Information Transmission in the Auditory Brainstem Figure 8. Distortion as a function of synchrony and interspike interval variability. Black filled (N = 26 and 34 in chinchillas and cats, respectively) and open ‘‘up’’ triangles (N = 16 and 2 in chinchillas and cats, respectively) represent data from high-sync and Group 2 VCN neurons, respectively. ‘‘Down’’ triangles represent data from Group 3 VCN neurons (N = 18). Red dots show cat ANF data (N = 59). Data from chinchillas are in panels A and B, and from cats in panels C and D. doi:10.1371/journal.pone.0044286.g008 represent the strength of phase locking to the frequency of F1 ( = 300 Hz). Individual peaks in period histograms of high-sync neurons’ responses (Figs. 9B and 9D) are narrower than peaks in histograms of ANFs (Fig. 9A and 9D). r values computed from the responses of the two high-sync neurons are higher than the values computed from ANF responses. The two aforementioned prop- erties of high-sync neurons are consequences of their outstanding temporal resolution. Figures 9E and 9F exhibit normalized SACs obtained from ANFs’ (red dashed lines) and high-sync neurons’ responses (thin lines). (SACs in Fig. 9E were obtained from the same data used to compute the period histograms in Figs. 9A and 9B; similarly, Fig. 9F was computed from data used to construct histograms in Figs. 9C and 9D.) SACs depicted in Figs. 9E and 9F consist of a 300-Hz oscillation (i.e., three periods in 10 ms). In other words, there is a representation of the frequency of F1 in the responses of ANFs and high-sync neurons in Fig. 9. With the exception of the peak at the origin, the largest peak in the SACs occurs at a time lag corresponding to 10 ms, i.e., the period of f0. The largest peak in a SAC always occurs at the origin and its amplitude is referred to as the correlation index [7]. Uncorrelated spikes yield a unit value in a normalized SAC. Individual peaks in SACs computed from high-sync neurons’ responses were narrower than those obtained from ANFs. Bandwidths of the central peaks, evaluated at half of their maximum value, for the high-sync SACs in Figs. 9E and 9F were 0.25 and 0.67 ms. Bandwidths of ANFs in Figs. 9E and 9F were 0.92 and 1.03 ms. Figure 10 contrasts the neural representations of the vowel /e/ in the responses of high-sync and Group 3 VCN neurons. Period histograms derived from responses to /e/ of three high-sync neurons are shown in Figs. 10A–10C. The duration of each period histogram, 10 ms, equals the period of f0. The frequency of F1 ( = 500 Hz) is similar to the neurons’ CF, which is listed on top of each figure along with THD and r values. (THDs were computed using the spectral amplitudes at F1, 2*F1, etc.). Each histogram consists of five peaks, which indicates a tendency of those neurons to follow the frequency of F1 ( = 5*f0). Fourier analysis of the period histograms in Figs. 10D–10F shows a predominant spectral component at the frequency of F1, which is marked with a star. There are also spectral peaks at frequencies corresponding to the harmonics of F1 (rectifier distortion products) [30,31]. Period histograms obtained from responses of three VCN neurons (Group 3) to /e/ are shown in Figs. 10G–10I and their respective Fourier transform amplitudes are shown in Figs. 10J– PLOS ONE | www.plosone.org 12 September 2012 | Volume 7 | Issue 9 | e44286