Neural interactions in unilateral colliculus and between bilateral colliculi modulate auditory signa

Front Neural Circuits. 2013; 7: 68. AbstractIn the auditory pathway, the inferior colliculus (IC) is a major center for temporal and spectral integration of auditory information. There are widespread neural interactions in unilateral (one) IC and between bilateral (two) ICs that could modulate auditory signal processing such as the amplitude and frequency selectivity of IC neurons. These neural interactions are either inhibitory or excitatory, and are mostly mediated by -aminobutyric acid (GABA) and glutamate, respectively. However, the majority of interactions are inhibitory while excitatory interactions are in the minority. Such unbalanced properties between excitatory and inhibitory projections have an important role in the formation of unilateral auditory dominance and sound location, and the neural interaction in one IC and between two ICs provide an adjustable and plastic modulation pattern for auditory signal processing.Keywords: inferior collicular neurons, excitatory interaction, inhibitory interaction, bilateral collicular interaction, auditory signal processingINTRODUCTIONIn sound reception, auditory signal processing has traditionally been explained by neural interactions of divergent and convergent projections within the ascending auditory system through the interplay between excitation and inhibition ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B52" rid="B52" class=" bibr popnode - Suga, 1997 - ). Auditory interactions can be found between neurons in one auditory nucleus, bilateral symmetrical auditory structures or nuclei, and even in auditory and non-auditory structures. This implies a neural modulation that plays an important role in maintaining the diversity and accuracy of auditory functions ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B36" rid="B36" class=" bibr popnode - Mei and Chen, 2010 - ). For example, all sound signals in the range of audible frequency can be perceived by ear, however, we only notice those sounds interested by us and other sound signals that assumed to have no biological significance are filtered by excitatory or inhibitory modulation during transmission upward to different auditory nucleus.Inferior colliculi (ICs), paired auditory structures, are located between the lower brainstem auditory nuclei and the auditory thalamus in the central auditory pathway. IC receives excitatory and inhibitory inputs from many lower auditory nuclei ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B1" rid="B1" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993144 - Adams, 1979 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B3" rid="B3" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993114 - Adams and Wenthold, 1979 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B6" rid="B6" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993136 - Brunso-Bechtold et al., 1981 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B2" rid="B2" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993118 - Adams and Mugnaini, 1984 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B44" rid="B44" class=" bibr popnode - Pollak and Casseday, 1989 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B11" rid="B11" class=" bibr popnode - Covey and Casseday, 1995 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B7" rid="B7" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993176 - Casseday and Covey, 1996 - ), contralateral IC ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B31" rid="B31" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993154 - Malmierca et al., 1995 - , http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B28" rid="B28" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993171 - 2009 - ) and from the primary auditory cortex (AC; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B14" rid="B14" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993175 - Games and Winer, 1988 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B16" rid="B16" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993168 - Herbert et al., 1991 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B41" rid="B41" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993134 - Ojima, 1994 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B48" rid="B48" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993161 - Saldaa et al., 1996 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B32" rid="B32" class=" bibr popnode - Malmierca and Ryugo, 2011 - ). IC functions as an important relay station, and not only analyzes and integrates sound signals in terms of amplitude, frequency, and time course, etc., from different sources, but also prepares to route these signals to higher level center ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B9" rid="B9" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993147 - Casseday et al., 1994 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B20" rid="B20" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993160 - Jen et al., 1998 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B56" rid="B56" class=" bibr popnode - Suga et al., 1998 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B22" rid="B22" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993143 - Jen and Zhang, 2000 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B24" rid="B24" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993174 - LeBeau et al., 2001 - ). A number of studies have shown that auditory signal processing and integration in ICs are significantly modulated by the massive descending corticofugal system which adjusts and improves ongoing collicular signal processing in multiple-parametric domains but also reorganizes collicular auditory maps according to the acoustic experience ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B20" rid="B20" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993127 - Jen et al., 1998 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B23" rid="B23" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993165 - Jen and Zhou, 2003 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B45" rid="B45" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993123 - Popelar et al., 2003 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B63" rid="B63" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993119 - Yan et al., 2005 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B65" rid="B65" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993132 - Zhou and Jen, 2007 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B27" rid="B27" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993135 - Ma and Suga, 2008 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B53" rid="B53" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_342212452 - Suga, 2008 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B54" rid="B54" class=" bibr popnode - Suga et al., 2010 - ). However, few studies have characterized how neural circuits in or between ICs can affect collicular auditory signal processing and integration. Therefore, in this article, we review recent findings and focus mainly on neural interactions either in one IC or between two ICs.EFFECT OF INTERACTIONS BETWEEN NEURONS IN ONE IC IN THE AUDITORY SIGNAL PROCESSINGThere are extensive intrinsic connections between neurons in one IC such that the IC neurons are likely to be a major source of inputs to other IC cells ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B49" rid="B49" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993173 - Saldaa and Merchn, 1992 - ). Such intercollicular fibers contribute to the formation of the known fibrodendritic laminae in one IC ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B18" rid="B18" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993172 - Herrera et al., 1988 - , http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B19" rid="B19" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993152 - 1989 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B42" rid="B42" class=" bibr popnode - Oliver and Schneiderman, 1991 - ). How do the neurons inside one auditory center interact with each other? Little is known about this interaction, but immunocytochemical localization demonstrated that one IC contained considerable amounts of glutamic acid, glycine, and glutamate decarboxylase (GAD), an enzyme that catalyzed the decarboxylation of glutamate to -aminobutyric acid (GABA), although some of these molecules could have an extrinsic origin ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B3" rid="B3" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993113 - Adams and Wenthold, 1979 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B43" rid="B43" class=" bibr popnode - Ottersen and Storm-Mathisen, 1984 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B58" rid="B58" class=" bibr popnode - Vetter and Mugnaini, 1984 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B40" rid="B40" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993164 - Moore and Moore, 1987 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B46" rid="B46" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993155 - Roberts and Ribak, 1987 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B8" rid="B8" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993156 - Caspary et al., 1990 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B35" rid="B35" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993126 - Merchn et al., 2005 - ). The presence of these excitatory and inhibitory transmitters suggests extensive interactions and modulations between neurons in one IC, because excitation and inhibition are the two most important neural interactions that modulate auditory signal processing by increasing and decreasing responses of auditory neurons. To study the effect of neural interactions on sound amplitude and frequency selectivities of IC neurons, the auditory responses including the rate-intensity function (RIF) and frequency tuning curve (FTC) of each IC neuron in two simultaneously recorded IC neurons (or paired neurons) were examined under two-tone stimulation conditions. A pair of electrodes was used to simultaneously record two IC neurons in the same iso-frequency lamina or different iso-frequency (non-iso-frequency) laminae of the IC (Figure ). A modulating tone with the best frequency (BF) of one of the paired IC neurons was delivered prior to a probe tone. This two-tone stimulating paradigm provided an opportunity to examine how a neuron activated by its BF sound might affect the response of the other neuron in amplitude and frequency domains. In particular, this procedure allows us to study the possible correlation of each pair of neurons in signal processing. For example, when a pair of IC neurons was stimulated by their two BF tones, the response of one IC neuron was either inhibited (two-tone suppression, Figure ) or facilitated (two-tone facilitation, Figure ) by the other. It has been reported that the proportion of neurons inhibited by interactions between simultaneously recorded neurons was always higher than that of facilitated neurons ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B21" rid="B21" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993146 - Jen et al., 2002 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B60" rid="B60" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993140 - Wu and Jen, 2008 - ). Thus, the high level of inhibition in IC is basically similar to that in other reports ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B57" rid="B57" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993166 - Vater et al., 1992 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B51" rid="B51" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993117 - Suga, 1995 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B13" rid="B13" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993153 - Fuzessery and Hall, 1996 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B64" rid="B64" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993148 - Zhou and Jen, 2000 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B25" rid="B25" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993133 - Lu and Jen, 2002 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B34" rid="B34" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_342212449 - Mayko et al., 2012 - ).Responses of two pairs of simultaneously recorded IC neurons.(A,B) Sketches showing the experimental arrangement for simultaneous recording two pairs of IC neurons. a and b represent a pair of IC neurons in iso-frequency laminae while c and d are another ...Further testing of inhibitory interactions on responses of the paired neurons revealed that the percent two-tone suppression of auditory responses decreased significantly with BF and recording depth differences between paired IC neurons ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B21" rid="B21" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993111 - Jen et al., 2002 - ). This observation is similar to a study in which auditory spatial selectivity of IC neurons was studied under two-tone stimulation conditions ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B64" rid="B64" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993128 - Zhou and Jen, 2000 - ). It was proposed that this phenomenon might be caused by the tonotopic organization of IC neurons, and that inputs from neurons with small BF differences arrive earlier with less attenuation than neurons with large BF differences ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B21" rid="B21" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993131 - Jen et al., 2002 - ). On the other hand, this observation also suggests a gradient of decreasing two-tone suppression along the dorsoventral axis of the IC ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B50" rid="B50" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993145 - Schreiner and Langner, 1997 - ). However, the neural basis underlying this observation remains to be explored.Because the two-tone stimulation was based on the BFs of two simultaneously recorded neurons, two-tone suppression and facilitation might be thought to be http://vagueexcerpt8577.shutterfly.com/vagueexcerpt8577 - http://vagueexcerpt8577.shutterfly.com/vagueexcerpt8577 - caused by interactions between the two simultaneously recorded neurons activated by their respective BF sounds. Since IC neurons are tonotopically organized, interactions between the IC neurons are actually interactions between frequency laminae or bands. For a pair of IC neurons simultaneously recorded in big brown bat, a sound with the BF of one neuron could modulate the frequency tuning of another neuron by sharpening or broadening its FTC ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B59" rid="B59" class=" bibr popnode - Wu et al., 2004 - ). The pairs of neurons involved in frequency band interaction are not only within the same frequency band, but also across different frequency bands. The sharpening degrees of neurons within the same frequency band are higher than those of neurons across frequency bands. It was also found that the strength of frequency band interactions was weaker near the BF but gradually increased with frequency away from the BF of FTC ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B59" rid="B59" class=" bibr popnode - Wu et al., 2004 - ). Moreover, FTCs of neurons with a BF of 2030 kHz are most strongly sharpened which is similar to that observed in the chinchilla ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B5" rid="B5" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993159 - Biebel and Langner, 2002 - ).These data suggest that IC neurons are highly correlated during frequency analysis such that frequency selectivity of the IC neurons is improved through inhibition while the spectrum of frequency sensitivity of other IC neurons is enhanced through excitation.To further explore the mechanism underlying the effect of two-tone suppression on the responses of two simultaneously recorded neurons, bicuculline (an antagonist of GABAA receptor) was applied to one of the paired IC neurons in big brown bat to abolish GABAergic inhibition (Figure ). Using a pair of neurons (A and B, for example), when bicuculline was applied to neuron A, its number of impulses was greatly increased (Figure Aa vs Aa+bic), and the two-tone suppression was completely removed in neuron A (Figure Aa+bic vs Aa+bic b), but was stronger in neuron B (Figure Bb a vs Bb a+bic). Thus, the degree of response inhibition decreased in the bicuculline-applied neuron but increased in the paired neuron, suggesting that GABAergic inhibition directly mediated the inhibitory interactions between two simultaneously recorded or paired IC neurons ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B60" rid="B60" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993125 - Wu and Jen, 2008 - , http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B61" rid="B61" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_342212454 - 2009 - ). However, for another pair of neurons C and D, the number of impulses greatly increased following bicuculline administration to neuron C (Figure Cc vs Cc+bic), but the two-tone suppression was only partly abolished in neuron C (Figure Cc+bic vs Cc+bic d), and was slightly increased in neuron D (Figure Dd a vs Dd c+bic). A previous study in big brown bat indicated that IC neurons with GABAA receptors are mostly distributed in the dorsomedial region but are sparsely distributed in the ventrolateral region which is mostly distributed with neurons containing glycine receptors ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B12" rid="B12" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993129 - Fubara et al., 1996 - ). Therefore, the degree of GABA-mediated two-tone suppression would progressively decrease along the dorsoventral axis of the IC. In brief, when an IC neuron is excited, it may inhibit other neighboring neurons to stand out as the best in the neurons through inhibitory interaction. These inhibitory interactions between neurons in one IC improves auditory sensitivity during auditory signal processing.Two-tone suppression on responses of two pairs of simultaneously recorded IC neurons before and during bicuculline (bic) application. Presentation of a counterpart sound decreased the firing rates of each IC neuron in the (A,B) pair or (C,D) pair (Aa, ...BILATERAL COLLICULAR INTERACTION IN AUDITORY SIGNAL PROCESSINGMany previous studies have clearly shown the anatomical connections between two ICs through the commissure of IC (CoIC). Injecting retrograde tracer in one IC demonstrated that commissure neurons in the central nucleus of IC (ICc) sent projections or fibers to the central nucleus, dorsal and lateral cortices of opposite IC. The commissural fibers ending in the contralateral IC to the injection point formed a laminar plexus that was symmetrical to the ipsilateral plexus, and interconnected mirror symmetric regions of the ICs representing similar frequency bands ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B49" rid="B49" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993157 - Saldaa and Merchn, 1992 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B31" rid="B31" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993163 - Malmierca et al., 1995 - ). Even in the ICc, retrograde labeling of neurons demonstrated that commissural neurons send a divergent projection to the whole extent of the contralateral lamina, which resulted in a V-shaped axonal plexus that covered most of the ICc laminae and extended into the dorsal and lateral cortices. However, the density of the labeled commissurally projecting neurons was weighted toward a point that matched the position of the corresponding tracer injection into the contralateral IC, which is consistent with a point-to-point pattern (Figure ). The coexistence of point-to-point and divergent projections suggest that CoIC is likely to be involved in interactions between specific regions of corresponding frequency band laminae as well as in integration across the laminae. ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B28" rid="B28" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993130 - Malmierca et al., 2009 - ).Schematic wiring diagrams of the commissural connections. In the central nucleus of IC (ICc), the retrograde labeling of neurons demonstrated that an injection into one point on the lamina (dotted circle, left IC) retrogradely labeled neurons over the ...Modulation of response of IC neuron (ICMdu) during focal electrical stimulation of the contralateral IC neuron (ICES). (A,B) Focal electrical stimulation of one ICES neuron produced inhibition of two ICMdu neurons but produced facilitation of another ...In addition, after the focal electrical stimulation was delivered for 30 min, a long term shift in an IC neurons BF was induced which remained for as long as 150 min and decreased with time ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B10" rid="B10" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993141 - Cheng et al., 2013 - ). Therefore the bilateral collicular interaction modulates both auditory signal processing and auditory plasticity of IC neurons that is similar to the corticofugal modulation of IC neurons ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B20" rid="B20" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993112 - Jen et al., 1998 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B26" rid="B26" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993150 - Ma and Suga, 2001 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B55" rid="B55" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993120 - Suga et al., 2002 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B62" rid="B62" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993124 - Yan and Ehret, 2002 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B23" rid="B23" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993151 - Jen and Zhou, 2003 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B63" rid="B63" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993116 - Yan et al., 2005 - ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B65" rid="B65" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993142 - Zhou and Jen, 2007 - ). Since the BF-dependent modulation of bilateral collicular interaction is not entirely comparable to the egocentric selectivity of corticofugal modulation, further studies are required to determine whether the modulation effect of bilateral collicular interactions might also be mediated through corticofugal feedback loops.Interestingly, following reciprocal electrical stimulation of pairs of neurons, respectively, in two ICs, we found that the bilateral collicular interaction was either reciprocal or unilateral. However, after HRP deposits were made in CoIC, regions of the IC supplying fibers to the commissure were not the main targets for the terminals of these fibers, which suggested that interconnections of the ICs through their commissure were complementary, rather than reciprocal ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630329/#B4" rid="B4" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_307993170 - Aitkin and Phillips, 1984 - ).PROSPECTSNeural interactions are of great interest because of their contribution to sensory information processing, neural functional integration and neural modulation. As for the auditory midbrain, neural interactions were found both in one IC and between two ICs, even in unilateral iso-frequency and non-iso-frequency laminae as well as bilateral corresponding and non-corresponding frequency laminae. Generally, there is a large percentage of inhibitory interactions but a small percentage of excitatory interactions, which is likely because of the presence of many inhibitory interneurons. These excitatory and inhibitory interactions in or between ICs modulate auditory signal processing in amplitude and frequency domains, and provide an adjustable and plastic modulation pattern for the auditory signal processing of ICs. However, many details, such as neural plasticity of the structure and function as well as cellular and synaptic mechanisms of the neural modulation underlying neural interactions in auditory signal processing, remain unclear and require further study. We have sufficient reasons to believe that new knowledge about the various neural interactions will be obtained with successive studies. Thus, the studies of neural interactions in one IC and between two ICs are in the ascendancy.Conflict of Interest StatementThe authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.AcknowledgmentsWe thank the anonymous reviewers for commenting on an earlier version of this manuscript. This work was supported by the grants (#31070971, #30970972) from the National Science Foundation of China.REFERENCESAdams J. C. (1979). Ascending projections to the inferior colliculus.J. Comp. Neurol.183519538 [ http://www.ncbi.nlm.nih.gov/pubmed/759446" target="pmc_ext" ref="reftype=pubmed&article-id=3630329&issue-id=218651&journal-id=640&FROM=Article%7CCitationRef&TO=Entrez%7CPubMed%7CRecord&rendering-type=normal - PubMed - ]Adams J. C., Mugnaini E. (1984). Dorsal nucleus of the lateral lemniscus: a nucleus of GABAergic projection neurons.Brain Res. 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