Neurons in the auditory cortex (AC) receive convergent excitatory and inhibitory inputs from the lower auditory nuclei. Interaction between these two opposing inputs shapes different response properties of AC neurons. In this study, we examined how this interaction might affect the frequency tuning curves (FTCs), number of impulses and latency of AC neurons in the big brown bat, Eptesicus fuscus, using a probe (excitatory tone) and a masker (inhibitory tone) under different stimulation conditions. Excitatory FTCs of AC neurons were either V-shaped, closed (i.e. upper threshold) or double-peaked. Inhibitory FTCs were obtained either at both flanks or only at the low or high flank of excitatory FTCs. Application of bicuculline, an antagonist for Q-aminobutyric acid A receptors, produced expansion of excitatory FTCs into predrug inhibitory FTCs. Inhibition of probeelicited responses occurred when a masker was presented at certain intertone intervals. Maximal inhibition typically took place when a masker was presented within 4 ms prior to the probe. During maximal inhibition, a neuron had the minimal number of impulses and the longest response latency. Inhibition became stronger with increasing masker intensity but became weaker with increasing intertone interval. Biological significance of these data is discussed.

This study examined auditory responses of two simultaneously recorded neurons in the central nucleus of bat inferior colliculus (IC) under two-tone stimulation conditions. We specifically examined how a sound within the excitatory frequency tuning curve (FTC) of one IC neuron might affect responses of the other IC neuron in amplitude and frequency domains. Under this specific two-tone stimulation condition, responses of 82% neurons were suppressed and their excitatory FTCs sharpened. Responses of the other 18% neurons were facilitated and their excitatory FTCs broadened. Two-tone suppression was greater at low than at high stimulus amplitudes. Two-tone suppression also decreased with increasing recording depth and best frequency (BF) difference between each pair of neurons. The suppressive or facilitatory FTC of a neuron plotted under two-tone stimulation conditions was always within the excitatory FTC of the other neuron. Two-tone suppression or two-tone facilitation was weak near the BF but became increasingly strong with frequencies away from the BF. Biological significance of these findings is discussed.

This article reviews our recent studies of brief and short-term corticofugal modulation of signal processing in the central nucleus of the inferior colliculus (ICc) by electrical stimulation in the primary auditory cortex (AC). When cortical electrical stimulation was synchronized with an acoustic stimulus, auditory responses of ICc neurons were either inhibited or facilitated and the modulative effect typically vanished within 5^10 s after the stimulation. When cortical electrical stimulation synchronized with an acoustic stimulus was repetitively delivered for 30 min, corticofugal modulation of collicular responses typically persisted up to 40 min after the stimulation. In the frequency domain, cortical electrical stimulation decreased the excitatory frequency tuning curves (FTCs) and asymmetrically increased the lateral inhibitory FTCs of corticofugally inhibited ICc neurons but produced the opposite effect on corticofugally facilitated ICc neurons. Cortical electrical stimulation facilitated auditory responses of neurons in the external nucleus of the inferior colliculus (ICx) while electrical stimulation in the ICx decreased auditory responses of ICc neurons. Auditory responses of simultaneously recorded ICx and ICc neurons varied in opposite ways during cortical electrical stimulation or drug application to recorded ICx neurons. In the amplitude domain, cortical electrical stimulation compressed rate amplitude functions so as to increase the slope of rate^amplitude functions of ICc neurons. This modulative effect decreased with increasing stimulus amplitude. The possible biological relevance of these findings is discussed.

有关听中枢神经元纯音前掩蔽效应的神经表征已进行了大量研究，但是，噪声前掩蔽尤其是间断噪声前掩蔽效应的神经表征却鲜有报道。本研究观察了自由声场条件下，昆明小鼠下丘神经元在持续与间断噪声前掩蔽条件下对纯音探测声的反应。共记录到96个下丘神经元，测量了其中51个神经元在不同声刺激条件下的强度-放电率函数。结果显示，掩蔽声强度分布较广（探测声阈下21dB至阈上19dB之间）。在将近一半的神经元中，间断噪声的前掩蔽效应比持续噪声强（Ⅰ型，45.10%，P<0.001），但也有少数神经元其间断噪声的掩蔽效应较持续噪声的弱（Ⅲ型，17.65%，P<0.001)，部分神经元无显著性差异（Ⅱ型，37.25%，P>0.05）。无论Ⅰ型还是Ⅲ型神经元，持续噪声和间断噪声均在探测声强度较低时产生较强的抑制效应，随着探测声强度的升高，抑制效应逐渐降低（P<0.001）； 同时，持续噪声和间断噪声之间前掩蔽效应差异亦不复存在（P>0.05）。此外，当掩蔽声由持续噪声换为间断噪声后，部分Ⅰ型神经元掩蔽时相的类型发生转变，其中最主要的转变为由前期抑制转变为均衡抑制（53.85%，7/13）。对下丘神经元声反应的时间域以及强度域，持续与间断噪声具有分化性前掩蔽效应，提示噪声前掩蔽并非简单的神经元发放压抑源，某些主动性神经调制机制可能参与了噪声条件下时相声信息的编码过程。

Using bats as a model auditory system, we studied corticofugal control of auditory sensitivity of neurons in the inferior colliculus. We demonstrate for the first time that the corticocollicular pathway continuously regulates acoustic signal processing in the inferior coiliculus by increasing the threshold, reducing the auditory spatial response area, and sharpening the frequency tuning curve of recorded inferior collicular neurons. Regulation of auditory sensitivity of recorded inferior collicular neurons was observed when the corticocollicular pathway was activated by electrical stimulation in the auditory cortex. The effect of this corticofugal regulation of auditory sensitivity in inferior collicular neurons can also be produced by ionophoretical application of GABA to the collicular recording site. This regulation of ascending acoustic information by commands originating from higher brain centers may provide the bat with a mechanism to actively control acoustic signal processing and thus optimize acoustic signal analysis.