The Auditory & Vestibular Systems

1. The Auditory & Vestibular Systems Psychology 355

2. Introduction Sensory Systems • Sense of hearing, audition • Detect sound • Perceive and interpret nuances • Sense of balance, vestibular system • Head and body location • Head and body movements Psychology 355

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 • Range: 20 Hz to 20,000 Hz • Pitch: High and Low • Intensity: Difference in pressure between compressed and rarefied patches of air Psychology 355

4. The Nature of Sound Psychology 355

5. The Nature of Sound Psychology 355

6. The Structure of the Auditory System Psychology 355

7. The Structure of the Auditory System • Auditory pathway stages • Sound waves • Tympanic membrane • Ossicles • Oval window • Cochlea fluid • Sensory neuron response • Brain stem nuclei output • Thalamus to MGN to A1 Psychology 355

8. The Middle Ear Components of the Middle Ear Psychology 355

9. Sound Force Amplification by the Ossicles Pressure: Force by surface area 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 Psychology 355

10. The Inner Ear Anatomy of the Cochlea Psychology 355

11. The Inner Ear Anatomy of the Cochlea Psychology 355

12. The Inner Ear Physiology of the Cochlea Pressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulges out Psychology 355

13. The Inner Ear The Organ of Corti Psychology 355

14. The Inner Ear Cilia Psychology 355

15. The Inner Ear Cilia Psychology 355

16. The Inner Ear Psychology 355

17. The Inner Ear Transduction by Hair Cells Sound: Basilar membrane upward reticular lamina up stereocilia bends outward Psychology 355

18. The Inner Ear Psychology 355

19. 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 Psychology 355

20. The Inner Ear The Basilar Membrane Structural properties: Wider at apex, stiffness decreases from base to apex Psychology 355

21. The Inner Ear Psychology 355

22. Central Auditory Processes Auditory Pathway • More synapses at nuclei than visual pathway, more alternative pathways • Anatomy • Dorsal cochlear nucleus, ventral cochlear nucleus, superior olive, inferior colliculus, MGN, lateral lemniscus, auditory nerve fiber • Primary pathway: Ventral cochlear nucleus to superior olive to inferior colliculus to MGN to auditory cortex Psychology 355

23. Auditory Pathway Psychology 355

24. Auditory Pathway Psychology 355

25. Central Auditory Processes Response Properties of Neurons in Auditory Pathway • Characteristic frequency Frequency at which neuron is most responsive • Response More complex and diverse on ascending auditory pathway in brain stem Psychology 355

26. Encoding Sound Intensity and Frequency • Encoding Information About Sound Intensity • Firing rates of neurons • Number of active neurons • Stimulus Frequency, Tonotopy, Phase Locking • Frequency sensitivity: Basilar membrane • Frequency: Highest at base, lowest at cochlea apex • Tonotopy: Systematic organization of characteristic frequency within auditory structure Psychology 355

27. Encoding Sound Intensity and Frequency Phase Locking Consistent firing of cell at same sound wave phase Psychology 355

28. 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 Psychology 355

29. Mechanisms of Sound Localization • The Sensitivity of Binaural Neurons to Sound Location • Monaural: Sound in one ear • Binaural: Sound at both ears • Superior olive: Cochlear nuclei input to superior olive, greatest response to specific interaural delay Psychology 355

30. 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, activity in right cochlear nucleus, first impulse far • Impulses reach olivary neuron at the same time summation action potential • Localization of Sound in Vertical Plane • Sweeping curves of outer ear Psychology 355

31. Mechanisms of Sound Localization A given binaural neuron indicates the amount of phase disparity between inputs from the left and right ear. Psychology 355

32. Auditory Cortex • Acoustic Radiation • 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 • Neuronal Response Properties • Frequency tuning: Similar characteristic frequency • Isofrequency bands: Similar characteristic frequency, diversity among cells Psychology 355

33. Auditory Cortex Principles in Study of Auditory Cortex Tonotopy, columnar organization of cells with similar binaural interaction Psychology 355

34. The Vestibular System • Importance of Vestibular System • Balance, equilibrium, posture, head, body, eye movement • The Vestibular Labyrinth Lateral line Organs Small pits or tubes Function Sense vibration or pressure changes Psychology 355

35. Head Rotation Head AngleLinear Acceleration The Vestibular System Psychology 355

36. The Vestibular System The Otolith Organs Psychology 355

37. The Vestibular System The Otolith Organs Psychology 355

38. The Vestibular System • The Semicircular Canals • Function: Detect head movements • Structure • Crista: Sheet of cells where hair cells of semicircular canals clustered • Ampulla: Bulge along canal, contains crista • Cilia: Project into gelatinous cupula • Kinocili oriented in same direction so all excited or inhibited together • Semicircular canals: Filled with endolymph Psychology 355

39. The Vestibular System Psychology 355

40. The Vestibular System • Push-Pull Activation of Semicircular Canals • Three semicircular canals on one side • Helps sense all possible head-rotation angles • Canal: Each paired with another on opposite side of head • Push-pull arrangement of vestibular axons: Rotation causes excitation on one side, inhibition on the other Psychology 355

41. 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 Psychology 355

42. The Vestibular System Vestibular Pathology • Drugs (e.g., antibiotics) can damage vestibular system • Effects: • Trouble fixating on visual targets • Walking and standing difficult Psychology 355

43. 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 Psychology 355

44. Concluding Remarks Hearing and Balance • 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 Psychology 355

45. End of Presentation Psychology 355