Perception WK 10 - Hearing

Sound

Sound is a pressure wave in the air

Can be split up into 3 main auditory percepts:

• Loudness - Sound Pressure Level

• Pitch - Frequency

• Timbre - Shape of sound wabe

Loudness

Measured by Sound Pressure Level (SPL) in decibels (dB)

Characterised by the amplitude of a sound wave on an oscillograph

Perceived loudness is the difference between 2 auditory stimuli is (Phons)

60 dB is normal speech loudness

Loudness can also depend on frequency (sometimes we cannot hear something if it is as a really high frequency)

Pitch

The frequency of sound waves measured in Hz

1 Hz = 1 Oscillation

Metameric concept - can appear to be the same pitch with different instruments

Timbre

Characterised by sound wave shape (pure tone vs complex sound)

• Pure tone = one consistent pitch; complex sound = contains mixes of frequencies to form a complex waveform

Determined by harmonics (the integer multiple of the fundamental frequency)

• Fundamental frequency = The lowest pitch in the harmonics which determine the perceived pitch → determines pitch

• Harmonics = The overall higher frequencies than the fundamental frequency in whole complex sound → determines timbre

Outer ear

Pinna (outer outer), Concha (outer)

These areas receive sound waves and funnel them into the auditory meatus and then the tympanic membrane (ear drum)

Selectively filters sound frequencies to provide cues about source elevation

Middle ear

Contains 3 ossicles: Malleus, Incus and Staples

These transfer the vibrations received from the tympanic membrane onto the labyrinth containing fluid (perilymph)

The pressure boosts up to 200x for impedance matching: being able to transfer sound through liquid instead of air

Inner ear

Contains cochlea

Stirrups moves like a piston into the labyrinth which sends vibrations through the perilymph into the cochlea

Cochlea

The cochlea contains two pathways sending signals up and down

The vibrations within the two pathways send signals to the organ of corti (sitting along the whole basilar membrane in between the two pathways) by moving stereocilia (hair cells) against the tectorial membrane and thus transferring neural signals

Basilar membrane acts as a frequency analyser, converting sound waves into precise neural signals

Inner hair cells

Sends signals to higher cerebral levels

One line of hair cells within the basilar membrane

About 95% of hair cells are within the organ of corti/auditory nerve

There about 3,500 inner hair cells in humans

Outer hair cells

3 lines of outer hair cells

Receives input from upper cerebral levels to modulate the signals sent by inner hair cells → either dampens (to decrease damage) or acentuates signal

Mechano-electrical transduction

The sterocilias’ movement triggers the K+ channels to open and therefore depolarise (due to the + ion in the cell)

This triggers an action potential to send through the auditory nerves

Auditory nerve

Sends action potentials to the brain

Each auditory nerve fires at peak displacement of the basilar membrane (i.e. when the basilar membrane’s hair cells move at a certain angle)

Phase locking: the neural responses synchronise to the stereocilias’ vibrations, resulting in a burst of action potential. Each auditory nerve is tuned to a “best frequency” which is the ideal frequency to synchronise and send an action potential

Frequency decomposition along the Basilar Membrane

Processing of frequencies vary along the basilar membrane (this is frequency decomposition)

The base processes higher frequencies, while the apex processes low frequencies

Topogaphic organisation = this frequency decomposition carries on up to the cerebral levels (A1 etc)

Tonotopy → the specific name for this frequency organisation along the auditory pathway

“Place” model

Zone of maximum excitation on the basilar membrane

WHERE the neuron fires

“Rate” model

The timing in between each neuron firing

The maximum action potential is shared among multiple neurons as one neuron cannot deal with all of the auditory processing/firing

HOW FAST neurons fire over time

Auditory pathway

Cochlear nucleus (brainstem) → Superior olivary complex (pons; passes to other side of brain) → inferior colliculus (midbrain) → Medial geniculate nucleus (thalamus) → Auditory cortex

Some information stays on the same side of brain (ipsilateral) for sound localisation

Auditory core region

A1, Rostral core, Rostrotemporal core

• A1 is the first order of auditory reception and deals with raw sounds, rostral and rostrotemporal get more complex

• Has topographic organisation which moves from low to high frequencies in each region

• Characteristic frequency = Dominant frequency in a system’s spectrum. Measured by frequency (Hz) : Amplitude (dB SPL)

Surrounded by belt and parabelt - deal with higher order auditory processing (e.g. speech)

Narrowly tuned neuron vs broadly tuned neuron

Narrowly tuned neuron only responds to a particular frequency (good for identifying precise frequencies)

• Characteristic frequency is precisely defined

Broadly tuned neuron responds to a range of different frequencies (good for complex sounds)

• Characteristic frequency is defined, but there are other frequencies in background