of the first two spectral prominences (formants F1 or F2), whispered vowels do not provide a compelling pitch percept because there is no f0 (Fig. 12A). Temporal analysis of the neuronal responses was thus performed by estimating SACs, whose waveforms consist of a damping oscillation (Figs. 12C and 12E). The main periodicity in the oscillation was approximately the same as 1/CF (arrows in Figs. 12C and 12E). Dominant component analysis [32] performed on whispered-vowel responses of ANFs and high-sync neurons shows that the largest spectral component in the response occurs at the unit’s CF (Fig. 13C). Certain features of SACs, such as CI and bandwidths of the central peak (Fig. 14), indicate that temporal processing in the responses to vowels by high-sync neurons is enhanced relative to the processing performed by ANFs. Functional relevance Although it is generally thought that the speech envelope is the most important carrier of information, the temporal fine structure (TFS) of speech has been the subject of recent work (e.g., [44]) and might be particularly important for speech recognition in the presence of fluctuating background noise. TFS is encoded in the temporal structure of ANF responses and depends on the amount of phase locking to the stimulus waveform. Because high-sync units enhance the amount of phase locking present in ANFs, the role of high-sync units in encoding TFS might be considerable. In fact, results in Figs. 9–12 show that, in general, synchronization to F1 of vowels is higher for high-sync neurons than for ANFs. The regular timing in the response of high-sync neurons is evident in the spectral representation of the spike train. For example, amplitude References 1. Galambos R, Davis H (1943) The response of single auditory-nerve fibers to acoustic stimulation. J Neurophysiol 6: 39–57. 2. Rose JE, Brugge JF, Anderson DJ, Hind JE (1967) Phase-locked response to low- frequency tones in single auditory nerve fibers of the squirrel monkey. J Neurophysiol 30: 769–793. 3. Frisina RD (2001) Subcortical neural coding mechanisms for auditory temporal processing. Hear Res 158: 1–27. 4. Goldberg JM, Brown PB (1969) Response of binaural neurons of dog superior olivary complex to dichotic tonal stimuli: some physiological mechanisms of sound localization. J Neurophysiol 32: 613–636. 5. Johnson DH (1980) The relationship between spike rate and synchrony in responses of auditory-nerve fibers to single tones. J Acoust Soc Am 68: 1115– 1122. 6. Joris PX, Carney LH, Smith PH, Yin TCT (1994a) Enhancement of neural synchronization in the anteroventral cochlear nucleus. I. Responses to tones at the characteristic frequency. J Neurophysiol 71: 1022–1036. 7. Louage DHG, van der Heijden M, Joris PX (2005) Enhanced temporal response properties of anteroventral cochlear nucleus neurons to broadband noise. J Neurosci 25: 1560–1570. 8. Rhode WS, Smith PH (1985) Characteristics of tone-pip response patterns in relationship to spontaneous rate in cat auditory nerve fibers. Hear Res 12: 159– 168. 9. Temchin AN, Ruggero MA (2010) Phase-locked responses to tones of chinchilla auditory nerve fibers: implications for apical cochlear mechanics. J Assoc Res Otol 11: 297–318. 10. Temchin AN, Recio-Spinoso A, van Dijk P, Ruggero MA (2005) Wiener kernels of chinchilla auditory–nerve fibers: Verification using responses to tones, clicks, and noise and comparisons with basilar-membrane vibrations. J Neurophysiol 93: 3635–3648. 11. Rhode WS, Smith PH (1986a) Encoding timing and intensity in the ventral cochlear nucleus of the cat. J Neurophysiol 56: 261–286. 12. Rhode WS, Smith PH (1986b) Physiological studies of the dorsal cochlear nucleus of the cat. J Neurophysiol 56: 287–307. 13. Blackburn CC, Sachs MB (1989) Classification of unit types in the anteroventral cochlear nucleus: PST histograms and regularity analysis. J Neurophysiol 62: 1303–1329. 14. Bourk TR (1976) Electrical responses of neural units in the anteroventral cochlear nucleus of the cat. Ph.D. dissertation, MIT, Cambridge MA. 15. Lavine RA (1971) Phase-locking in response of single neurons in cochlear nuclear complex of the cat to low-frequency tonal stimuli. J Neurophysiol 34: 467–483. Information Transmission in the Auditory Brainstem of the spectral component at 2*F1 relative to the amplitude at F1 is larger for high-sync neurons than for ANFs. The distorted representation of the stimulus waveform that is provided by high-sync neurons raises the question of the relevance of this type of neuron in the processing of environmental sounds— speech included. Globular and spherical bushy cells, whose responses to tones can be considered as those of high-sync neurons [19,24], send their main projections to binaural brainstem nuclei involved in sound localization [45]. There is, however, anatomical and physiological evidence that suggest other roles for high-sync neurons. Globular bushy cells provide an excitatory input to the medial nucleus of the trapezoid body, which in turn projects to the ventral nucleus of the lateral lemniscus (VNLL), a predominantly monaural center involved in tasks related to temporal processing and pattern recognition of sounds [46,47]. The low variability in FSLs measured in responses to tones of high-sync neurons might provide an indication of the ability of those neurons to detect stimulus changes. Acknowledgments I am indebted to the two anonymous reviewers for all their many useful comments. Thanks to W.S. Rhode and A.N. Temchin for reading drafts of this manuscript. W.S. Rhode participated in some of the experiments with cats, and K.R. Kluender provided the synthetic and spoken vowel samples used in this research. Author Contributions Conceived and designed the experiments: AR-S. Performed the experi- ments: AR-S. Analyzed the data: AR-S. Wrote the paper: AR-S. 16. Winter IM, Palmer AR (1990) Responses of single units in the anteroventral cochlear nucleus of the guinea pig. Hear Res 44: 161–178. 17. Carney LH (1990) Sensitivities of cells in anteroventral cochlear nucleus of cat to spatiotemporal discharge patterns across primary afferents. J Neurophysiol 64: 437–456. 18. Paolini AG, FitzGerald JV, Burkitt AN, Clark GM (2001) Temporal processing from the auditory nerve to the medial nucleus of the trapezoid body in the rat. Hear Res 159: 101–116. 19. Joris PX, Smith PH (2008) The volley theory and the spherical cell puzzle. Neuroscience 154: 65–76. 20. Stein RB, Gossen ER, Jones KE (2005) Neuronal variability: noise or part of the signal? Nat Rev Neurosci 6: 389–397. 21. Knox CK (1970) Signal transmission in random spike trains with applications to the statocyst neurons of the lobster. Kybernetik 7: 167–174. 22. Recio A, Rhode WS (2000) Representation of vowel stimuli in the ventral cochlear nucleus of the chinchilla. Hear Res 146: 167–184. 23. Rhode WS (1999) Vertical cell responses to sound in cat dorsal cochlear nucleus. J Neurophysiol 82: 1019–1032. 24. Rhode WS (2008) Response patterns to sound associated with labeled globular/ bushy cells in cat. Neuroscience 154: 87–98. 25. Mardia KV, Jupp PE (2000) Directional statistics. Chichester: Wiley. 26. Rhode WS (1998) Neural encoding of single-formant stimuli in the ventral cochlear nucleus of the chinchilla. Hear Res 117: 39–56. 27. Joris PX, Louage DH, Cardoen L, van der Hejden M (2006). Correlation index: A new metric to quantify temporal coding. Hear Res 216–217: 19–30. 28. Joris PX, Smith PH, Yin TCT (1994b) Enhancement of neural synchronization in the anteroventral cochlear nucleus. II. Responses in the tuning curve tail. J Neurophysiol 71: 1037–1051. 29. Stein RB (1970) The role of spike trains in transmitting and distorting sensory signals. In: Schmitt FO, editor. The neurosciences: second study program. New York: Rockefeller University Press. pp. 597–604. 30. Blackburn CC, Sachs MB (1990) The representations of the steady-state vowel sound /e/ in the discharge patterns of cat anteroventral cochlear nucleus neurons. J Neurophysiol 63: 1191–1212. 31. Young ED, Sachs MB (1979) Representation of steady-state vowels in the temporal aspects in the discharge patterns of populations of auditory-nerve fibers. J Acoust Soc Am 66: 1381–1403. 32. Sinex DG, Geisler CD (1983) Responses of auditory-nerve fibers to consonant- vowel syllables. J Acoust Soc Am 73: 602–615. 33. Kiang N Y-S, Watanabe T, Thomas EC, Clark LF (1965) Discharge patterns of single fibers in the cat’s auditory nerve. MIT Press, Cambridge MA. PLOS ONE | www.plosone.org 20 September 2012 | Volume 7 | Issue 9 | e44286