Applied Psychoacoustics Lecture 1: Anatomy and Physiology of the human auditory system Jonas Braasch

Overview of the Human Ear

Outer Ear Pinna: External cartiledge –Provides direction dependent frequency cues for sound localization through spectral filtering –Position can be actively controlled by some mammals (e.g., cat) Meatus (Auditory Canal) –Pathway to the middle ear, approx. 7mm diameter, 27mm length –Amplifies sounds in the range of 2000 to 5000 Hz through resonance (approx. 10 – 15 dB)

Simulation of the sound pressure wave in the ear canal frontal, 2.7 kHz 2 lateral, 10 kHz 3 rear, 2.7 kHz

Photo of ear drum

Middle Ear

Middle Ear

Tympanic Membrane –Sound pressure vibration is trancduced into mechanical oscillation and passed on to the malleus –protects ear (e.g, water, wind) Ossicles –Malleus, incus, stapes (hammer, anvil, and stirrup) –are the Smallest bones in human body Muscles –Stapedius muscle (connected to the stapes) –Tensor tympani muscle (connected to the malleus) –are the smallest muscles in the human body Oval Window –connection to the cochlea Eustachian Tube –connects the middle ear to the throat for pressure relief

Function of the middle Ear Is an impedance transformer Without it difference in densities of air and the cochlear liquid would result in lossy energy transfer Pressure increase the pressure between the oval window and the ear drum by nearly a factor of 30 –Amplitude ratio ear (drum/stapes) ~1.3:1 –Area ratio (ear drum /oval window): ~20:1

Acoustic Reflex Transmission can be attenuated in the middle ear by stiffening the Stapedius muscle and the tensor tympani muscle to protect the inner ear Is controlled by the auditory system and react to loud sound exposure

Arrangement to measure the pressure-force transfer function of a middle ear (RUB-IKA)

Vibration of the ossicular chain

Inner Ear Semicircular canals Cochlear Auditory Nerve

Basilar Membrane

The Traveling Wave in the Basilar Membrane

Frequency Mapping on the BM Logarithmic Frequency Mapping

Traveling Wave Simulation Hz Tone 1000-Hz Tone 4000-Hz Tone

Neurotransmitters are chemicals that enable communication between two neurons are released from one neuron at its presynaptic nerve terminal and cross the synapse, a small gap, to the receptor of the second neuron

Connecting the ear to the auditory pathway 95% of auditory nerve fibres (Type-I fibres: large diameter, myelinated) innervate IHCs ( to a single IHC) sending information to the CNS 5% (Type-II fibres: thin, unmyelinated) innervate OHCs (each fibre innervating OHCs)

Tonotopic organization of auditory nerve and cochlear nucleus

Definition Tonotopy (from greek tono- and topos = place: the place of tones) is the spatial arrangement of where sound is perceived, transmitted, or received. It refers to the fact that tones close to each other in terms of frequency are represented in topologically neighbouring neurons in the brain. from Wikipedia

Cell Response Types Primary-like (PL) Primary-like, notch (PL-N) Phase-lock (onset) Onset, lock (O-L) Chopper

Cell types time Acoustic stimulus Primary-like chopper onsetPrimary-like, notch

Tuning Curves

Post Stimulus Time (PST) Histogram

Phase Locking sound pressure action potential

Physiological Coordinate System DirectionDescription LateralAway from the midline MedialToward the midline BilateralOn both sides of the body or head IpsilateralOn the same side of the body or head ContralateralOn the opposite side of the body or head