Masking by harmonic complexes in birds: behavioral thresholds and cochlear responses Robert J. Dooling a; *, Micheal L. Dent a , Marjorie R. Leek b , Otto Gleich c a Department of Psychology, University of Maryland, College Park, MD 20742, USA b Army Audiology and Speech Center, Walter Reed Army Medical Center, Washington, DC 20307, USA c ENT Department, University of Regensburg, 93042 Regensburg, Germany Received 10 July 2000; accepted 31 October 2000 Abstract Thresholds for pure tones embedded in harmonic complexes were measured behaviorally and physiologically for three species of birds, and physiologically in gerbils. The harmonic maskers were generated using the Schroeder-phase algorithm, characterized by monotonically increasing or decreasing phase across frequency. Previous work has shown that these stimuli produce large differences in masking in humans but not budgerigars. In this study, we show that for two additional species of birds, the patterns of masking were similar to those shown for budgerigars, with masking differing only slightly for the two Schroeder-phase waveforms, and in the opposite direction from that demonstrated in humans. Amounts of masking among species corresponded qualitatively to differences in their critical ratios. Evoked potential measurements in birds and gerbils indicated responses that were consistent with the behaviorally measured thresholds in birds and humans. Results are interpreted in light of differences in frequency selectivity and cochlear temporal processing across species. ß 2001 Elsevier Science B.V. All rights reserved. Key words: Masking ; Bird ; Harmonic complex ; Cochlear microphonics 1. Introduction In human listeners, Smith et al. (1986) reported large di¡erences in masking by harmonic complexes that were identical in long-term amplitude spectra, but var- ied in their phase spectra. Depending on harmonic phases, these complexes may show di¡erences in mask- ing of 20 dB or more (Kohlrausch and Sander, 1995; Carlyon and Datta, 1997a,b; Summers and Leek, 1998). Recently, Leek et al. (2000) reported that these large masking di¡erences in response to maskers with di¡erent phase spectra were not found in behavioral studies in budgerigars (Melopsittacus undulatus, a small Australian parrot). Leek et al. reviewed a number of possible functional and structural di¡erences between the auditory systems of birds and humans that might contribute to this intriguing ¢nding. A major source of these disparate masking results may be the di¡erences observed in patterns of frequency resolution in the avi- an and mammalian inner ears. In the current study, the generality of the budgerigar masking ¢ndings to other bird species was explored, with particular attention to di¡erences in frequency selectivity among the species. In addition, electrophysiological responses to harmonic complexes in several species of birds and one mammal were measured, in order to explore these masking di¡er- ences at the cochlear level. The harmonic maskers used here and in previous masking experiments had component starting phases calculated from an algorithm developed by Schroeder (1970). The negative Schroeder equation produces com- ponent phases monotonically decreasing with increasing harmonic frequency, while the positive Schroeder equa- tion results in phases monotonically increasing with frequency. The positive Schroeder waveform is the time-reversed version of the negative Schroeder wave- form, and within each period of the waveforms, the instantaneous frequency sweeps upward or downward, 0378-5955 / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII:S0378-5955(00)00249-5 * Corresponding author. Tel.: +1 (301) 405-5925; Fax: +1 (301) 314-9566; E-mail: dooling@psyc.umd.edu Hearing Research 152 (2001) 159^172 www.elsevier.com/locate/heares