Abstract
A subset of neurons in the cochlear nucleus (CN) of the auditory brainstem has the ability to enhance the auditory nerve's temporal representation of stimulating sounds. These neurons reside in the ventral region of the CN (VCN) and are usually known as highly synchronized, or high-sync, neurons. Most published reports about the existence and properties of high-sync neurons are based on recordings performed on a VCN output tract—not the VCN itself—of cats. In other species, comprehensive studies detailing the properties of high-sync neurons, or even acknowledging their existence, are missing.Examination of the responses of a population of VCN neurons in chinchillas revealed that a subset of those neurons have temporal properties similar to high-sync neurons in the cat. Phase locking and entrainment—the ability of a neuron to fire action potentials at a certain stimulus phase and at almost every stimulus period, respectively—have similar maximum values in cats and chinchillas. Ranges of characteristic frequencies for high-sync neurons in chinchillas and cats extend up to 600 and 1000 Hz, respectively. Enhancement of temporal processing relative to auditory nerve fibers (ANFs), which has been shown previously in cats using tonal and white-noise stimuli, is also demonstrated here in the responses of VCN neurons to synthetic and spoken vowel sounds.Along with the large amount of phase locking displayed by some VCN neurons there occurs a deterioration in the spectral representation of the stimuli (tones or vowels). High-sync neurons exhibit a greater distortion in their responses to tones or vowels than do other types of VCN neurons and auditory nerve fibers.Standard deviations of first-spike latency measured in responses of high-sync neurons are lower than similar values measured in ANFs' responses. This might indicate a role of high-sync neurons in other tasks beyond sound localization.
Highlights
In response to low-frequency sinusoidal sounds, auditory nerve fibers (ANFs) generate trains of action potentials, or spikes, that are synchronized to the stimulating waveform—a property usually known as phase locking [1,2]
Total harmonic distortion (THD) estimates computed from responses of chinchilla high-sync neurons are higher than those obtained from ventral portion of the CN (VCN) neurons in Groups 2 and 3, THD estimates are independent of their coefficient of variation (CV) values, regardless of the type of VCN neuron
CV values computed from the responses of some high-sync neurons, resemble those computed from the responses of ANFs
Summary
In response to low-frequency sinusoidal sounds, auditory nerve fibers (ANFs) generate trains of action potentials, or spikes, that are synchronized to the stimulating waveform—a property usually known as phase locking [1,2]. A discrepancy exists, among published studies regarding synchronization limits of VCN neurons with CFs below approximately 1 kHz. Some studies have found that phase-locking strength exhibited by VCN neurons deteriorates or remains constant relative to ANFs [11,13,14,15,16]. The work of others [6,7,17,18], has shown an enhancement in phase locking relative to the auditory nerve (AN), at least for CFs below 1 kHz
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