Sound frequency (pitch, tone) measured in hertz (cycles per sec)

Transcription

1 Psych 2200, Class 15 Hearing and the Auditory System Oct 20, 2015 **ONLINE LECTURE See webpage or for link** Sam office hours this week wed 11-12p, thurs 1-4p, Friday 1-2p. Sound frequency (pitch, tone) measured in hertz (cycles per sec) If 2 msec (0.002 sec) is the interval, then the stimulus frequency is 500 Hz (1/ sec = 500 Hz) 1

2 Sound amplitude (loudness, sound pressure (SPL)), measured in decibels (db) soft loud Sound Intensity (db) Ticking of Watch (20) Whisper (30) Normal Speech (50-60) Car Traffic (70) Chain Saw (110) Jackhammer (120) Jet Engine (130) A plot of frequency X minimum db audible = audiogram AMPLITUDE FREQUENCY 2

3 EXTERNAL (skin & air) MIDDLE (bones & air) INNER (cochlea & fluid) Semicircular canals code vestibular info, and send axons to the vesibular nerve which joins with the auditory nerve to become the vestibulocochlear cranial nerve 3

4 Scala vestibuli, perilymph Scala media, endolymph Reissner s membrane Tectorial membrane Spiral axons Scala tympani, perilymph Organ of Corti Basilar membrane Vibrations in the bony stapes (connects to oval window) cause vibrations in the perilymph (like squeezing and bulging in a water balloon). The fluid motion induces corresponding vibrations in underlying Basilar (and Reissner) membranes. Since the endolymph of the Scala media is in a separate compartment, the Tectorial membrane does not pick up fluid vibrations. The Organ of Corti contains inner and outer hair cells which have their base in the Basilar (vibrating) membrane and whose tips (bundles of serteocilia filled with actin) project into the Tectorial (non-vibrating) membrane. This mismatch leads to transduction. TRANSDUCTION FROM VIBRATION TO NEURAL IMPULSE hair cells (cilia) 1 Tectorial membrane Basilar membrane 2 Tectorial membrane hair cells (cilia) Basilar membrane Motion causes ion gates of hair cells open to K+, leading to graded depolarization of the hair cell Hair Cell Bi-polar spiral neuron 4

5 10/18/15 A side note on sensory transduction and action potentials For touch/pain & prioprioceptive (deep tissue) receptors, the receptor is part of the sensory neuron -- a special modification in the sensory neurons dendritic ending. This leads to direct action potentials in the sensory neuron when stimulated. Unipolar sensory neuron Sensory dendrite 5

6 For smell, (taste), hearing, and vision, the sensory receptors are NOT neurons. They produce graded depolarization only, but release transmitter when stimulated that leads to an action potential in an associated (secondary) neuron: Smell olfactory hair cell -> mitral neuron Hearing hair cell -> spiral neuron Vision photoreceptor/bipolar -> ganglion neuron Hair Cell, graded potential (or depolarization) Bi-polar spiral neuron can fire action potential Spiral dendrite contacts the hair cell. Tectorial membrane Basilar membrane Spiral axon runs in the cochlear and then vestibulocochlear (cranial) nerve. 6

7 Type I spiral neurons (95%) ennervate a single inner hair cell. Therefore each Type I neuron exhibits the prefered frequency of its hair cell. Type II small, unmyelinated spiral neurons branch to connect multiple outer hair cells, generally in the same row. 7

8 Outer hair cells don t detect sound -- they fine-tune the Organ of Corti by changing their sterocilia legth based on feedback from the brain. Individual type I axons in the auditory nerve are often called "auditory nerve fibers. Since each connects to only 1 hair cell, Each has its own characteristic frequency (CF) Auditory nerve fiber tuning curves The prefered frequency (most sensitive) is the characteristic frequency (CF), which is the tip of the above curve (each color=unique axon) 8

9 Apex= low Base= High The deformation ( ripple ) of the basilar membrane is a traveling wave. When motion of the stapes creates a sound wave in the fluid of the inner ear, each region of the basilar membrane ripples in response to this pressure. The part near the base, with its high resonance frequency, moves first --- followed by lower frequency (more apical) segments. Base= High Apex= low Apex=Low Base=High 9

10 SOUND WAVES AND THE BASILAR MEMBRANE -- MOVIE CLIP Hair cells with preferred frequency or characteristic frequency (CF) are organized in a low-to-high format (called TONOTOPY). This starts at level of hair cells in cochlea and is conserved all the way up primary divisions of structures in the auditory system. 10

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12 ENCODING FREQUENCY (tone, pitch) measured in hertz or cycles per sec Single spiral neurons CANT fire fast enough to encode high frequencies. The frequency encoding problem is solved by: 1) SPATIAL SEGREGATION (TONOTOPY). 2) PHASE LOCKING (collective firing of sub-groups of neurons in sync with frequency rate). soft loud ENCODING AMPLITUDE (intensity, loudness, sound pressure) measured in decibels Intensity encoding starts at the auditory nerve with: 1) NEURAL FIRING RATE 2) SPATIAL SPREAD Louder Louder 12

13 YIPES -- a lot of terms!! Which are the most important 1. Divisions of the ear -- outer (ear & ear-drum), middle (bones), inner (cochlea). 2. Parts of the cochlea -- oval window, perilymph, basilar & tectorial membranes, organ of corti, hair cells (inner & outer), spiral neurons. 3. Transduction at the hair cell -- stereocilia bend due to vibrations in the basilar membrane while tectorial membrane stays still. Bending causes depolarization, spiral neuron fires. 4. Tonotopy -- the basilar membrane is organized so that the base vibrates preferentially to high frequencies, and the tip to low frequencies. The hair cells for each region thus have a preferred or characteristic frequency (& so does their spiral neuron). 5. Encoding frequency -- tonotopy, phase-locking. 6. Encoding amplitude -- firing rate, spatial spread. 7. Auditory structures -- cochlea, cochlear nucleus, olivary nucleus, inferior colliculus, medial geniculate nucleus, A1, A2. AMPLITUDE MODULATION (AM) 13

14 FREQUENCY MODULATION (FM) FM vs AM signals /ba/ /da/ Frequency (Hertz) 4,000 3,000 2,000 1, F 4 F 3 F 2 F 1 TIME (ms) Spectrograph for consonant-vowel (CV) syllables /ba/ and /da/. 14

15 DOLPHINS USE SOUND TO SEE -- MOVIE CLIP 15

16 Classification of hearing loss relates to severity (complete deafness vs hearing-impaired); mechanism (peripheral vs central); and timing. Congenital - Peripheral (abnormalities of the hearing apparatus) - Central (abnormalities in auditory nerve or brain regions) Progressive -- Due to degeneration of hair cells or nerves Injury-induced -- birth trauma (hypoxia); jet engine engineer Age-related hearing loss -- loss of tympanic membrane flexibility, hair-cell loss Mechanical (peripheral) abnormalities can sometimes be surgically fixed, or treated with cochlear implant (candidacy is best for congenitally deaf or low residual hearing children, or adults deafened post-lingually). Partial/progressive loss can be ameliorated with hearing aids. 16

17 A hearing aid can be used to boost a signal, if there is only a partial hearing loss ( hard of hearing ). A hearing aid cannot help if there is no sound processing to begin with. A cochlear implant can be used if hearing loss is profound and cannot be boosted by hearing aids. The cochlear implant has severe limitations including: -loss of any residual hearing due to destruction of hair cells; -effective mainly for recent loss (deaf children, and adults with hearing loss) 17

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19 For the profoundly deaf, a viable alternative is visual sign language (ASL, BSL, JSL many dialects). In fact, within the Deaf Community, many view deafness as a non-medical -- but rather, cultural -- affiliation, and celebrate the birth of deaf children. Many technological innovations (blinking alarms, text-phones) make this possible. Similar adaptations and cultural affiliation are not seen in the blind community. 19

20 For those with congenital or very-early acquired deafness, a re-organization of auditory cortex occurs. Remember -- Cortical plasticity is greatest in children Although the auditory area of the brain is not being used, deaf individuals do rely heavily on vision, for sign language and other information. In fact, in the congenitally deaf, Wernicke s area is re-allocated to visual speech (American Sign Language or other languages). Thus a stroke to Wernicke s in a congenitally deaf individual will cause difficulty interpreting ASL (or other sign languages) Wernicke s Broca s JSL = Japanese Sign Language (study done in Japan) Age-dependent plasticity in the superior temporal sulcus in deaf humans: a functional MRI study, BMC Neuroscience Norihiro Sadato1,2*, Hiroki Yamada2, Tomohisa Okada1, Masaki Yoshida4, Takehiro Hasegawa4, Ken-Ichi Matsuki4, Yoshiharu Yonekura5 and Harumi Itoh3 20