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Lecture 9

Lecture 9 - Feb 8.pdf

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McGill University
PSYC 211
Yogita Chudasama

PSYC211 Lecture 9 - Feb. 8 The Stimulus: • Sounds are vibrations of air molecules that are produced by objects • If the vibration ranges between 30 and 20,000 times per second, it stimulates receptor cells in the ear • Sound has three physical dimensions: • The pitch determines the frequency of the molecular vibrations. It is measured in hertz (Hz) or cycles per second • The loudness corresponds to the amplitude o intensity of the molecular vibrations • The timbre corresponds to the complexity of the sound. Provides information to the nature of the sound (i.e. train’s steam whistle) Anatomy of the Ear: • Unlike the eye, the ear does not mix sound waves that enter the ear; what you hear is two original tones. It is auditory system that helps develop the complexity of the tone and to identify the nature of the sound Sound is funneled through the pinna (the external ear) • • Sounds coming down the ear canal cause the tympanic membrane (the eardrum) to vibrate. These vibrations are transferred to the middle ear • The middle ear comprises thee ossicles (small bones of the middle ear: the malleus, incus and stapes) • The cochlea is part of the inner ear. It is a long coiled tube structure containing fluid. It also contains the receptors Cross Section Through Cochlea: • The cochlea is divided into three longitudinal divisions: scala vesibuli, scala media and scala tympani • The receptive organ is the organ of Corti. It consists of the basilar membrane, the tectorial membrane and hair cells • The hair cells are the auditory receptors • Fine cilia extensions of the hair cells attach to the tectorial membrane • Sound waves cause the basilar membrane to move relative to the tectorial membrane which bends the cilia of the hair cells. This bending of the cilia produces receptor potentials From the Ear to the Primary Auditory Cortex: • The organ of Corti sends auditory information to the brain by the cochlea nerve • The axons enter the cochlear nuclei where hey synapse • The axons of the cochlear nuclei then enter the superior olivary complex • Axons from superior olivar complex pass through a bundle of fibres (lateral lemniscus) and enter the inferior colliculus • The axons then pass to the medial geniculate nucleus of the thalamus which make their way to the primary auditor cortex of the temporal lobe Tonotopic Representation: • The major principle of cochlear coding is that different frequencies produce maximal stimulation of hair cells at different points on the basilar membrane • Like the basilar membrane, the auditory cortex is also organized according to frequency, i.e. different parts of the auditor cortex respond best to different frequencies • This organization of different frequencies of sound that are represented in different places of the auditory cortex is known as tonotopic representation Perception of Complex Sounds: • A principle function of the auditory system is to identify the sound • Perception of complex sounds is accomplished by neural circuits in the auditory cortex: • Axons have voltage-dependent potassium channels that produce very short action potentials • Terminal buttons are large and therefore release large amounts of glutamate • Postsynaptic membrane contains neurotransmitter dependent ion channels which act rapidly and produce strong EPSPs • Terminal buttons synapse with the membrane of the soma of the postsynaptic neuron. This minimizes the distance between the synapse and the axon The Primary Auditory Cortex: • The primary auditory cortex is hidden on the upper bank of the lateral fissure • The belt region is the first level of auditor association cortex • The parabelt region is the second level of auditory association cortex The “What” and “Where” Streams: Where Vision and Audition Converge • Dorsal stream – terminates in the posterior parietal cortex. Involved in sound localization (“where”) • Ventral stream – terminates in the temporal lobe. Involved with analysis of complex sounds (“what”) • Damage to the auditory areas will lead to auditory agnosia Perception of Music: Music is a special and complex form of auditory processing • • Particular combinations of musical notes can be perceived as happy, sad, pleasant, unpleasant, consonant, dissonant, etc. • Music perception requires: • Recognition of sequences of notes, • Rules that govern permissible pitches, • Rhythmic structure, • Memory capacity • Different regions of the brain are involved in different aspects of musical perception: • Recognition of harmony – inferior frontal cortex • Underlying beat - right auditory cortex • Rhythmic patterns superimposed on the rhythmic beat – left auditory cortex • Musical timing and movements – cerebellum and basal ganglia Recognition of Complex Auditory Sounds: (Lewis et al., 2004) • Presented subjects with “recognizable” recordings of environmental sounds (e.g. tools, pouring liquids, dropping objects) – activated region of the ventral stream • Sounds were presented backwards as well (preserved complexity but difficult to recognize) – does not activate auditor cortex Amusia: • After sustaining brain damage, patient I.R. developed complete amusia, the inability to perceive or produce melodic music • She could recognize different emotions expressed in music • She could also recognize environmental sounds, converse and understand speech • However, she was unable to tell the difference between consonant (e.g. harmony; pleasant sounding) and dissonant (unstable, transitional) music Birdsong: Auditory Communication in Birds • Bids raised in different regions acquire different dialects Echolocation: Auditory Communications in Bats • Echolocating bats use soun
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