Neuroscience 500 What is sound?

Dec 12 2007 • Pressure waves generated by vibrating air molecules.

• Propagate in 3 dimensions The Auditory and Vestibular Systems • Molecules of air alternatively compressed and released (rarefaction) Dr. Brian D. Corneil [email protected]

Features of sound waves 200 Hz 500 Hz

1. Amplitude (measured in dB) 1 kHz Loudness 5 kHz 2. Frequency (measured in Hz) Pitch 10 kHz 3. Waveform (e.g., simple versus 15 kHz complex). Timbre 16 kHz 4. Phase 17 kHz

18 kHz

19 kHz

20 kHz

Capacity of . Part 1 Speech Capacity of auditory system. Part 2

Most sounds and complex Loudness can vary over a waveforms composed of 12 range of 120 dB (about 10 ) multiple frequencies Sensitive to vibrations down to the diameter of an atom of gold The auditory system performs a type of Fourier Transform, breaking such complex Frequency can vary between waveforms into the component 20-20,000 Hz frequencies

1 The human is a sense organ specialized for and balance Capacity of auditory system. Part 3 Outer Middle ear ear

• CNS can respond up to ~1,000 Hz max Vestibular apparatus (for balance)

Pinna

Nerve • How then can the auditory system: 1. Represent frequencies greater than 1,000 Hz? 2. Represent loudness over a 120 dB range?

3. Decode complex waveforms? (for hearing)

Tympanic membrane (ear drum)

Outer ear • Pinna – Helps direct sound into ear canal, • Interface between air in and fluid amplifying certain frequencies (eg, 3kHz) in inner ear – Aids in sound localization of vertical targets • Fluids usual have a much higher impedance than air (~99% of sound waves • Auditory canal in air reflect off water) – Funnels and modulates incoming sounds • The Middle Ear acts as an impedance matcher, so that sound waves in the air • Tympanic membrane () are changed into pressure waves in the – converts air pressure changes into inner ear fluid mechanical vibrations

Middle ear • The inner ear is where the mechanical • Essentially an amplifier energy is transduced into neural signals • 3 small bones (the “ossicles”: , and ) transmit • Inner ear has two main sound vibrations mechanically to components: the cochlea – cochlea (for hearing). • Amplification due to: – 1. Leverage (malleus moves more than – vestibular labyrinth stapes) – 2. Force focusing device (Tympanic area > stapes footplate) • Both are filled with , which has a high K+ concentration

2 Transducing auditory signals:• The cochlea has three The canals separated by -Rests on membranes – scala vestibuli -Contains the hair cells receptors – scala tympani – scala media () • Basilar membrane forms base of scala media • Movement of stapes displaces oval window

• This sends a pressure wave, which is released at of hair cells embedded within • The pressure wave displaces the basilar membrane

Inner Hair transduction: Auditory transduction: auditory and vestibular systems -Mechanically-linked K+ • The shearing channels

motion of the -Bending of stereocilia hair cells towards (largest generates a one) opens channel, depolarizing , neural signal in increasing the auditory neurotransmitter release nerve - Bending in opposite direction will hyperpolarize hair cell, decreasing neuotransmitter release

Frequency mapping = Tonotopy How does the encode high frequencies > 1kHz? • Both hair cells and auditory nerve fibres respond best to a narrow range of frequencies • Activation of a particular nerve fibre is interpreted at a particular frequency, regardless of rate of action potential Hair cell Auditory nerve fibre

Sound produces a travelling wave along the basilar membrane

The location of maximum displacement varies with frequency

Tonotopy is maintained throughout auditory system

3 How does the brain represent loudness Central Auditory Pathways over a 120 dB range? • Ascending pathways pass • Loudness coding cannot be a simple result through several different of increases in neural firing rate nuclei before reaching the cortex

• Several other mechanisms: • Parallel pathways

– Multiple sets of with different • All information leaving the thresholds ear projects bilaterally – Recruitment of additional neurons as loudness increases

How does the brain localize Interaural time differences sound? • Sound takes longer to reach one ear than • Several cues available to localize sound another direction • Timing differences are typically very small (e.g., ~700 μs). Humans are – Interaural time differences sensitive to differences of around 10 μs (1 degree) – Interaural intensity differences • Better for low frequency sounds < 3kHz – Pinna cues • Binaural “Coincidence detectors” in auditory brainstem

Interaural intensity differences Pinna cues

• Head “shadows” sound: “Head-related transfer function” • Time and intensity cues greater differences for only give information more lateralized sounds about horizontal position, not the vertical plane • More effective for high frequency sounds > 3kHz • The convolutions of the pinna sets up a • Due to binaural characteristic pattern of interactions in auditory reflections brainstem • Sound stimuli must contain multiple frequencies

4 Primary auditory cortex: Percepetion Aphasia = language disorder

Caused by focal brain lesions (e.g., strokes or head injury)

Wernike’s aphasia: impaired comprehension. “Nonsense” words

Broca’s aphasia: Non-fluent, effortful speech. Good comprehension

- Located on superior temporal gyrus - Tonotopically organized - “Stripes” of neurons excited by both (“EE cells”), alternating with stripes excited by one ear and inhibited by the other (“EI cells”)

Wernicke’s aphasia Broca’s aphasia

Types of hearing loss 1. Conductive: Sound is unable to be transmitted through Cochlear prosthetic outer or middle ear. A mechanical defect. • Array of tiny electrodes implanted into - extremely loud sounds rupture eardrum or damage ossicles scala tympani. Between 8-20 electrodes - infection fills middle ear with fluid “wound” into cochlea -ear wax! • Incoming sounds analyzed by receiver 2. Sensorineural: damage to structures of inner ear that circuitry, and decomposed into affects hair cells, or to auditory nerve (nerve deafness). frequency components A transductive and/or peripheral defect. • Extracellular stimulation excites - extremely loud sounds damage Organ of Corti nearby auditory nerve fibers - ototoxic drugs that damage hair cells • Exploits tonotopic organization of - presbycusis (aging) due to atherosclerotic damage to frequency microvasculature of inner ear • Users must relearn the auditory world, -auditory nerve tumour but can have amazing long-term 3. Central: Damage to auditory pathways upstream from recovery of function. cochlea. A defect in the Central Nervous system. - e.g., tumours or strokes in the central auditory pathways

5 The

• Provides and body position in space

5 minute break?? • Bodies move in 6 degrees of freedom: 3 rotational 3 translational

How is this information derived from the vestibular organs?

The sense of balance originates within the Organs Vestibular Labyrinth

Each labyrinth consists of 2 otolith organs (the and the ) and 3

The otolith organs are two membranous sacs call the utricle and the saccule. • Semi-circular canals: sensitive to angular rotations In each, a portion of the membrane is thickened (the macula) • Otolith organs: sensitive to linear accelerations and static and contains the hair cells innervated by neurons of the 8th nerve position of the head relative to gravity

Otolith transduction What bends the hairs? At rest, the vestibular afferents have a baseline firing of ~100 Hz When the head moves, the inertia of the crystals bends the hair cells in the opposite direction When the head tilts, gravity “pulls” the crystals downwards

Rules of transduction similar to auditory system: Toward kinocilium Æ increase APs freq. • The hairs of the hair cells project into the , a Away from kinocilium Æ decrease AP freq. gelatinous material embedded with calcium carbonate stones (otoconia = ear stones). A given vestibular afferent can therefore signal motion or tilt in two directions • Hair cell transduction in the vestibular organs is the same as in the cochlea

6 Orientation of utricle and saccule TILT-TRANSLATION AMBIGUITY • With the head upright, the macula of the utricle senses • Linear accelerometers detect net acceleration regardless of horizontal translations (left/right its source (Einstein’s equivalence principle) and front/back) • For the otolith organs, this introduces an ambiguity between tilt and linear acceleration • The macula of the saccule is on the side, and sense vertical Real world examples? translations (up/down and front/back).

• In each macula, the hair cells Pilots must learn to ignore are oriented in all possible perception of excessive tilt directions. All possible tilt or during period of high translation directions can linear acceleration… therefore be represented across Arrows point towards kinocilium both sides of the head

Functional anatomy of semicircular canals How the semicircular canals sense rotations

Each canal has a swelling called the ampulla

A pliable membrane called the cupula spans and seals the inner diameter of the ampulla.

The stereocilia of the hair cells project • 3 semicircular canals per side into the cupula • One lies approximately in the horizontal plane. The others, the anterior and the posterior, are aligned vertically and lie perpendicular to each other

How the semicircular canals sense rotations Canals have a preferred direction of rotation, and work in pairs • When there is a change in head • Three canals per side, oriented roughly orthogonal to rotation, the endolymph lags behind each other. because of inertia, distorting the cupula • Each of the 6 canals is best activated by a different direction of head rotation (rotation toward the canal • Bending of the stereocilia towards increases afferent firing). the kinocilium increases the firing of vestibular afferents • The canals work together in a push-pull organization. When one canal is maximally active, the other is • Bending away from the kinocilium maximally inhibited. decreases firing of the vestibular afferents • For example, for rightward rotation, excitation occurs • Like the , canal afferents in the right horizontal canal, and inhibition in the left have a baseline firing rate of ~ 100 horizontal canal Hz, and can therefore signal rotations • All possible directions of head rotation are covered, in two directions and hence can be unambiguously encoded

7 Neural circuit for the VOR The Vestibular-Ocular Reflex (VOR) 3 arc: Vestibular afferent Æ interneuron Æ abducens nerve • The VOR stabilizes the retinal image during head rotations. An additional interneuron to the oculomotor nucleus ensures conjugate eye movement 3 • When the head rotates in one direction, the eyes 3 rotate at the same speed in the opposite SEQUENCE OF EVENTS: direction (ideal VOR gain = 1). 1. Leftward head rotation increases firing of afferents from left horizontal canal 2 2. 2nd-order neuron in vestibular nucleus 1 • This keeps gaze stable in space projects to contralateral abducens nucleus 3. Contraction of right lateral rectus muscle, and left medial rectus muscle (via interneuron in the MLF)

NYSTAGMUS (Greek = “Nod”) CAROLIC TESTING OF VESTIBULAR FUNCTION

-Lie subject on back (places horizontal canals vertical) -Cold water causes convection currents -Eye position of subject rotated clockwise at a constant rate in that decrease firing the dark -Warm water causes -Slow phase: driven by VOR, matches speed of rotation convection currents that increase firing -Quick phase: saccadic eye movements in direction of head motion to “reset” position of eye in orbit

CAROLIC TESTING OF VESTIBULAR FUNCTION - Used to test brainstem in unconscious patients Additional Resources - Assess VOR: Look for direction of slow and fast phase of and conjugate eye movements • Chpt 13 and 14 of “Neurosciences, 2nd ed”, Purves et al 2001 -Indicative of brainstem function (pictures below show slow phase) • Chpt 30, 31 and 40 of “Principles of Neural Science, 4th ed”, Kandel, Schwartz and Jessel

• Great animations available at: http://www.physpharm.fmd.uwo.ca/undergrad/sensesweb/ (For this website, concentrate on what we talked about in class!)

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