The auditory system allows humans to perceive sound. Sound waves enter the outer ear and vibrate the eardrum, transmitting vibrations through three small bones to fluid in the inner ear. Hair cells in the cochlea convert vibrations into nerve signals about sound frequencies and intensities. The brain processes these signals to perceive qualities like pitch and loudness. Damage to hair cells or nerves can cause hearing loss while cortical damage may impact speech understanding.

1. The Auditory System

2. The Nature of Sound Sound is a longitudinal wave Vibrations cause compressions & rarefactions Medium is usually air, but sound travels in any elastic medium All waves have basic characteristics: Amplitude Frequency Wavelength These properties have particular names with reference to sound Amplitude = loudness or volume Frequency = pitch

3. Properties of Waves

4. Sound Frequency Units of frequency are cycles/second (complete waves/second) Measured in hertz (Hz) Humans with normal hearing can hear in the frequency range 20 Hz to 20,000 Hz

5. Path of Sound Sound waves enter the outer ear They pass along the ear canal to the eardrum Sound waves bounce off the eardrum, making it vibrate. The eardrum is connected to 3 tiny bones, the ossicles The vibrations pass along these bones. The 3 rd of these bones, the stapes, presses against the oval window in the cochlea. The vibrations pass into the fluid inside the cochlea. Here, they shake thousands of tiny hairs that stick into the fluid from hair cells. As the hairs vibrate, the hair cells generate nerve signals The nerve signals travel along the auditory nerve to the hearing center of the brain.

6. A Diagrammatic Representation

7. Structure of the Ear The ear is divided into three parts: The outer ear The middle ear amplifies the sound and transfers it to the inner ear The inner ear converts the sound waves into action potentials and transmits them to the brain.

8. Divisions of the Ear

9. Structures of the Ear 1.Ossicles 2. Semicircular canal 3. Cochlea 4. Auditory nerve 5. Eustachian tube 6. Middle ear 7. Tympanic membrane 8. Auditory canal

10. The Outer Ear The pinna – the visible part of the ear directs sound waves into the middle ear involved in localizing sounds in vertical plane Auditory canal – The tube connecting the center of the pinna with the tympanic membrane channels sound into the middle ear can be analyzed (to first approximation) as a open-closed pipe. Both the pinna and auditory canal impose filtering characteristics based on unique shapes.

11. The Middle Ear Tympanic Membrane – Ear drum: cone-shaped membrane that converts sound to mechanical vibration Ossicles - three serial bones that conduct sound vibrations from the tympanic membrane to the cochlea Eustachian Tube – connects the middle ear and throat equalizes pressure on either side of tympanic membrane

12. The Ossicles Three small bones conduct sound vibrations from the tympanic membrane to the oval window of the cochlea The malleus – attached to the tympanic membrane The incus – connects the malleus to the stapes, The stapes – the foot of the stapes is attached to the oval window Also known as the hammer, anvil, & stirrup Ossicles act as a mechanical transformer, converting pressure on the eardrum to pressure on the oval window Amplifies signal 20 – 30x.

13. Picturing the Ossicles

14. Attenuation The acoustic reflex Loud noise triggers two sets of muscles: Stapedius muscle Tensor tympani One tightens the eardrum The other pulls the stirrup away from the oval window. Change sound conduction from the tympanic membrane to the cochlea

15. The Inner Ear Semicircular canals control balance The cochlea A double-walled, fluid-filled tube, curled into a snail shell shape with 2 ½ turns transforms pressure variations to neural impulses. Organ of Corti Inside the cochlea Contains actual receptors, hair cells Bone conduction to inner ear is also significant .

16. The Middle & Inner Ear

17. The Cochlea Two major chambers scala tympani & scala vestibuli Stapes footplate sits on the oval window opens onto the scala vestibuli at the base of the cochlea At the apex, the scala vestibuli communicates with the scala tympani via a hole, the helicotrema At the base, the scala vestibuli ends at the round window which is covered by a membrane

18. Picturing the Cochlea

19. The Cochlea in Cross-section This section shows the coiling of the cochlear duct (1) which contains endolymph, and the scala vestibuli (2) and scala tympani (3) which contain perilymph. The red arrow is from the oval window, the blue arrow points to the round window. Within the modiolus, the spiral ganglion (4) and auditory nerve fibres (5) are seen.

20. A Single Turn

21. Frequency Processing Occurs in the cochlea The base of cochlea processes high frequencies The apex processes low frequencies From the base to the apex of the cochlea the frequency that produces a maximal deformation of the basilar membrane decreases Auditory nerve fibers are ``tuned'' to different center frequencies.

22. Picturing Frequency Processing

23. Cochlear Fluids Scala tympani & scala vestibuli are filled with perilymph Inner chamber, scala media , is filled with endolymph Inwardmovement of the stapes footplate causes bulging of the round window membrane because the fluid in the cochlea is incompressible The stria vascularis secretes endolymph Perilymph is like CSF, high Na + , low K + Endolymph is very unusual, high K + , low Na + There is a voltage in the scala media , the endocochlear potential of about +80mV

24. Organ of Corti Part of the cochlea A helical band between the outer wall of the bony cochlea and the inner bony covering of the modiolus (central axis of the helix) Contains the receptors of hearing: hair cells Layers: Basilar membrane Hair cells rest on the basilar membrane Reticular lamina – top rigid surface that supports the stereocilia of the hair cells Tectorial membrane – gelatinous mass with internal fibers that sits on top of stereocilia

25. Picturing the Organ of Corti

26. Hair Cells The Organ of Corti contains 16,000 - 20,000 hair cells along its 37 millimeter length. Each hair cell has many cilia which bend with the vibrations of the basilar membrane.

27. Transduction Sound input causes a traveling wave in the basilar membrane and the organ of Corti Upward movement of organ of Corti deflects stereocilia away from the modiolus Downward movement of organ of Corti deflects them toward the modiolus This deflection is reflected in the receptor potential of the inner hair cells

28. Inner Hair Cells Cell body below the reticular lamina sits in normal extracellular fluid high Na + , low K + Top surface, bearing stereocilia sits in endolymph high K + , low Na + In silence, mechanically gated potassium channels at tips of the stereocilia are partly open Resting potential is about -70mV E K , the potassium equilibrium potential is 0mV because of the high concentration of potassium both inside the cell and in the endolymph

29. Transduction of Inner Hair Cells Deflection of the stereocilia either fully opens or fully closes the potassium channels The mechanism is the mechanical springs (filaments) connecting the stereocilia When open, depolarization results inward rushing potassium tends to move the membrane potential toward 0mV = E K When closed, hyperpolarization results Depolarization opens voltage-gated calcium channels Calcium mediates release of synaptic vesicles containing glutamate onto auditory nerve neurites Each IHC is innervated by about 10 auditory nerve fibers

30. Outer Hair Cells - The Cochlear Amplifier Main purpose of OHCs is not to stimulate auditory nerve fibers, but to change the mechanical properties of the organ of Corti affects transduction in IHCs Stimulation of OHC causes inward movement of potassium This contracts motor proteins in the cell wall and shortens the cell pulls reticular lamina closer to basilar membrane and causes the stereocilia of the IHCs to bend more - Thus a cochlear amplifier

31. Modifying Outer Hair Cell Response Blocking the action of the OHC motor proteins by drugs or sound damage reduces the sensitivity of the cochlea The actions of the OHCs can be modified by efferent nerve fibers from the brain the brain can modulate the sensitivity of the cochlea Ototoxic effects of antibiotics occur because they damage the OHCs and reduce the sensitivity of the cochlea. IHCs are not affected directly by antibiotics.

32. Picturing Hair Cell Movement

33. Processing Auditory Signals in the Brain Two major pathways: the dorsal pathway the ventral pathway Pathways are complex and connections not well understood. Ventral Auditory Pathway: begins in the ventral cochlear nucleus, travels through the Superior olive to the inferior colliculus and MGN to the auditory cortex.

34. Major Structures of the Ventral Auditory Pathway Spiral ganglion - spiral band of auditory nerve cell bodies in wall of modiolus Auditory nerve - fibers enter modiolus and exit toward the brainstem Ventral cochlear nucleus - brainstem nucleus - ipsilateral innervation, monaural response properties Superior olive - each side is innervated from both ventral cochlear nuclei - binaural response properties Medial geniculate nucleus - next to LGN, auditory thalamic relay nucleus

35. Other Important Structures Acoustic radiation - fibers from MGN to A1 - auditory cortex Auditory cortex - A1, Brodmann area 41, superior surface of temporal lobe Secondary auditory cortices - e.g. Wernicke's area, etc.

37. Tonotopic Maps Frequency sensitivity is caused by properties of the Basilar Membrane. The map of sound frequency from the basilar membrane in cochlea is preserved like the retinotopic map of visual system When neurons synapse, they do so in an organized pattern based on characteristic frequency. Systematic organization of characteristic frequency is called tonotopy There are tonotopic maps on the basilar membrane, the MGN, the auditory cortex, and within each of the nerve relay nuclei. Tonotopy allows for the location of the impulse to indicate frequency.

38. Sound Intensity Louder sounds cause the basilar membrane to vibrate with greater amplitude More intense stimuli produce movements of the basilar membrane over a greater distance, which leads to the activation of more hair cells More action potentials occur because of the greater movement of the basilar membrane

39. Coding Sound Intensity Two ways to code sound intensity Number of active neurons and firing rates of neurons Population code as sound intensity increases, the deflections of the basilar membrane stimulating IHCs broaden more and more IHCs are activated Rate code as sound intensity increases, the receptor potential in IHCs grows larger the auditory nerve fibers fire faster Together they tell the brain the value of sound intensity This produces the sensation of loudness

40. Coding of Sound Frequency Place code – according to the tonotopic map, different frequency sounds cause deflections of the basilar membrane at different places in the cochlea – which IHCs are activated indicates what the sound frequency is Phase locking – for sound frequencies below 4,000 Hz, the timing of action potentials in the auditory nerve is locked to the cycle of compression & rarefaction in the sound wave timing of action potentials codes for sound frequency Together these two codes produce the sensation of pitch

41. Frequency Variation For very low frequencies (below 200 Hz), only phase locking codes frequency This is because there aren't dedicated fibers For medium frequencies (200-4000 Hz), both place code and phase locking code frequency For high frequencies (4000-20,000 Hz), only place code indicates sound frequency phase locking stops

42. Localizing Sound in the Horizontal Plane Time differences between ears For frequencies 20-2000 Hz, the phase locking in firing patterns from the two ears are compared The difference in timing between them specifies the location of the sound source Intensity differences between ears For frequencies above 2000 Hz, the head produces a significant shadow on the sound waves The differences in intensity between the two ears are compared to localize the sound

43. Localizing Sound in the Vertical Plane Works as well with one ear as with two ears Covering up the pinna eliminates this capability Comparison of direct and secondary reflected sound paths from the wrinkles on the pinna enables us localize vertically

44. The Auditory Cortex Layers similar to visual cortex 6 of them A1 has a tonotopic map with low frequencies represented anteriorly and high frequencies represented posteriorly Most A1 neurons are sharply tuned for frequency All are binaural Some are excited by both left and right ears (EE) Some are excited by one ear and inhibited by the other ear (EI)

45. Cortical Modules in the Auditory Cortex Each vertical column has cells sensitive to the same frequency Adjacent columns in anterior-posterior direction change frequencies in order - tonotopy Adjacent columns in lateral-medial direction change from EE to EI to EE like ocular dominance columns Analogous to cortical modules in Area 17

46. Cortical Damage Unlike the visual system, damage to auditory cortex often has little effect on basic hearing More often ability to understand speech or some other complex ability is lost Damage to cochlea, auditory nerve, or cochlear nuclei are more typically causes of deafness

47. Auditory Disorders Conduction deafness blockage in sound conduction: wax in ear, disarticulated ossicles, stiffening of insertion of stapes footplate into oval window. Usually correctable with surgery Nerve deafness damage to hair cells or auditory nerve fibers from tumors, ototoxic drugs, loud sounds, etc. No treatment for nerve deafness, but partial loss can be compensated for by various hearing aids. Prevention is important.

48. Tinnitus Ringing in the ears A common phenomenon Caused by hyperactivity of cochlear amplifier The sounds of tinnitus are actually occurring in the cochlea and one is simply hearing them they originate in cochlea and mask incoming external sounds May indicate sound damage, cochlear disease or vascular abnormalities Tinnitus after rock concerts is very common (and not healthy)!

49. Auditory Perception Like vision, auditory sensations are organized and interpreted in the brain to create auditory perceptions Like visual perception, auditory perception is relative Brain also makes “assumptions” Basis of auditory illusions: Shephard Tones Tri-tone illusion McGurk Effect

50. Shepard's Tones Circularity of judgment of relative pitch These tones eliminate all relative pitch discrimination information. As a result, when played in sequence, each tone sounds higher than all tones preceding it and lower than all tones following it (and vice versa when the sequence is played in the opposite order). Since there are only twelve tones in the sequence, played in a continuous loop, every tone sounds both higher and lower than every other at some point in the sequence.

51. The Tritone Effect Although pitch discrimination cues have been removed from Shepard's Tones, proximity information remains. 2 consecutive tones are always separated by a single semitone. Although you can't determine which is higher based on the tones alone, your choice is that the second tone is either one semitone higher or eleven semitones lower in pitch than the first. It is natural for the smaller distance to be selected. What if the proximity cue was removed? If the second tone played is either half an octave higher or half an octave lower than the first Result is the tri-tone effect (The midpoint of the octave is called the tritone )

52. The Risset Scale This is actually a single octave of twelve notes! Each note, however, is actually a chord. Each chord is comprised of six individual notes from six different octaves. The notes of each chord have the same pitch (6 C's, 6 D's) - but they are played at 6 different volumes. This creates ambiguous information for the listener. The Risset Scale blends each tone from this special octave into the next tone, over and over again. This blending, combined with the complex and ambiguous tonal information of each note, creates the illusion of an endlessly rising or descending tone.

53. McGurk Effect What am I saying? Alternate between looking at the talking head while listening, and listening with your eyes shut. Most adults (98%) think they are hearing "DA" a so called "fused response" the "D" is a result of an audio-visual illusion In reality you are hearing the sound "BA", while you are seeing the lip movements "GA".

55. The Scale Illusion A scale with successive tones alternating from ear to ear The scale is played simultaneously in ascending and descending form When a tone from the ascending scale is in the right ear, a tone from the descending scale is in the left ear, and vice versa When heard through earphones produce a number of illusions

56. A Variant Scale Illusion When listening to this pattern through loudspeakers, notice that when each channel is played separately, it appears to shift dramatically in pitch, but when both channels are played together, two smooth melodies are heard. The brain creates order out of chaos.

57. Name That Tune Knowledge of a piece of music influences what we hear All of the notes of a well known tune are correct, but the tones are distributed randomly across three octaves In the second clip, the notes are the same, but now they are all in one octave