NEURAL ENCODING OF SINGLE-FORMANT STIMULI IN THE CAT .1. RESPONSES OFAUDITORY-NERVE FIBERS

1. We have studied auditory responses to a set of speech-related narro wband sounds, single-formant stimuli (SFSs), in populations of auditor y nerve fibers (ANFs). An analytic method was developed to extract the envelope of temporal discharge patterns of the ANF responses to nonsi nusoidally modulated stimuli, whose spectra have multiple clusters of components. Such responses are often encountered in the auditory syste m when complex stimuli are used and have traditionally been studied by analyzing the fundamental component of the responses. 2. The envelope modulation in the SFSs is shown to be represented by the response pat terns of ANFs. When the whole ANF population is considered, the inform ation on modulation in stimulus envelope does not disappear at the hig hest sound level tested at all best frequencies (BFs) we studied (1 - 10 kHz). The representation is the best at medium sound levels and deg rades at high sound levels. Low/medium-spontaneous rate (SR) ANFs show ed greater envelope modulation in their responses at high sound levels than do high-SR ANFs. The quality of the representation at high sound levels is, on average, proportional to BF threshold of an ANF. On the basis of populations of ANFs with all SRs, the envelope modulation in the SFSs is represented over a wide range of sound levels. 3. We foun d that low-BF ANFs differ from high-BF ANFs in representing envelope m odulation in the SFSs. For ANFs with BFs less than approximately 6 kHz , information on stimulus envelope is not only contained in spectral c omponents near direct current but also in components at the vicinities of frequencies equal to BF and its multiples. In fact, for ANFs with BFs <3 kHz, the contribution from spectral components centered at BF t o overall response modulation is greater than that from spectral compo nents near direct current. These findings indicate that, by using meas ures solely based on the fundamental component, the amount of modulati on in the responses to narrowband stimuli is underestimated for low-BF ANFs. 4. Off-BF stimulation of ANFs with SFSs was found to result in increased envelope modulation in responses at high sound levels. The f urther away the stimulus is centered relative to unit BF, the greater the modulation it induces, provided that the stimulus is capable of ex citing the unit. An SFS centered as close as 15% off unit BF can produ ce a significant increase in the modulation of responses at very high sound levels. Therefore ANFs whose BFs differ from the center frequenc y of narrowband stimuli provide additional sources of envelope modulat ion at high sound levels. 5. Detailed analysis of response envelope sh owed that low/medium BF ANFs differ systematically from high-SR ANFs i n coding SFSs. Besides having higher modulation, low/medium-SR ANFs we re found to have a higher envelope peak height and a lower envelope mi nimum at high sound levels. In addition, the envelope latency is longe r in low/medium-SR ANFs than in high-SR ANFs. These results have direc t implications for convergence configurations at the cochlear nucleus.