values computed from responses to CF tones are ,0.9. Vector strength values (i.e., mean plus one standard deviation) computed from responses of spherical bushy cells in gerbil (Fig. 5 in [35]) are lower than those shown here for high-sync neurons (Figs. 7A and 7C). An apparent small percentage of individual r values in another recent study in the gerbil (Fig. 8 in [36]) are above 0.9. It is not clear from the two aforementioned studies, however, what the r values are in responses to CF tones. Nevertheless, works by Dehmel et al. [35], Kuenzel et al. [36] and Paolini et al. [18] demonstrate an enhancement in temporal processing by VCN cells relative to ANFs, in spite of their apparent lower vector strength values. Nonmonotonic RIFs have been found in AVCN units in the gerbil [37], presumably due to the overlapping of postsynaptic inhibitory areas with excitatory ones. The proportion of non- monotonic RIFs in chinchillas is smaller than (30%) but comparable to that found in gerbils (almost 50%). Results from another gerbil study [38] showed only monotonic RIFs, probably because recordings in [38] were performed in the caudal end of the AVCN and those in [37] originated from the rostral end of the AVCN. All the RIFs measured in high-sync units in cats were monotonic, yet inhibitory sidebands were present in some units (e.g., Fig. 4E). Morphology of highly synchronized neurons Labeling experiments [6,24] have indicated that globular and spherical bushy cells show high neural synchronization and entrainment values. PSTHs and receptive fields of high-sync neurons shown in this work resemble low-CF neurons described by Rhode and Smith [11]. Those neurons, which were named ‘‘onset units with low CFs’’ (Olf) by Rhode and Smith, have high r values (in the 0.8–0.99 range) and can entrain to stimulus frequencies up to around 1 kHz. The authors thought of those neurons as representing ‘‘the low end of the frequency spectrum of onset cells.’’ Octopus cells in cats, whose responses to sound display an onset pattern, are located in the posterior end of the ventral cochlear nucleus [11,39]. It appears that low-CF onset units and high-sync neurons, which are thought to be bushy cells, share many characteristics in their responses to tones and broadband stimulation. Although marking of the recording site was not done in this study, placement of the electrode was documented, and based on histological processing in some animals it was judged that recordings originated from either the posterior end of the AVCN or the nerve root area. Because globular bushy cells in the chinchilla are located in the anterior end of the PVCN and the posterior end of the AVCN [40], VCN recordings in this work likely originate from globular bushy cells. Recordings from the rostral end of the AVCN have yielded units with strong phase- locking in their responses to CF tones, but their vector strength values were not published [41]. Possible mechanisms of synchronization enhancement Results of computer simulations have shown that a ‘‘coincidence detection’’ mechanism is capable of enhancing synchrony and entrainment [19]. A postsynaptic neuron implementing this type of mechanism generates an action potential if a certain number of inputs arrive at the same time or within a given time window of very short duration. A relatively large number of inputs is needed for coincidence detection [19], which makes it viable as a model for globular bushy cells. Such neurons receive a large amount of ANF inputs. Information Transmission in the Auditory Brainstem Spherical bushy cells, which receive inputs from only a few auditory nerve fibers, also show enhanced synchronization over ANFs, at least in cats [19]. Simulating the synchronization enhancement properties of spherical bushy cells using coincidence detection has been proven difficult [19]. Distortion in the representation of the stimulus waveform by high-sync neurons The representation of the stimulus waveform by a train of action potentials is usually distorted [21]. There are many ways to provide a quantitative measure of the distortion in the represen- tation of the stimulus waveform. When the stimulating waveform is periodic, for example, the distortion can be visualized using period histograms, such as those that appear in Fig. 1. The process of fitting a sinusoidal function to a given period histogram and then estimating the error in the fit would yield a measure of the distortion. In this manuscript Eq. 1 was used to quantify harmonic distortion. Figures 8,10, 11 and 14 display THD values for ANFs and VCN neurons. Although all units encode the stimulating waveform with relatively large amounts of distortion, high-sync neurons exhibit the largest amounts, thus agreeing with a previously published assertion [19]. Previous studies of the transmission of information by spike train carriers have linked interspike variability to high THD [20,21]. For example, in response to a sinusoidal excitation, it has been argued [21] that Poisson processes carry information about the stimulus waveform with less harmonic distortion than ‘‘regular’’ point process, i.e., process with less variable interspike interval times. Given that ANF discharges can be modeled reasonably well using modified Poisson processes [42], it was not surprising that THD values obtained from responses of high-sync neurons were larger than those estimated from ANF responses or other types of VCN neurons (Fig. 8). The relationship between THD and interspike variability, however, is a complex one. CVs of Group 2 neurons overlap those obtained from responses of high-sync neurons (Fig. 8B), yet THD values for the latter type of neurons are larger than for the former. There is also a considerable amount of overlap between the CVs obtained from the responses of ANFs and high-sync neurons in the cat (Fig. 8D), yet THDs measured in high-sync neurons are always the largest. In general, distortion in the representation of the stimulus waveform in the neural responses presented in this work depends on the amount of synchronization to the stimulus frequency (Figs. 8A and 8C), rather than on interspike variability. As with the entrainment index and vector strength [6], THD and vector strength are different measures of the timing of the response. (THD computations are based on the results of a Fourier transformation of an autocorrelation function. Two neural point processes may have the same vector strength but different autocorrelation functions.) Responses to vowel stimuli Synchronization to either f0 or F1 is higher, on average, for high-sync neurons than for ANFs or other types of VCN neurons (Figs. 9 and 10). Temporal enhancement by high-sync neurons is thus also shown in their responses to speech-like stimuli. Period histograms based on responses of high-sync neurons are narrower than the peaks in the period histograms obtained from responses of other types of VCN neurons. This translates into a poorer representation of the stimulus waveform. Responses of high-sync neurons and ANFs to the whispered /i/ vowel were measured in the cat. Although it has been suggested [43] that perceived pitch of whispered vowels is determined by one PLOS ONE | www.plosone.org 19 September 2012 | Volume 7 | Issue 9 | e44286