Bilateral Collicular Interaction:Modulation Of Auditory Signal Processing In Amplitude Domain

In the ascending auditory pathway, the inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from many lower auditory nuclei, intrinsic projections within the IC, contralateral IC through the commisure of the IC and from the auditory cortex. All these connections make the IC a major center for subcortical temporal and spectral integration of auditory information. In this study, we examined bilateral collicular interaction in modulating amplitude-domain signal processing using electrophysiological recording, focal electrical stimulation and ionophoresis. The responses of IC neurons (abbreviated as ICMdu neurons) of mice(Mus musculus, Km) were recorded before, during and after the electrical stimulating of contralateral IC neurons (abbreviated as ICES neurons). The possible biological significance and neural pathways underlying the bilateral collicular interaction were discussed.1. Fifty ICMdu neurons were classified as two groups according to the best frequency (BF) difference between ICMdu and ICES neuron. For simplicity,26ICMdu neurons were called bilateral corresponding frequency lamina (BCFL) neurons only if the BF difference was within2kHz. Otherwise24ICMdu neurons were called bilateral non-corresponding frequency lamina (BNCFL) neurons. During focally electrical stimulation of the contralateral IC neurons,14.0%IC neurons were facilitated and located in BCFL while86.0%IC neurons were inhibited and widespreadly located in BCFL (38.0%) and BNCFL (48.0%). The facilitatory modulation of bilateral collicular interaction increased the number of impulses and dynamic ranges (DRs) of facilitated IC neurons but decreased the slope of their rate-amplitude functions (RAFs) for wider amplitude responses to sound stimulus. Otherwise, the inhibitory modulation of bilateral collicular interaction decreased the number of impulses and DR of inhibited IC neurons but increased the slopes of their RAFs for sharper sensitivity to sound amplitude. Moreover the degree of inhibitory modulation was greater in BCFL neurons than in BNCFL neurons.2. Seventy ICMdu neurons were studied for bilateral collicular interaction. Focal electrical stimulation of ICES produces widespread inhibition (87.1%) and focused facilitation (12.9%) of responses of contralateral ICMdu neurons. This bilateral collicular interaction produces a decrease in the response magnitude and an increase in the response latency of inhibited ICmdu neurons but produces an opposite effect in the response of facilitated ICMdu neurons. This modulation effect is most effective at low sound amplitude and is dependent upon the time interval between the acoustic and electric stimuli. At the optimal inter-stimulus interval, the inhibited ICMdu neurons have the smallest response magnitude and the longest response latency during focal electrical stimulation while the opposite effects are observed for the facilitated ICMdu neurons. The focal electrical stimulation of ICES compresses or expands the RAF of contralateral ICMdu neurons to improve the sensitivity in the change or range of amplitude. This bilateral collicular interaction is found to be either reciprocal or unilateral by alternatively electical stimulating paires neurons in both IC. The focal electrical stimulation also produces a shift in a neuron’s minimum threshold and dynamic range for as long as150minutes. The degree of bilateral collicular interaction is dependent upon the difference in BF between the electrically stimulated IC neurons and the affected IC neurons. The results suggest that bilateral collicular interaction is mainly to change the ratio between excitation and inhibition during signal processing so as to sharpen the amplitude sensitivity of IC neurons. Respective ionophoretic application of bicuculline and kynurenic acid to inhibited and facilitated neurons shows that bilateral collicular interaction induced by focal electrical stimulation can be eliminated or only partly abolished.The role of bilateral collicular interaction in amplitude-domain signal processing is to mudulate the amplitude signal processing by sharpening the amplitude sensitivity through wide spread inhibition and by enhancing response of facilitated IC neurons for better amplitude responsiveness to tuned sound stimulus through the more focused point-to-point connections. The small proportion of bilateral collicular facilitatory interaction between neurons in BCFL observed in our study may be involved in the formation of binaural property of EE (excitation-excitation) neurons. Conversly, the large proportion of bilateral collicular inhibitory interaction between the neurons in BCFL and BNCFL can perhaps match a high proportion of the El (excitation-inhibition) neurons and may be involved in the formation of binaural property of El neurons. The unbalanced property between excitatory and inhibitory projections have very important role in the formation of unilateral auditory dominance and sound location. The bilateral collicular interaction modulates the amplitude signal processing of IC neuron on the basis of topographic projections between two ICs, and it may be also invovled in acoustic-experience-dependent plasticity in the IC. The bilateral collicular interaction which was mediated by GABAergic and glutamatergic receptors provide an adjustable and plastic modulation pattern for auditory signal processing of ICs.