Studies On The Bilateral Collicular Interaction:Modulation Of Auditory Processing In Frequency Domain

The inferior colliculus (IC) occupies a pivotal position in the ascending auditory pathway. It can receive and integrate excitatory and inhibitory inputs from virtually all bilateral lower brainstem auditory nuclei and from auditory cortex (AC) and medial geniculate body (MGB). Additionally, at the same level of auditory pathway, the two ICs in the left and right divisions and neurons within one IC are interconnected to each other by a bundle of fibers called commissure of the inferior colliculus (CoIC) and intrinsic projections, respectively. All these connections make the IC a major center for subcortical temporal and spectral integration of auditory information. In the present study, we investigated the effects of bilateral collicular interaction on modulation of auditory signal processing in frequency domain. The results obtained are summarized as followings:(1) In this study, the responses of123ICMdu neurons to sound stimulation were recorded before and during focal electrical stimulation of ICES.Focal electrical stimulation of ICES neurons decreased the number of impulses (30-100%, average:55±20%) but increased the response latency (0-10ms, average:2.3±2.5ms) of63(51%) inhibited ICMdu neurons. Conversely, focal electrical stimulation of ICES neurons increased the number of impulses (33-200%, average:90-50%) but decreased the response latency (0-5ms, average:1.3±1.5ms) of25(20%) facilitated ICMdu neurons. Focal electrical stimulation of ICES neurons did not appear to modulate the response of the remaining35(29%) unaffected ICMdu neurons.(2) Among the three groups of neurons with different efficient, most (42/63,67%) of inhibited ICMdu neurons had BF difference (between ICMdu neurons and ICES neurons) larger than2kHz, while most(16/25,64%) of facilitated ICMdu neurons had BF difference smaller than2kHz. On the other hand, most (22/35,63%) of unaffected ICMdu neurons had BF difference larger than8kHz. Statistic analysis showed that the BF difference among these three groups of neurons differs significantly (one-way ANOVA, P<0.001).(3) Focal electrical stimulation of ICES neurons sharpened the FTC of inhibited ICMdu neurons and broadened the FTC of facilitated ICMdu neurons. Sharpening or broadening of the FTC of ICMdu neurons is typically asymmetrical indicating varying degree of inhibition or facilitation at each modulated frequency. In addition, focal electrical stimulation of ICES neurons produced a shift of the BF of ICMdu neurons toward that of ICES neurons when the BF difference between ICMdu and ICES neurons is larger than2kHz. Focal electrical stimulation of ICES neurons did not produce a BF shift of the ICMdu neurons when BF difference is smaller than2kHz. The sharpened or broadened FTC and the shifted BF of ICMdu neurons induced by focal electrical stimulation gradually returned to the control FTC measured before ICES focal electrical stimulation over a period of150min.(4) For the BF shift, latency shift, and percent change in Q10value of ICMdu neurons induced by ICES focal electrical stimulation, they all changed regularly with the alteration of BF difference and recording depth difference between ICMdu neurons and ICES neurons. Specifically, BF shift significantly increased while latency shift and percent change in Qio value significantly decrease with BF and depth difference increasing. The results suggests that the modulation of bilateral collicular interaction on frequency processing of ICMdu neurons are dependent on the relationship in frequency tonotopical structure between the two ICs, not on the frequency tonotopical structure of single IC.