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A. A fundamental property of hearing is the decomposition of complex sounds\ninto perceptually distinct frequency components. Each receptor cell in the\ncochlea and most centrally located neurons respond only to a limited range of\nfrequencies. The individual frequency channels are spatially organized on the\ncortical surface. This consistent topographical pattern provides a framework for\nthe investigation of other functional organization principles, e.g., the functional\nproperties of neurons in the six cortical layers and the responsiveness of\nneurons to complex sounds. The frequency specific features of inhibition should\nplay an important role in shaping a neuron\u2019s response to complex behaviorally\nrelevant stimuli.\nPhysiological and immunocytochemical evidence indicates a layer-dependent\norganization of inhibitory circuits in the neocortex. To investigate the\ncontribution of GABAergic inhibition to frequency tuning in the different cortical\nlayers, single and multi units were recorded in near-radial penetrations before\nand during iontophoretic application of the GABAA-receptor antagonist\nbicuculline in the auditory cortex of the lightly anaesthetized gerbil (Meriones\nunguiculatus). Bicuculline generally increased the spontaneous neuronal activity\nand enhanced and prolonged onset responses to sound. Application of\nbicuculline often resulted in a shift of the most sensitive frequency of the\nneurons\u2019 receptive fields and a decrease of threshold (5.5 dB). A broadening of\nthe frequency tuning evident by lower Q40dB values was observed in 63% of the\nunits. In units with several peaks in their tuning curve or clearly separated\nresponse areas, bicuculline application removed inhibitory gaps in the receptive\nfields and created single-peaked tuning curves. The influence of bicuculline on\nthe receptive field size was not significantly layer-specific but tended to be most\npronounced in layers V and VI. In layer VI, "silent" neurons were frequently\nfound that responded to sound only when GABAergic inhibition was\nantagonized. From the analysis of postembedding GABA immunocytochemistry,\nthe proportion of GABAergic neurons was found to be maximal in layers I and\nV, and the number of GABAergic perisomatic puncta (axon terminals) on cell\nsomata peaked in layer V. The influence of bicuculline was compared with the effects of two-tone\nsuppression. It was found that in some units, the effects of suppression could\nbe partially mediated by intracortical GABAergic inhibition.\nIn some units in layers IV, V, and VI, additionally to the initial excitatory activity\nin response to stimulus onset, a second, long-lasting excitatory response\noccurred several hundred milliseconds after the stimulus. This late response\nwas not dependent on stimulus duration and could be enhanced or elicited by\nGABAA blockade. The fact that several, rhythmically occurring late responses\nwere elicited by the application of bicuculline suggests that recurrent excitatory\nnetworks can become entrained by small modifications of inhibition.\nB. In the natural environment, acoustic signals like animals\u2019 communication\nsound or human speech is often masked by background noise. Amplitude\nfluctuations are often superimposed upon environmental sounds on their path of\ntransmission which can lead to a distinct temporal structure of the sound.\nFurthermore, many natural background sounds are often temporally structured.\nVertebrates have evolved mechanisms to exploit amplitude modulations in\nbackground noise to improve signal detection. Psychophysical and behavioral\nexperiments have shown that amplitude-modulated background noise\n(comodulated noise) is less effective as a masker than unmodulated noise\nbands of the same bandwidth, a phenomenon called comodulation masking\nrelease (CMR). This phenomenon has been extensively studied in human\npsychoacoustics. However, the underlying neural mechanisms are still debated.\nAnimal models in which a direct comparison of the neuronal response and the\nbehaviorally measured performance is possible could increase our\nunderstanding of the underlying mechanisms. CMR could be demonstrated\nbehaviorally and neurophysiologically in a songbird, however, models for\nmammals are still lacking. In behavioral experiments, Kittel et al. (2000)\ndemonstrated CMR in the gerbil. In the present study, using acoustic stimuli that\nwere identical with those of a behavioral experiment, a neural correlate of CMR\nwas described in the auditory cortex of the gerbil and compared with the\nbehavioral data.\nIn this study of neural mechanisms of masking release in the primary auditory\ncortex of the anaesthetized gerbil, I determined neural detection thresholds for 200-ms test tones presented in a background of band-pass amplitude\nmodulated (50 Hz) noise maskers of different bandwidth (between 50 and 3200\nHz). Neural release from masking caused by comodulated band-pass noise was\nevident at the level of the gerbil\u2019s primary auditory cortex. On average, the\nlargest masking release (median 6.9 dB) was found for a masker bandwidth of\n3200 Hz. This is less than the median masking release of 15.7 dB observed in\nthe behavioral study in the gerbil. For most masker bandwidths, however, a\nsmall fraction of the neurons exhibited a masking release that was close to or\neven larger than the behavioral masking release. The observation that the\nrelease from masking increased as a function of the masker bandwidth\nindicates that spectral components remote from the signal frequency enhance\nthe signal detection. However, there was no correlation between the neurons\u2019\nfilter bandwidths and the amount of masking release. Thus, neuronal masking\nrelease in the gerbil primary auditory cortex could be attributed to both signalmasker\ninteractions across different frequency channels and also to\nmechanisms that act within a single frequency channel. The gerbil appears to\nbe a suitable animal model for additional studies comparing behavioral and\nphysiological performance in the same species. These studies could increase\nour understanding of the perceptual mechanisms that are useful for the analysis\nof auditory scenes.