Information Transmission in the Auditory Brainstem Figure 7. Enhancement of vector strength and entrainment in VCN neurons’ discharges relative to ANFs’. Filled triangles in panels A and B denote vector strength values and entrainment indices, respectively, obtained from responses to 60-dB SPL tones at CF of a population (N = 26) of high-sync neurons (i.e., Group 1) in chinchillas. Similar results are in panels C and D for a population (N = 34) of cat high-sync neurons. Black and red continuous lines in panel C represent average vector strength values for ANFs in cats and chinchillas, respectively. Results of the same temporal analysis performed on Group 2 neurons are shown using open triangles in panels A–D (N = 21 in chinchillas; N = 2 in cats). Open circles in A represent rmaxvalues for high-sync and Group 2 neurons (see text). Red dots in panels A and C show rmax data for a population of chinchilla (N = 207) and cat (N = 63) ANFs. Red dots in panel D represent entrainment indices for ANFs. doi:10.1371/journal.pone.0044286.g007 than ANFs. Neurons with the highest THD values in Fig. 1 had the lowest CVs. Results from the population analysis, however, show a more complex relationship between distortion and interspike variability (Figs. 8B and 8D), as measured using the CV. Although THD estimates computed from responses of chinchilla high-sync neurons (filled triangles in Fig. 8B) are higher than those obtained from VCN neurons in Groups 2 and 3 (open triangles in Fig. 8B), THD estimates are independent of their CV values, regardless of the type of VCN neuron. Similarly in cats, THDs for high-sync neurons are larger than those yielded by ANFs (Fig. 8D). CV values computed from the responses of some high-sync neurons, however, resemble those computed from the responses of ANFs. Scatter plots of THD-vs.-r (Figs. 8A and 8C) show a near increasing monotonic relationship, at least for r.0.5. THD computations for lower synchronization values are less reliable due to very low signal-to-noise ratios. Results in Fig. 8 thus demonstrate that distortion in the neural representation by ANFs and VCN neurons is not related to interspike variability. Responses to vowel sounds Neural responses to synthetic and spoken whispered vowels were obtained in chinchillas and cats, respectively. Examples of a temporal analysis performed on two ANFs and two high-sync neurons are shown in Fig. 9. Period histograms (period = 1/f0) in Figs. 9A and 9C were computed from the responses to the vowel / i/ of two ANFs in chinchillas. Vector strength values in each plot PLOS ONE | www.plosone.org 11 September 2012 | Volume 7 | Issue 9 | e44286