NEURAL ENCODING OF SINGLE-FORMANT STIMULI IN THE CAT .2. RESPONSES OFANTEROVENTRAL COCHLEAR NUCLEUS UNITS

1. We have studied responses of anteroventral cochlear nucleus (AVCN) units to single-formant stimuli (SFS), in an effort to make quantitati ve comparisons with responses observed in auditory-nerve fibers (ANFs) to the same stimuli (Wang and Sachs 1993) and to reveal some of the s ignal processing mechanisms at the AVCN. Single-unit recordings and su bsequent analyses were performed on each type of commonly recorded uni ts, namely primarylike (Pri), primarylike with notch (PN), sustained c hopper (ChS), transient chopper (ChT), and onset chopper (OnC), as wel l as a few onset (On) units, from the AVCN in anesthetized cats. The r esponses were obtained at a wide range of sound levels and at a freque ncy range of 1-10 kHz. Modulation in the envelopes of discharge patter ns was quantified by a measure called modulation depth. 2. At moderate to high sound levels, most AVCN units were found to have enhanced mod ulation depth compared with that of ANFs, although the degree of enhan cement varies among different types. All AVCN units, except Pri type, showed an enhancement in modulation depth over that of the highest of ANFs at moderate to high sound levels in the order of(from the highest to the lowest) On, OnC, ChT/PN, and ChS. Specifically, 1) modulation depth in Pri units was comparable to that of high spontaneous rate (SR ) ANFs at low sound levels and to that of low/medium SR ANFs at high s ound levels (in dB SPL). When sound level was normalized by unit thres hold, Pri units, on average, exhibited only limited enhancement in env elope modulation at high sound levels (>80 dB re threshold); 2) PN uni ts showed substantially enhanced modulation depth over that of all SR groups of ANFs at moderate to high sound levels in dB SPL or dB re thr eshold scales; 3) significant enhancement in modulation depth was seen in both ChS and ChT units, with a slightly higher modulation depth in ChT type across sound levels (in dB SPL or dB re threshold); 4) modul ation depth of OnC units was higher than those of primarylike (Pri and PN) and chopper (ChS and ChT) units at a wide range of sound levels; 5) responses from a limited sample of On units showed the highest modu lation depth among all types of AVCN units. 3. Detailed analysis revea led that the enhanced modulation depth in the responses of AVCN units is the result of increased envelope peak height and decreased envelope minimum, relative to those of ANFs. In particular, 1) PN units achiev ed this enhanced modulation depth mainly by a reduction in the envelop e minimum and a small increase in the envelope peak height; 2)the enha ncement in ChS and ChT units resulted from decreased envelope minimum and substantially increased envelope peak height; 3) the greatly reduc ed envelope minimum in OnC units is the crucial factor in the observed enhancement. Some of OnC units also showed significantly increased en velope peak height; 4) high modulation depth in On units was caused by an extremely low envelope minimum. Data presented in this study also showed that AVCN units can have different average discharge rates and yet still exhibit enhanced modulation depth. 4. The envelope latency o f responses to SFS in AVCN units exhibited properties similar to the f irst spike latency determined by tonal stimuli. ChS/ChT units were fou nd to have longer latency than Pri/PN units do at all frequencies stud ied. For both Pri/PN and ChS/ChT units, envelope latency decreased as sound level increased. However, there was a greater reduction in laten cy from low to high sound level in ChS/ChT units than in Pri/PN units. 5. The results of the present study suggest convergence of multiple A NFs on an AVCN cell as a mechanism for the observed increment in envel ope peak height. The effect of convergence is stronger at high sound l evels if inputs from low/medium SR or off-best frequency (BF) ANFs are present. Two mechanisms are suggested to explain the reduction in env elope minimum: 1) a threshold effect due to temporal integration of su bthreshold excitatory postsynaptic potentials (EPSPs) and 2) somatic i nhibition that changes the discharge threshold in an AVCN cell. The ab ove three mechanisms are proposed as potential candidates responsible for the observed modulation enhancement.