Responses to dichotic broadband noise stimulation in chinchilla ventral cochlear nucleus
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The ventral cochlear nucleus (VCN) is traditionally thought of as part of a monaural nucleus that serves as a relay between the auditory nerve fibers (ANFs) and the higher auditory centers. Recently, emerging evidence substantiates the existence of binaural interaction at this level (Babalian et al., 1999 & 2001; Needham and Paolini, 2003 & 2006; Paolini et al., 2004; Shore et al., 2003; Sumner et al., 2005). To investigate the role of the VCN in binaural processing, the present study examined extracellular responses elicited by dichotic broadband noise stimulation recorded with silicon-substrate multichannel probes. The results revealed that the majority of units had a reduction in their ipsilaterally-driven activities as interaural level difference (ILD; contra re: ipsi intensity) increased, while a small percentage (< 5%) showed an increase in firing. When the stimulus onset of the contralateral signal was manipulated relative to that of the ipsilateral signal in the ILD paradigm, the inhibitory effect was the strongest when the two signals coincided in time (contra - ipsi onset < 10-ms). Surprisingly, a significant reduction in spike count re: ipsilateral-stimulation-only baseline was also found when the onset of the ipsilateral signal began 10-ms after the offset of the contralateral signal (i.e., a 10-ms silent gap between the two signals). The finding suggests that contralateral influence may outlast the duration of stimulation, and that the VCN has the ability to continuously monitor preceding as well as ongoing bilateral activities. In general, the overall results of the present study were compatible with in vivo guinea pig data that showed a significant reduction in spontaneous rate to contralateral stimulation (Shore et al., 2003). The temporal effect was in the scale of milliseconds (ms), which may suggest that the VCN is less sensitive compared to the binaural responses of superior olivary complex (SOC). Based on the time course of the strongest contralateral effect observed, the two most likely sources involved are the commissural fibers between the two CN and the collaterals of the MOC fibers. Combined with other DCN literature (e.g., Joris and Smith, 1998; Davis, 2005), the study further supports CN as the first central binaural processing nucleus in the auditory pathway.