Transcript November 3
The Auditory System Audition (Hearing) Transduction of physical sound waves into brain activity via the ear. Sound is perceptual and subjective. Structure of the ear. Pathways from the ear to the cortex. MGN (medial geniculate nucleus of the thalamus) The Nature of Sound When displaced by a moving object, air becomes compressed. Vibrations produce periodic patches of compressed air. Frequency is the number of such patches per second (Hz). Intensity is the amount of air pressure (dB, decibels). Physics of Sound Frequency – Number of cycles completed by a wave in a given amount of time Low Frequency High Frequency Physics of Sound Amplitude – Physical strength of a wave High Amplitude Low Amplitude Pitch, Loudness, and Timbre Pitch – Sensory characteristic of sound produced by the frequency of the sound wave Loudness – Sensory characteristic of sound produced by the amplitude (intensity) of the sound wave Timbre – Quality of a sound wave that derives from the wave’s complexity Anatomy of Auditory Perception Outer ear – directs sound waves to tympanic membrane, pinna localizes sound. Middle ear – amplifies the wave and transmits it to the fluid-filled inner ear. Eustachian tube – equalizes pressure and protects ear from loud noise Inner ear – cochlea transduces sound waves into neural signals. Auditory Pathways Auditory receptors in cochlea exit via auditory nerve. Brain stem neurons at superior olive permit sound localization. Separate pathways for each ear up the brain stem. MGN Auditory cortex Tonotopy Portions of the basilar membrane and frequency selective. Frequencies maintain their relation to each other in the MGN and auditory cortex. Phase locking represents low frequencies, tonotopy and phase locking both represent mid level frequencies, tonotopy alone is useful at highest levels. Attenuation Reflex Muscles contract to make the ossicles more rigid, reducing sound transmission to the inner ear – protects the ear. Operates more at low frequencies. Prevents saturation, making high frequency sounds more discernible. Makes speech easier to understand in a noisy environment. Sound Localization Interaural time delay Interaural intensity difference (sound shadows) Detected at superior olive Works at low frequencies Works at high frequencies Pinna localizes sounds vertically (from above and below)