Transformation of temporal discharge patterns in a ventral cochlear nucleus stellate cell model: Implications for physiological mechanisms

1. We have simulated responses of stellate cells in the anteroventral cochlear nucleus (AVCN) to single-formant stimuli (SFSs) with the use of recorded auditory-nerve fiber (ANF) responses as inputs. In particular, two important features of temporal discharge patterns, the phase locking to best frequency (BF) tones and to stimulus envelopes, were examined in the model output. Our earlier experimental studies with SFSs found an enhancement of the envelope modulation in AVCN chopper units, presumably recorded from stellate cells, as compared with that of ANFs. 2. We simulated in the model three mechanisms for the enhancement in envelope modulation proposed earlier by us, namely, convergence of ANFs, temporal summation and inhibitory input. It was found that the convergence of multiple ANFs alone did not always lead to an enhancement in modulation depth, but was necessary for the model to produce other physiologically plausible envelope features: the temporal summation of subthreshold events can lead to an increase in modulation depth; and the somatic inhibition effectively reduced the envelope minimum and, as a result, increased the modulation depth. In addition, we found that, given the same input configuration, the closer the inputs were located to the soma, the greater modulation depth they produced at the model output. 3. Different types of convergence of ANF inputs were tested in our model. It was found that the convergence of both low and high spontaneous rate (SR) ANFs resulted in an enhancement in modulation depth over a wider range of sound level than that due to the convergence of ANFs from the same SR group. However, to achieve a modulation depth higher than that of the low SR ANFs, as seen in real chopper units, a 'weighted summation' with a stronger influence from the low SR ANFs was found to be necessary at high sound levels. Simulations with off-BF inputs indicated that the information on the envelope modulation carried by the ANFs whose BFs are away from the stimulus carrier frequency can be used by a chopper unit to achieve higher modulation depth at high sound levels. 4. The BF phase locking in the model output was examined for various input configurations with the use of phase-locked ANF spike trains as inputs. We found that the temporal summation of subthreshold events can significantly reduce the amount of phase locking in the model output. This provides another mechanism to explain the degraded phase locking in real chopper responses, in addition to the low-pass filtering effect that has been addressed in current literature.