Browsing by Subject "Inferior colliculus"
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Item Binaural mechanism revealed with in vivo whole cell patch clamp recordings in the inferior colliculus(2010-12) Li, Na, 1980 Oct. 2-; Pollak, G. D. (George D.), 1942-; Huk, Alex; Priebe, Nicholas; Golding, Nace; Zakon, Harold; Morrisett, RichardMany cells in the inferior colliculus (IC) are excited by contralateral and inhibited by ipsilateral stimulation and are thought to be important for sound localization. These excitatory-inhibitory (EI) cells comprise a diverse group, even though they exhibit a common binaural response property. Previous extracellular studies proposed specific excitatory and/or inhibitory events that should be evoked by each ear and thereby generate each of the EI discharge properties. The proposals were inferences based on the well established response features of neurons in lower nuclei, the projections of those nuclei, their excitatory or inhibitory neurochemistry, and the changes in response features that occurred when inhibition was blocked. Here we recorded the inputs, the postsynaptic potentials, discharges evoked by monaural and binaural signals in EI cells with in vivo whole cell recordings from the inferior colliculus (IC) of awake bats. We also computed the excitatory and inhibitory synaptic conductances from the recorded sound evoked responses. First, we showed that a minority of EI cells either inherited their binaural property from a lower binaural nucleus or the EI property was created in the IC via inhibitory projections from the ipsilateral ear, features consistent with those observed in extracellular studies. Second, we showed that in a majority of EI cells ipsilateral signals evoked subthreshold EPSPs that behaved paradoxically in that EPSP amplitudes increased with intensity, even though binaural signals with the same ipsilateral intensities generated progressively greater spike suppressions. These ipsilateral EPSPs were unexpected since they could not have been detected with extracellular recordings. These additional responses suggested that the circuitry underlying EI cells was more complex than previously suggested. We also proposed the functional significance of ipsilaterally evoked EPSPs in responding to moving sound sources or multiple sounds. Third, by computing synaptic conductances, we showed the circuitry of the EI cells was even more complicated than those suggested by PSPs, and we also evaluated how the binaural property was produced by the contralateral and ipsilateral synaptic events.Item Motion selectivity as a neural mechanism for encoding natural conspecific vocalizations(2010-12) Andoni, Sari; Pollak, G. D. (George D.), 1942-; Pillow, Jonathan; Fiete, Ila; Priebe, Nicholas; de la Llave, RafaelNatural sound, such as conspecific vocalizations and human speech, represents an important part of the sensory signals animals and humans encounter in their daily lives. This dissertation investigates the neural mechanisms involved in creating response selectivity for complex features of natural acoustic signals and demonstrates that selectivity for spectral motion cues provides a neural mechanism to encode communication signals in the auditory midbrain. Spectral motion is defined as the movement of sound energy upward or downward in frequency at a certain velocity, and is believed to provide the auditory system with an important perceptual cue in the processing of human speech. Using the Mexican free-tailed bat, tadarida brasiliensis, as a model system, this research examined the role of selectivity for spectral motion cues, such as direction and velocity, in creating response selectivity for specific features of the social communication signals emitted by these animals. We show that auditory neurons in the midbrain nucleus of the inferior colliculus (IC) are specifically tuned for the frequency-modulated (FM) direction and velocities found in their conspecific vocalizations. This close agreement between neural tuning and features of natural conspecific signals shows that auditory neurons have evolved to specifically encode features of signals that are vital for the survival of the animal. Furthermore, we find that the neural computations resulting in selectivity for spectral motion are analogous to mechanisms observed in selectivity for visual motion, suggesting the evolution of similar neural mechanisms across sensory modalities.Item The response of inferior colliculus neurons in the Mexican free-tailed bat to species-specific calls(2000-12) Klug, Achim Egbert; Pollak, G. D.Item The roles of inhibition in hierarchical processing in the auditory system and the response features of inferior colliculus neurons revealed by in vivo whole cell recordings(2006) Xie, Ruili; Pollak, George D.The inferior colliculus (IC) receives inputs, which include both excitatory and inhibitory projections, from almost all lower auditory nuclei and thus is the major integration center in the auditory pathway. This dissertation evaluated three questions in order to understand the roles of inhibition in shaping response features of IC neurons, and how the influences of inhibition in the IC compare to the roles that inhibition plays in shaping the response properties of neurons in two lower nuclei, the dorsal nucleus of the lateral lemniscus (DNLL) and intermediate nucleus of the lateral lemniscus (INLL). The first question asks what response features are expressed by cells in the DNLL and INLL, and to what degree those features are shaped by inhibition. The second question asks whether the response properties of IC cells are inherited from lower nuclei and to what degree are they created in the IC by the interactions of excitatory and inhibitory projections. To investigate these questions, I recorded single-unit activity evoked by the same set of stimuli in the three nuclei both before and while inhibition was blocked by iontophoresis of receptor blockers. The results showed that the response features of DNLL and INLL neurons are homogeneous and weakly influenced by inhibition. The response features of IC neurons, however, are heterogeneous and markedly shaped by inhibitory inputs. Thus, DNLL and INLL properties were either largely inherited or shaped by the convergence of excitatory projections, while IC response features were largely created in the IC by inhibition. The third question asks what intrinsic and synaptic features underlie sound-evoked response properties in IC neurons. These features were evaluated with in vivo whole-cell recordings. Consistent with extracellular studies, a variety of tuning features were revealed among the IC population by whole-cell recordings, but the spectral extent of subthreshold excitatory and inhibitory inputs was surprisingly broad. Particular attention was given to a subset of IC neurons with special features in synaptic responses, discharge features and their intrinsic properties. These features provide unique insights into how sound-evoked synaptic response features and intrinsic properties might interact to produce the discharge features of these novel cells.