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THE AUDITORY SYSTEM the Central Auditory System

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THE AUDITORY SYSTEM the Central Auditory System THE AUDITORY SYSTEM The Central Auditory System • Each auditory nerve sends information to the cochlear nucleus. • From there, projections diverge to many different pathways. The Central Auditory System • There are many parallel pathways in the auditory brainstem. • The binaural system receives input from both ears. • The monaural system receives input from one ear only. Each set of auditory pathways has a specialized function The cochlear nucleus is the first stage in the central auditory pathway • Each auditory nerve fiber diverges to three divisions of the cochlear nucleus. This means that there Are three tonotopic maps In the cochlear nucleus. Each map provides a separate set of pathways. Each division of the cochlear nucleus contains specialized cell types • Different cell types receive different types of synaptic endings, and respond differently to the same stimulus. Neurons in the cochlear nucleus (and at higher levels) transform the incoming signal in many different ways. • The auditory nerve input is strictly excitatory (glutamate). Some neurons in the cochlear nucleus are inhibitory (GABA or glycine). • Auditory nerve discharge patterns are converted to many other types of temporal pattern. • There is a progressive increase in the range of response latencies. Changes in response pattern • The stereotyped auditory nerve discharge is converted to many different temporal response patterns. • Each pattern emphasizes different information. Increase in range of latencies • Each synapse adds approximately 1 ms latency. • Any integrative process that requires time adds latency. The superior olive and processing of sound localization cues • Cues for sound localization in the horizontal plane: • Interaural level difference • Interaural time difference Interaural Intensity Differences (IIDs) • For wavelengths less than head diameter, the head casts a partial or total sound “shadow”. • The difference in sound intensity between the two ears is the interaural intensity difference (IID). • IID varies as a function of sound source position Interaural Time Differences (ITDs) • For wavelengths greater than head diameter, there is no sound shadow, but the sound may reach the two ears at slightly different times. • The difference in timing between the two ears is the interaural time difference (ITD). • ITD varies as a function of sound source position. The superior olivary complex receives input from both cochlear nuclei • The medial superior olive (MSO) receives excitatory input from both ears. • The lateral superior olive (LSO) receives excitatory input from the ipsilateral ear and inhibitory input from the contralateral ear. The duplex theory of sound localization • The cues that are available suggest that we use two different mechanisms for localizing sound sources: • Interaural Intensity Differences (ILDs) for high frequency sounds • Interaural Time Differences (ITDs) for low frequency sounds VIA AUDITIVA CORTEZA AUDITIVA TÁLAMO AUDITIVO COLÍCULO INFERIOR NÚCLEOS DEL LEMNISCO LATERAL NÚCLEOS COCLEARES COMPLEJO OLIVAR SUPERIOR ESTRUCTURA DE LOS NÚCLEOS DEL LEMNISCO LATERAL DEL GATO NDLL VNLL Coronal Plane IC IC Petrol Station DNLL VNLL From CN and SOC From NLL Neurotrasmitters TEMPORAL PROCESSING includes • Neural representation of time-varying stimulus features • Generation of a temporal pattern of neural activity from stimulus properties that are not temporal in nature. • Time-based neural computations or other operations. Neural Representation of Time-Varying Information: Some Examples Onset or sustained responses provide time markers for temporal features of a stimulus, e.g. onset time and/or duration. There may be synchrony of responses within a neural population. onset sustained Many neurons at early stages of the auditory system respond in a phaselocked pattern to each cycle of a low frequency tone or each cycle of a tone that is periodically modulated in amplitude or frequency. The temporal pattern of response provides real-time information about the fine structure of sound. What Happens to Temporal Patterns of Neural Activity in Sensory Systems? • The original temporal pattern that directly reflects stimulus timing may be retained. Any neuron can transmit this information. • The original temporal pattern may be converted to a place code. This means that activity in a specific neuron signals that a specific temporal pattern occurred. The activity in this case does not need to reflect stimulus timing properties. • The original temporal pattern may be converted to a hybrid spatio-temporal code in which specific neurons are active and fire in specific temporal patterns. The Central Nervous System Performs Storage, Retrieval, and Recombination of Temporal Information. • Short-term storage provides context or sets up a pattern of expectation. • Long-term storage provides templates for recognition of commonly encountered temporal patterns. • Specific patterns of sensory stimuli can evoke specific spatiotemporal patterns of motor activity. All of the auditory structures and pathways in the central nervous system participate in some aspect of temporal processing. Green arrows show the cochlear nucleus (CN) and inferior colliculus (IC). They also indicate the plane of section in the next slide where the laleral lemniscus nuclei are shown. The Nuclei of the Lateral Lemniscus (NLL) are an important part of the pathway for analyzing temporal patterns of sound. Frontal section through the brain of a bat shows the large NLL complex. In animals such as the cat, the nuclei of the lateral lemniscus are smaller relative to total brain size, and cell types are not as well segregated. VIA AUDITIVA CORTEZA AUDITIVA TÁLAMO AUDITIVO COLÍCULO INFERIOR NÚCLEOS DEL LEMNISCO LATERAL NÚCLEOS COCLEARES COMPLEJO OLIVAR SUPERIOR ESTRUCTURA DE LOS NÚCLEOS DEL LEMNISCO LATERAL DEL GATO NDLL VNLL 31 kHz. CASE OO/122 Fluorescein 110µm VCN 30.5 kHz. 160µm Rodhamine 200µm Deep DCN 100µm Case 00/034 Case 97/114 CASECASE 00/12200/122 Coronal DCNDCN-30.5-30.5 kHz.kHz. Sagittal Horizontal Slicing VCNVCN-31-31 kHz.kHz. (270 µm. thick) D M V D D R M M C CASECASE 00/12100/121 Coronal DCNDCN-1.8-1.8 kHz.kHz. Sagittal Horizontal Slicing DCNDCN-1.7-1.7 kHz.kHz. (200 µm. thick) D M V D D M M C C Coronal Sagittal Horizontal L C CASECASE 00/12100/121 DCNDCN-1.8-1.8 kHz.kHz. D D DCNDCN-1.7-1.7 kHzkHz M C CASECASE 97/11497/114 Coronal VCNVCN-3-3 kHz.kHz. Sagittal Horizontal Slicing DCNDCN-6-6 kHz.kHz. (230 µm. thick) D M R V M D D M C 97/114 00/121 3 kHz. vs. 6kHz. 1.8 kHz. vs. 1.7kHz. Segregated Overlaping axonal plexus axonal plexus Lateral view D D C C 1.8 kHz. vs. 1.7kHz. 3 kHz. vs. 6kHz. TONOTOPICAL ORGANIZATION OF CNC AFFERENCES 30.5 / 31 kHz. 1.7/1.8 kHz. WIRING DIAGRAM Dorsal Middle Ventral ESTRUCTURA DE LOS NÚCLEOS DEL LEMNISCO LATERAL DEL GATO ESTRUCTURA DE LOS NÚCLEOS DEL LEMNISCO LATERAL DEL GATO ESTRUCTURA DE LOS NÚCLEOS DEL LEMNISCO LATERAL DEL GATO ESTRUCTURA DE LOS NÚCLEOS DEL LEMNISCO LATERAL DEL GATO The intermediate and ventral divisions of the NLL (INLL and VLL) are predominantly monaural, receiving excitatory input from the contralateral cochlear nucleus (AVCN and PVCN) and inhibitory input from the medial nucleus of the trapezoid body (MNTB) The IC receives direct projections from the cochlear nucleus (DCN and AVCN) as well as projections via the INLL and VNLL. 6 µm 20 µm Neuron Types‐DNLL Neuron Types‐VCLL The lower brainstem auditory pathways perform a number of transformations that are important for temporal processing: • Change from excitation to inhibition • Change in discharge pattern • Increase in response latency • Selectivity for behaviorally relevant sound patterns. Change From Excitation to Inhibition All input from the auditory nerve is glutamatergic and excitatory. Fibers diverge to different cell types which in turn provide direct or indirect excitatory or inhibitory input to cells in the NLL complex. The firing of the NLL neuron (or IC neuron) is determined by the relative strengths and timing of convergent inputs, which may add together or cancel each other. Each division of the NLL has characteristic cell types and characteristic discharge patterns. Sustained, non- adapting response Single-spike onset response Chopper response The range of latencies increases at each stage of processing up through the level of the IC. There is considerable overlap in the ranges of neurons’ latencies at every stage of the central auditory system. This means that a single brief sound causes a prolonged wave of activity that is simultaneously present at all stages. Delay lines Neurons with different latencies form systems of delay lines that provide a mechanism for comparing or otherwise integrating information about sounds that occur at different times. This is important for analyzing temporal patterns, including speech. The nuclei of the lateral lemniscus provide both excitatory and inhibitory inputs to the IC. These inputs have different latencies and time courses. Blue = monaural Red = binaural A single neuron in the IC may receive many inputs with very different characteristics. There is convergence of excitatory and inhibitory inputs with different latiencies, time courses, and tuning. As a result of this convergence, many neurons in the IC are selective for specific temporal features of patterns of sound. IC neurons may be selective for: • Amplitude • Sound duration • Interstimulus interval • Direction of frequency modulation • Rate of sinusoidal amplitude modulation • Multiple parameters of sinusoidal frequency modulation Synaptic inputs interact with IC neurons’ intrinsic properties. The resulting computations are important for creating selectivity to temporal features, including those important for speech. IC neurons act as coincidence detectors (or anti-coincidence detectors) in that they respond only to certain combinations of inputs that arrive in specific time relationships. A Case Study in Temporal Pattern Analysis: Delay-Tuned Neurons A Some neurons in the IC respond selectively to two sounds separated by B a specific interval, or “delay” between one sound and the next (A- C C). They respond poorly or not at all to a single sound (D). D Another Case Study: Duration Tuning.
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