representation of the vowel /i/ is also larger for high-sync neurons than for ANFs (Fig. 14C). Discussion This report documents for the first time the existence of highly synchronized neurons in the VCN of the chinchilla as well as the responses of high-sync neurons to vowel sounds in chinchillas and cats. The use of high-impedance glass pipettes in this research to record extracellular action potentials from the VCN introduced difficulties in distinguishing between responses of ANFs and VCN neurons, but at the same time made it possible to record from high-sync neurons without electrical contamination [6]. The use of metal electrodes would avoid ANF recordings [33], but that type of electrode can pick up unwanted local-field potentials when recording from units with high amounts of phase locking. Responses of highly synchronized neurons to CF tones exhibit vector strength values above 0.9, which represents an improve- ment over the synchronization limits of ANFs (Fig. 7). Preliminary results in the encoding of synthetic vowels in the temporal structure of the responses of high-sync neurons also indicate an enhancement relative to the AN’s temporal processing (Fig. 9). An enhancement of synchrony was also evident in the responses to tones and whispered vowels of high-sync neurons in the cat. Responses of high-sync neurons and ANFs to the whispered /i/ value show differences in the temporal structures of their responses. Shuffled autocorrelograms constructed from the responses of high-sync neurons exhibit a better temporal precision than SACs of ANFs. FSLs computed in the responses to tones in cat high-sync neurons are smaller than FSLs in the responses of ANFs. Temporal properties of highly synchronized neurons in chinchilla and cat Responses to CF tones of high-sync VCN neurons in the chinchilla consist of trains of action potentials that are well synchronized to one phase of the stimulating waveform (Figs. 1 and 2). For CFs,500–600 Hz, r values computed from responses to CF tones can be above 0.9 (Fig. 7A). By contrast, ANF responses to CF tones in that frequency range are always below 0.9 (Fig. 7A, [9],[10]). These results indicate that highly synchronized neurons do exist in the chinchilla and provide an enhancement in the temporal representation provided by ANFs. (Cat ANF responses to single tones can also be highly synchronized to the stimulus frequency [i.e., r.0.9] provided that the stimulus frequency is well below the unit’s CF [24,28].) Plots of vector strength as a function of CF for highly synchronized neurons in the chinchilla display a low-pass characteristic with a cutoff frequency of around 500–600 Hz (Fig. 7A). This is lower than in the cat (Fig. 7C), where CFs of high-sync neurons can be as high as 1000 Hz. Responses of ANFs in cats and chinchillas evoke similar rmax values for CF,600 Hz (inset in Fig. 7A [9]). For CFs.600 Hz, however, r values in cats are larger than in chinchillas. Therefore, the reduced CF range of high-sync neurons in chinchillas could be a consequence of temporal processing limitations in the auditory nerve of the chinchilla. Some high-sync VCN neurons fire one action potential at almost every stimulus period (Figs. 1B, 3A and 4A). In other words, the responses of those neurons are entrained to the stimulus frequency. Entrainment has been shown previously by Joris et al. [6] and Rhode [24] in the responses of cat VCN neurons, but never before in a rodent. Entrainment to CF tones occurs in the chinchilla up to CF,600 Hz (Fig. 7B). Information Transmission in the Auditory Brainstem Standard deviation of FSLs measured from responses to CF tones of high-sync neurons are below 1–2 ms (Fig. 6E) and also below the sFSL measured in most ANF responses. This decrease in the variability of the time of occurrence of the first spike in response to a tone represents another type of temporal enhance- ment of ANF responses. Responses to CF tones of high-sync neurons in the cat yield rmax values that are larger than those measured in ANFs (Fig. 7B). This enhancement has been found by some authors [6,7,17] but not others [11,13–16]. Considering the large amount of data collected by those authors using the same laboratory species, it is difficult to explain the differences in their findings. It is possible, however, that previous studies [11,13–16] did not target the VCN region(s) in which high-sync neurons seem to be located. Spherical and globular bushy cells, which are capable of high synchronization and entrainment [6,19,24], are found mostly in the nerve root area and in the rostral end of the AVCN, respectively [34]. In addition, because of phase-locked local field potentials, the common use of metal electrodes to record from the VCN might have prevented the recordings of high-sync neurons in previous studies. Joris and colleagues [6,7,19] recorded from the ventral acoustic stria, one of the three CN output tracts, which allowed for the sampling of AVCN neurons, especially from bushy cells. The proportion of chinchilla high-sync neurons (26%, 26/99) is much lower than the percentage (75%) found in cats [6] for the same CF range. One possible explanation for this discrepancy is that whereas recordings performed by Joris [6] originated mostly from bushy cells, in the present study no particular VCN region was targeted and thus fewer bushy cells were sampled. Discharge rate properties of highly synchronized neurons High levels of neural synchrony and entrainment in high-sync neurons affect their firing-rate profiles. Receptive fields in Figs. 2E, 2F, 3E, 3F, 4E and 4F contain maximum firing rates similar to the stimulating frequency. (For example, the maximum firing rate in Fig. 3F is approximately 350 spikes/s, which corresponds to the stimulus frequency value.) Depending on the neuron’s CF, the frequency that evokes such high firing rates could be either below or above CF. Receptive fields of neurons with a certain level of spontaneous activity might exhibit firing rates that are below the spontaneous activity of the neuron. One consequence of the firing rate reduction is that receptive fields of high-sync units are narrower than those of ANFs, especially at medium to high intensity levels. This decrease in firing rate in response to a single tone is probably due to inhibitory mechanisms occurring at off-CF frequencies. It is likely that inhibitory process also occur at CF frequencies, at least in chinchilla, since approximately 30% of units classified as Group 1 or 2 in chinchillas have RIFs that were considered nonmono- tonic. Shifts in the center of gravity of receptive fields in ANFs of the chinchilla have been shown before [9]. Changes in firing rate (DRmax) are much larger in the VCN than in the auditory nerve (Fig. 1). Such differences in firing rate probably stem from entrainment properties of high-sync neurons as well as inhibitory mechanisms described in the previous paragraph. Profiles of shifts in the center of gravity as a function of CF shown in receptive fields of ANFs indicate a zero shift at around 1 kHz [9], which is different from the equivalent value for VCN neurons (around 500– 600 Hz in Fig. 6A). Highly synchronized neurons in other rodent species Previous studies of temporal processing by AVCN neurons in the guinea pig [16] and rat [18] indicate that vector strength PLOS ONE | www.plosone.org 18 September 2012 | Volume 7 | Issue 9 | e44286