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Auditory Midbrain

Auditory Midbrain. Nuclei of the Lateral Lemniscus Inferior Colliculus. Lemniscal vs Extralemniscal Pathways. Lemniscal pathway is tonotopically organized. like the geniculostriate pathway of the visual system, has components of both the where and the what pathway.

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Auditory Midbrain

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  1. Auditory Midbrain Nuclei of the Lateral Lemniscus Inferior Colliculus

  2. Lemniscal vs Extralemniscal Pathways • Lemniscal pathway is tonotopically organized. • like the geniculostriate pathway of the visual system, has components of both the where and the what pathway. • Extralemniscal pathway is like the retinotectal pathway: mediates motor orientation reflexes (e.g. head turning). It is mainly a part of the "where" pathway.

  3. Lateral Lemniscus: Organization, Projections • Dorsal nucleus (DNLL) • Ventral nucleus (VNLL) • Intermediate nucleus (INLL; not all spp).

  4. Lateral Lemniscus DNLL • DNLL • Binaural (input from MSO and LSO). • GABAergic. • Bilateral projection to IC. • VNLL and INLL • Monaural • Glycinergic/GABAergic. • Ipsilateral projections to IC INLL VNLL

  5. VNLL • Weakly tonotopic, very broad tuning. • Input from (contra) CN: • octopus cells (PVCN) • Spherical bushy cells (AVCNa) • Multipolar cells (AVCN) • “Onset” responses: single spike, latency short (3-5 ms) and precisely locked to stimulus. • No response, or high-pass response to AM • Coding of acoustic transients at microsecond time resolution.

  6. VNLL • “Onset” responses: single spike, latency short (3-5 ms) and precisely locked to stimulus. • Chopper responses: precise intrinsic rhythm. • No response, or high-pass response to AM • Coding of acoustic transients at microsecond time resolution.

  7. INLL • Tonotopic, but broadly tuned • Phasic, chopper, pauser responses. • Few latency-constant cells like VNLL. • Encodes low to intermediate frequencies of AM, FM

  8. Output of NLL • DNLL projects bilaterally to IC. • INLL and VNLL project ipsilaterally to IC • As a group, NLL provide over 60% of afferents to IC. • NLL projections are overwhelmingly inhibitory (glycine/GABA).

  9. Inferior Colliculus: Organization, Projections commissure • Central nucleus (ICC) • Lateral (or external) nucleus (ICL or ICX) • Dorsal cortex (DC) or pericentral nucleus DC ICL ICC

  10. Receives ascending projections from nearly all brainstem auditory nuclei, and from the contralateral ICC. Disynaptic from NLL (largest %) Monosynaptic from contralateral DCN Polysynaptic from SOC bilaterally (where pathway). IC on each side interconnected via “commissure” of IC. Important modulatory inputs from serotonergic Raphe nuclei, noradrenergic locus coeruleus. Inferior Colliculus

  11. Inputs to the IC Inputs are tonotopically organized Convergence of Brainstem in IC

  12. IC Outputs • ICC projects mainly ipsilaterally to the auditory thalamus (medial geniculate body)

  13. ICL (ICX) • ICL receives input from ICC binaural “where” pathway • ICL is multimodal: some cells respond to sound and neck and ear movements via somatosensory afferents of neck and pinna muscles.

  14. Dorsal Cortex • DC is the primary target of corticofugal pathway, mediating (attentional?) shifts in responsiveness. • Sparse ascending input from the auditory brainstem.

  15. IC Organized Tonotopically • ICC is tonotopically organized with a single map of frequency. • Convergence of inputs from brainstem within isofrequency laminae creates complex response properties and feature gradients.

  16. Response Properties in ICC • ICC neurons receive both excitatory and inhibitory projections.

  17. Inhibition Dominates IC • Brainstem inhibitory projections are largely glycinergic (GLY). • GLY expression increases, while GABA expression decreases, more ventrally (complementary pattern) GABA GLY

  18. Notable Modifications of Response Properties in ICC • Receptive Field shape • Sound level selectivity • Expansion of response latencies • Duration selectivity • Place representation of temporal patterns • Facilitation in cross-frequency interactions • Delay selectivity • Multimodality (touch, vision) • Significant influence of neuromodulators (serotonin, norepinephrine)

  19. Features Mapped within Isofrequency Contours • Threshold • Latency • Tuning sharpness (Q) • Binaural interactions • AM tuning (best modulation frequency)

  20. type V type I type O BF BF BF Rate Rate 100 50 100 0.25 0.5 2 2 4 8 16 1 4 8 16 32 Frequency (kHz) Receptive Fields in ICC Courtesy K. Davis V-shaped FTC I-shaped FTC O-shaped FTC

  21. Elaboration of Receptive Fields in ICC Receptive Fields show complex interactions of excitation and inhibition: Top: I type cell showing asymmetric facilitatory and inhibitory “sidebands” Bottom: O type cell with flanking inhibition, broad high threshold inhibition.

  22. Sound Level Selectivity • Dominance of inhibitory inputs creates non-monotonic rate-level functions in ICC (~ 50% of units). • Selectivity (tuning) for sound level, as well as sound frequency

  23. Range of Latencies Expands • ICC latency distribution highly expanded compared to brainstem nuclei • Two latency gradients are expressed: • Along tonotopic axis: Dorsolateral to ventromedial gradient of decreasing latency • Within isofrequency laminae: short latencies lateral, long medial. Cat

  24. GABA Inhibition Shapes Latency • GABAergic inhibition involved in generating longer latencies Bat

  25. Duration Tuning • ICC neurons (~30%) can be selective for sound duration • Blocking GABA reduces or eliminates duration tuning in most neurons.

  26. Selectivity for Complex Sounds Emerges in ICC • Many cells show specificity for biologically relevant sounds (e.g., communication sounds) • Selectivity for these sounds strongly influenced by local inhibitory circuitry.

  27. Vowel Formants • Vowels are discriminated based upon unique combinations of formants (harmonics)

  28. Facilitation, Combination Selectivity • Facilitatory (combinatorial) interactions between frequency channels first expressed in ICC. • May be basis of vowel discrimination in speech.

  29. Importance of FM in Speech • Many consonant-vowel transitions are frequency modulated. • Rate and direction of FM critical to discrimination of CV syllables

  30. Directional Selectivity • Frequency modulation (FM) induces a orderly shift of excitation along basilar membrane (akin to “motion”). Downward FM sweep Base (HF) Apex (LF) Upward FM sweep

  31. Directional Selectivity • ANFs respond to FM, but are non-directional • (Some) IC neurons are directionally selective for FM (e.g., downsweeps over upsweeps). ICC ANF

  32. Binaural Interactions MSO • Tonotopic projections from LSO, MSO, DNLL, co-variation of binaural interaction with tonotopy. • EE (ITD sensitive) neurons located dorsolaterally. • EI (IID sensitive) neurons located ventromedially. Low EE DNLL Medium EI LSO High

  33. Systematic Map of IID in ICC • Evidence suggests that each high-frequency iso-CF contour in ICC has systematic gradient of IID selectivity, forming a crude map of horizontal space.

  34. Systematic Map of IID in ICC • IID functions correlate with spatial receptive fields

  35. Systematic Map of IID in ICC • Systematic mapping of IID within isofrequency contours may represent location by a boundary of excitation.

  36. A Map of Auditory Space in Barn Owl ICX • Barn owl localizes prey using prey-generated sound. • ITDs used for horizontal localization, IIDs for vertical. • ICX neurons sensitive to both ITD and IlD, creating circular spatial receptive field.

  37. A Map of Auditory Space in Barn Owl ICX • Orderly arrangement of ITD/IID selectivity in ICX (MLD) creates a map of auditory space.

  38. Bimodal Space Map in SC • MLD (ICX) of owl projects to optic tectum (SC), just like in mammals. • Deep SC neurons are bimodal, with auditory and visual receptive fields. • Alignment of RFs is dominated by visual cues during early postnatal development; recalibrates binaural cues to head size changes.

  39. Moving sounds can alter spatial selectivity of neurons in ICC and AC. Spatial RF shifts toward source of motion. Auditory Motion

  40. Auditory Motion • Shift in RF may assist in predictive tracking of moving sound source.

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