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Chapter 7

Chapter 7: Audition

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PSYC 211
Yogita Chudasama

Stimulus: when object vibrates, causes air molecules to condense/rarefy, makes waves of 700 mph. if vibration btwn 30/20,000 times/sec, waves will stimulate receptor cells, be perceived as sound. Pitch: frequency measured in Hz (cycles/sec). Loudness: intensity, degree condensation/rarefaction differs. Timbre: nature of particular sound. Ear is analytical (not synthetic), because we hear both original tones instead of intermediate.  Anatomy: sound goes through pinna (outer) through canal to tympanic membrane (eardrum), which vibrates. Middle ear: bones are ossicles, vibrate from tympanic. Malleus (hammer) links w/ tympanic and transmits vibration via incus (anvil) and stapes (stirrup) to cochlea: has receptors, part of inner ear. Filled w/ fluid. Baseplate of stapes presses against membrane behind oval window (opening in bones around cochlea). Cochlea divided into scala vestibuli/media/tympani. Organ of Corti has hair cells anchored by Deiter’s cells to basilar membrane; cilia pass through reticular membrane, ends link tectorial membrane. Waves caused basilar to more relative to tectorial, which bends cilia, makes potentials. Vibrations on oval window cause basilar to bend, det. by frequency. Round window lets cochlear fluid move.  Auditory hair cells, transduction of auditory info: inner/outer cells of cochlear coils have cilia. Cells form synapses w/ dendrites of bipolar neurons. Waves cause basilar/tectorial membrane to flex, bending cilia. Actin covered by myosin filaments make cilia stiff, linked via tip links at insertional plaques. Bending of ciliareceptor potentials. Fluid around cells is K-rich. Each plaque has TRPA1 cation channel, which opens when bundles move, and membrane depolarizes, NT release increases.  Auditory pathway: o link w/ cochlear nerve: branch of auditory nerve, sends info Cortibrain. Each nerve has 50,000 afferent axons, thick/myelinated form synapses w/ inner ear cell, which forms synapse w/ 20 fibers. The rest form synapses w/ outer hair cells, 1:30 ratio, thin/unmyelinated. Inner hair cells necessary for normal hearing. Outer hair cells are effectors, alter basilar membrane and influence effects of vibrations on inner hair cells. Cochlear nerve has efferent too, from superior olivary complex in medulla (efferent fibers = olivocochlear bundle). Form synapses on outer hair cells, dendrites of inner hair cells. NT at afferent synapse = glutamate. Efferent buttons secrete ACh, which inhibits. o Central auditory system: axons enter cochlear nucleus of medullasuperior olivary complex lateral lemniscusinferior colliculus (dorsal midbrain)medial geniculate nucleus (thalamus) auditory cortex (temporal lobe). Tonotopic representation: relationship btwn cortex and basilar membrane responding to different frequencies. Core region is primary auditory cortex, on gyrus of dorsal surface of temporal lobe, 3 regions get separate tonotopic map of info from ventral division of medial geniculate: belt region (surrounds PAC, has 7 divisions, gets info from PAC and dorsal/medial divisions of medial geniculate nucleus), parabelt region (gets info from belt region).  Perception of Pitch: cochlea detects frequency 2 ways: moderate/high = place coding, low = rate coding. o Place coding: dif. locations on basilar membrane, frequency coded by active neurons. Kanamycin/ neomycin produce degeneration of hair cells, beginning at basal end of cochlea towards apical, parallels hearing loss, highest frequency goes first. Cochlear implants: restore hearing from damage to hair cells, electrode developed to duplicate place coding of pitch on basilar membrane – when different regions are stimulated, you perceive sounds w/ different pitches. Although basilar membrane codes for frequency along its length, not specific. Outer hair cells responsible for selective tuning, amplification of vibration of basilar membrane produced by sound waves. 221 MORE INFO o Rate coding: coded by rate of firing of neurons in auditory system, esp. lower frequencies. Stim. of single electrode w/ pulses of electricity  sensations of pitch proportional to frequency  Perception of loudness: cochlea sensitive, can detect sounds when eardrum vibrated less than diameter of H atom. Ear limited by noise of blood, not sensitivity of auditory system. Softest sounds move hair tip 1-100 pm, max response when moved 100 nm. Axons of cochlear nerve inform brain of loudness by altering rate of firing; louder soundsmore intense vibrations on eardrum/ossiclesmore intense shearing force on cilia of auditory hair cells release more NThigher rate of firing by cochlear nerve axons. This means pitch signaled by which neurons fire, loudness signaled by rate of firing. Loudness of low-frequency sounds is signaled by number of axons arising from active neurons. P222  Perception of timbre: we hear sounds w/ rich mixture of frequencies-complex timbre, lets us to distinguish types of sounds. Fundamental frequency: lowest, most intense frequency of, basic pitch. Has series of sine waves that includes fundamental frequency and many overtones, multiples of fundamental frequency. Different instruments make overtones w/ different intensities. When basilar membrane stimulation by sound, different portions respond to each of overtones, response produces unique anatomically coded pattern of activity in cochlear nerve, which is id’ed by circuits in auditory association cortex.  Perception of complex sounds: task of auditory sysem in identifying sound sources is pattern recognition, must recognize htat particular patterns of constantly changing activity belong to different sources. Perception of enviro sounds and location: circuits in auditory cortex that perform analysis must get accurate info, timing of changes in components of sounds must be preserved all the way to auditory cortex. Neurons that convey info to auditory cortex have special features that let them conduct info rapidly/accurately, b/c axons have special low-threshold voltage-gated K channels w/ short action potentials. Buttons are large, release large amount of glutamate, postsynaptic membrane has NT-dependent ion channels that are very quick, so synapses make strong EPSPs. Buttons form synapses w/ somatic membrane of postsynaptic neurons, minimizing distance from synapse to axon, and thus min delay in conducting info to axon of postsynaptic neuron. Judgments of pitch activate ventral regions (what) and judgments of location activated dorsal regions (where). Dorsal streams of visual/auditory systems overlap in parietal lobe. Recognition of complex enviro sounds activated region known to be involved in knowledge about world and objects. When sounds played backwards, still activated auditory cortex, but only recognize sounds activated region of left hemisphere on posterior middle temporal gyurs (activated by recognition of objections by seeing/hearing them). Lesions of auditory association cortex can produce auditory agnosias, not deafness. o Perception of music: special form of auditory perception, has sounds of various pitches/timbres played in particular sequence w/ underlying rythmrules vary in different cultures. Melody recognized by relative intervals between notes, not by absolute value. Musical perception requires recognition of sequences of notes, adherence to rules, harmonic combos, rhythmical structures, memory capacity. Analysis of music begins with subcortical auditory pathways and primary auditory cortex, more complex aspects analyzed by auditory association cortex. Different regions of brain vinvolved in different aspects of musical perception: inferior frontal recognizes harmony, right auditory involved in perception of underlying beat, left auditory perceives rhythmic patterns superimposed on rhythmic beat, cerebellum and basal ganglia involved in timing of rhythm. Amusia: inability to recognize music, but may still know if it’s happy or sad. Response of auditory cortex to tones played on piano was greater in musicians, related to age at which they began training (if earlier training then greater incrase in activation). Size of primary auditory cortex much larger in musicians, also volume of gray matter of anteromedial primary auditory cortex.  Somatosenses: provide info about what is happening on surface of body and inside it. Cutaneous (skin) sense include submodalities referred to as touch. Kinesthesia provides info about body position/movement, arises from receptors in joints, tendons, muscles. Organic senses arise from receptors in/around internal organs, give us un/pleasant sensations. Stimuli: pressure, vibration, heating, cooling, tissue damage. Feelings of poressure caused by mechanical deformation of skin. Kinesthesia providd by stretch receptors in skeletal muscles that report change sin muscle length to CNS and by stretch receptors in tendons that measure force exerted on muscles. Receptors in joints between adjacent bones respond to magnitude/direction of limb movement. Muscle length detectors give info to control movement.  Anatomy of skin and receptive organs: skin has subcutaneous tissue, dermis, and epidermis w/ various receptors throughout layers. Glabrous skin is hairless, found on fingertips, palms, bottom of foot. Hairy skin has unencapsulated nerve endings, Ruffini corpuscles (respond to indentation of skin), Pacinian corpuscles (respond to rapid vibrations, largest sensory end organs in body). Free nerve endings that detect painful stimuli and T changes are just below skin surface, other free nerve endings found in basketwork around base of hair follicles, around emergence of hair shafts from skin, fibers detect
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