech sounds. In a single-trial 2AFC task, subjects identified randomly presented FM sweeps by pressing one of two labeled keys (up or down). Subjects were significantly better at identifying upward sweeps than downward ones at rapid FM rates (6.2 oct./sec. - 25.0 oct/sec., parameterized as stimulus duration at constant bandwidth).