Abstract Pitch is a fundamental perceptual dimension of natural sounds, and pitch perception is crucial for understanding speech and music and for segregating concurrent sounds. Pitch perception degrades as component frequencies in a sound increase beyond 2-3 kHz, a trend that is usually assumed to reflect a corresponding loss of phase locking to temporal fine structure (TFS) in the auditory nerve above 2-3 kHz. However, recent psychophysical and physiological findings, including evidence that accurate pitch perception is possible at high frequencies and that phase locking may degrade in humans at lower frequencies than previously believed, have cast doubt on this link. Aim 1 of the proposed research tests the hypothesis that a general deficit in across-frequency comparison or integration of information, instead of the roll-off of phase locking, may explain why pitch perception degrades as frequency increases. To this end, performance will be measured in tasks that do not depend on across-frequency comparisons, where this hypothesis would predict equivalent performance at low and high frequencies, and in tasks that are designed to require across- frequency comparisons, where this hypothesis would predict poorer performance at high frequencies than at low frequencies. Aim 2 of the proposed research tests the hypothesis that this deficit in across-frequency comparison might also predict that segregation of concurrent sounds should be impaired at high frequencies, even when the segregation cues are not encoded via TFS information. To this end, we will measure the extent to which amplitude modulation and onset asynchrony cues promote segregation of single or multiple target components from within a complex tone. Here, our hypothesis predicts that performance should be poorer at high frequencies, especially when compared to matched control tasks that require no segregation. The behavioral results, which will be interpreted with the aid of ideal observer analysis of simulated auditory nerve responses, will provide valuable insight into the neural mechanisms that underlie complex pitch perception and concurrent sound segregation.