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 particu lar, two important features of temporal discharge patterns, the phase locking to best frequency (BF) tones and to stimulus envelopes, were e xamined in the model output. Our earlier experimental studies with SFS s found an enhancement of the envelope modulation in AVCN chopper unit s, presumably recorded from stellate cells, as compared with that of A NFs. 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 enhanceme nt in modulation depth, but was necessary for the model to produce oth er physiologically plausible envelope features; the temporal summation of subthreshold events can lead to an increase in modulation depth; a nd the somatic inhibition effectively reduced the envelope minimum and , as a result, increased the modulation depth. In addition, we found t hat, given the same input configuration,, the closer the inputs were l ocated to the soma, the greater modulation depth they produced at the model output. 3. Different types of convergence of ANF inputs were tes ted in our model. It was found that the convergence of both low and hi gh spontaneous rate (SR) ANFs resulted in an enhancement in modulation depth over a wider range of sound level than that due to the converge nce of ANFs from the same SR group. However, to achieve a modulation d epth higher than that of the low SR ANFs, as seen in real chopper unit s, 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 modulatio n carried by the ANFs whose BFs are away from the stimulus carrier fre quency can be used by a chopper unit to achieve higher modulation dept h at high sound levels. 4. The BF phase locking in the model output wa s examined for various input configurations with the use of phase-lock ed ANF spike trains as inputs. We found that the temporal summation of subthreshold events can significantly reduce the amount of phase lock ing in the model output. This provides another mechanism to explain th e degraded phase locking in real chopper responses, in addition to the low-pass filtering effect that has been addressed in current literatu re.