Neuroscience: Exploring the Brain, 3e

1. Neuroscience: Exploring the Brain, 3e Chapter 11: The Auditory and Vestibular Systems

2. Introduction • Sensory Systems • Sense of hearing, audition • Detect & localize sound • Perceive and discriminate • Sense of balance, vestibular system • Head and body location • Head and body movements

3. The Nature of Sound • Sound • Audible variations in air pressure • Sound frequency: Number of cycles per second expressed in units called hertz (Hz) • Cycle: Distance between successive compressed patches

4. The Nature of Sound • Sound • Range: 20 Hz to 20,000 Hz • Pitch: High pitch = high frequency; low frequency = low pitch • Intensity: High intensity louder than low intensity

5. The Structure of the Auditory System • Auditory System

6. The Structure of the Auditory System • Auditory pathway stages • Sound waves • Tympanic membrane • Ossicles • Oval window • Cochlear fluid • Sensory receptor and neuron response

7. Sound Force Amplification by the Ossicles Pressure: Force per surface area e.g. dynes/cm Greater pressure at oval window than tympanic membrane, moves fluids The Attenuation Reflex Response where onset of loud sound causes tensor tympani and stapedius muscle contraction Function: Adapt ear to loud sounds, understand speech better The Middle Ear 2

8. The Inner Ear • Anatomy of the Cochlea • Perilymph: Fluid in scala vestibuli and scala tympani • Endolymph: Fluid in scala media • Endocochlear potential: Endolymph electric potential 80 mV more positive than perilymph

9. The Inner Ear • Physiology of the Cochlea • Pressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulges out • The Response of Basilar Membrane to Sound • Structural properties: Wider at apex, stiffness decreases from base to apex • Research: Georg von Békésy • Endolymph movement bends basilar membrane near base, wave moves towards apex

10. The Inner Ear • Travelling wave in the Basilar Membrane

11. The Inner Ear • The Organ of Corti and Associated Structures

12. The Inner Ear • Transduction by Hair Cells • Research: A.J. Hudspeth. • Sound: Basilar membrane upward, reticular lamina up and stereocilia bends outward (towards largest)

13. The Inner Ear • The Innervation of Hair Cells • One spiral ganglion fiber: One inner hair cell, numerous outer hair cells • Amplification by Outer Hair Cells • Function: Sound transduction • Motor proteins: Change length of outer hair cells • Prestin: Required for outer hair cell movements

14. Central Auditory Processes • Auditory Pathway

15. Central Auditory Processes • Response Properties of Neurons in Auditory Pathway • Characteristic frequency: Frequency at which neuron is most responsive - from cochlea to cortex • Response Properties more complex and diverse beyond the brain stem • Binaural neurons are present in the superior olive

16. Encoding Sound Intensity and Frequency • Encoding Information About Sound Intensity • Firing rates of neurons • Number of active neurons

17. Encoding Sound Intensity and Frequency • Stimulus Frequency • Tonotopic maps on the basilar membrane, spiral ganglion & cochlear nucleus

18. Encoding Sound Intensity and Frequency • Phase Locking • Low frequencies: phase-locking on every cycle or some fraction of cycles • High frequencies: not fixed

19. Mechanisms of Sound Localization • Techniques for Sound Localization • Horizontal: Left-right, Vertical: Up-down • Localization of Sound in Horizontal Plane • Interaural time delay: Time taken for sound to reach from ear to ear • Interaural intensity difference: Sound at high frequency from one side of ear • Duplex theory of sound localization: • Interaural time delay: 20-2000 Hz • Interaural intensity difference: 2000-20000 Hz

20. Mechanisms of Sound Localization • Interaural time delay and interaural intensity difference

21. Mechanisms of Sound Localization • The Sensitivity of Binaural Neurons to Sound Location

22. Mechanisms of Sound Localization • Delay Lines and Neuronal Sensitivity to Interaural Delay • Sound from left side, activity in left cochlear nucleus, sent to superior olive • Sound reaches right ear later; delayed activity in right cochlear nucleus. • Impulses reach olivary neuron at the same time summation action potential

23. Barn Owl : Space Map in Inf. Colliculus This is a computational map!

24. Frog calls (Hylaregilla) Advertisement call Encounter call

25. Model of short-pass duration selectivity Excitation is delayed and inhibition increases in duration with longer-duration stimuli. Leary et al (2008)

26. Intracellular recordings from a short-pass duration-selective neuron Made in the auditory midbrain of a frog

27. Frog calls (Hylaregilla) Advertisement call Encounter call

28. Recordings from neurons selective for short and long intervals Neuron A – selective for short intervals Neuron B – selective for long intervals

29. Models of long-interval selectivity Model A – Each pulse produces an EPSP followed by an IPSP. IPSP and EPSP from following pulse overlap at fast rates (short intervals). Model B – There is depression of excitation at fast pulse repetition rates. Edwards et al (2008)

30. Human speech also has periodic AM flatounet.net

31. Auditory Cortex • Primary Auditory Cortex • Axons leaving MGN project to auditory cortex via internal capsule in an array • Structure of A1 and secondary auditory areas: Similar to corresponding visual cortex areas

32. Target distance and relative velocity

34. Auditory Cortex • Principles of Auditory Cortex • Tonotopy, columnar organization of cells with similar binaural interaction • Unilateral lesion in auditory cortex: No deficit in understanding speech. But, Localization deficit. • Lesion in striate cortex: Complete blindness in one visual hemifield • Different frequency band information: Parallel processing e.g. frequency-specific localization deficit assoc. with small lesion. • Neuronal Response Properties • Frequency tuning: Similar characteristic frequency • Isofrequency bands: Similar characteristic frequency, diversity among cells • Multiple computational maps- Bat auditory cortex.

35. The Vestibular System • Importance of Vestibular System • Balance, equilibrium, posture, head, body, eye movement • Vestibular Labyrinth • Otolith organs - gravity and tilt • Semicircular canals - head rotation • Use hair cells, like auditory system, to detect changes

36. The Vestibular System • The Otolith Organs: Detect changes in head angle, linear acceleration • Macular hair cells responding to tilt

37. The Vestibular System • The Semicircular Canal • Structure

38. The Vestibular System • Push-Pull Activation of Semicircular Canals • Three semicircular canals on one side • Helps sense all possible head-rotation angles • Each paired with another on opposite side of head • Push-pull arrangement of vestibular axons:

39. The Vestibular System • Central Vestibular Pathways

40. The Vestibular System • The Vestibulo-Ocular Reflex (VOR) • Function: Line of sight fixed on visual target • Mechanism: Senses rotations of head, commands compensatory movement of eyes in opposite direction • Connections from semicircular canals, to vestibular nucleus, to cranial nerve nuclei  excite extraocular muscles

41. The Vestibular System • The Vestibulo-Ocular Reflex (VOR)

42. Concluding Remarks • Hearing and Balance • Nearly identical sensory receptors (hair cells) • Movement detectors: Periodic waves, rotational, and linear force • Auditory system: Senses external environment • Vestibular system: Senses movements of itself

43. Concluding Remarks • Hearing and Balance (Cont’d) • Auditory Parallels Visual System • Tonotopy (auditory) and Retinotopy (visual) preserved from sensory cells to cortex code • Convergence of inputs from lower levels  Neurons at higher levels have more complex responses

44. End of Presentation

45. The Middle Ear • Components of the Middle Ear

46. Mechanisms of Sound Localization • Localization of Sound in Vertical Plane • Vertical sound localization based on reflections from the pinna