PRESERVATION OF AMPLITUDE-MODULATION CODING IN THE PRESENCE OF BACKGROUND-NOISE BY CHINCHILLA AUDITORY-NERVE FIBERS

Sound envelope temporal fluctuations are important for effective proce ssing of biologically relevant acoustic information including speech, animal vocalizations, sound-source location, and pitch. Amplitude modu lation (AM) of sound envelopes can be encoded in quiet with high fidel ity by many auditory neurons including those of the auditory nerve (AN ) and cochlear nucleus. From both neurophysiological and clinical pers pectives, it is critical to understand the effects of background maski ng noise on the processing of AM. To further this goal, single-unit re cordings were made from AN fibers in anesthetized chinchillas. Units w ere classified according to spontaneous firing rate (SR) and threshold . Best frequency (BF) pure-tone bursts and AM (10-500 Hz) tone bursts were employed as stimuli at several sound levels, both in quiet and in the presence of a continuous wideband noise. It was found that (1) in quiet, low SR AN fibers show the strongest AM coding, followed in ord er by medium SR and high SR fibers, respectively. (2) AN units of all three classes generally preserve their AM coding even in the presence of loud (O or +6 dB S/N) background noise and at high sound levels (ov er 75 dB SPL). (3) This preservation is usually achieved by lowering t he average firing rate proportionately to decreases in the synchronous (fundamental frequency) response. (4) For a few AN fibers, the AM cod ing increases or is reduced in the presence of the background noise. T hese findings suggest that AN preservation of AM coding in the presenc e of a continuous masking noise results from shifts in the operating r anges and firing rates of AN fibers resulting from cochlear nonlineari ties and adaptive mechanisms. (C) 1996 Acoustical Society of America.