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 High Amplitude Low Amplitude Amplitude – Physical strength of a wave

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 Detected at superior olive Works at low frequencies  Interaural intensity difference (sound shadows) Works at high frequencies  Pinna localizes sounds vertically (from above and below)