Audiology Unit 3: Psychoacoustics and Vestibular System

Vocabulary flashcards covering psychoacoustics, auditory measurement methods, auditory nerve physiology, the ascending central auditory pathway, and the vestibular system.

Psychoacoustics

The study of how humans perceive acoustic stimuli and the relationship between physical sound characteristics and psychological responses.

Method of Limits

A classical measurement method similar to clinical audiometry where the examiner controls stimulus level using alternating ascending runs and descending runs, determining threshold as the average of crossover points.

Ascending runs: begin below threshold and increase.

o   Descending runs: begin above threshold and decrease.

·         Threshold = average of crossover points.

Method of Adjustment

A measurement method where the subject changes continuously controls and adjusts the stimulus level until the sound becomes audible or inaudible, with threshold estimated from average crossover points.

Method of Constant Stimuli

A measurement procedure where multiple stimulus levels are presented randomly without ascending or descending runs, responses are plotted on a psychometric function, and threshold is defined as the stimulus level heard $50\%$ of the time.

Human hearing range

Approximately 20 Hz to 20,000 Hz, great sensitivity occurs between 2000-5000 Hz

Minimum Audible Pressure (MAP)

Hearing threshold measured monaurally (with one ear) using earphones, where acoustic pressure is assessed directly within the ear canal.

Minimum Audible Field (MAF)

Hearing threshold measured binaurally (in two ears) in a sound field; typically about $6\,\text{dB}$ better than Minimum Audible Pressure (MAP) due to binaural summation.

Upper limits of hearing

Uncomfrtobale loudness is ~100 dB SPL with pain/discomfort of 120-1240 dB SPL. If tehre is prolonged exposure above ~80dB SPL, it may damage hearing

Binaural Summation

The perceptual enhancement that occurs when listening with two ears instead of one, rendering Minimum Audible Field (MAF) approximately $6\,\text{dB}$ more sensitive than Minimum Audible Pressure (MAP).

Temporal Integration

The interaction between duration and intensity wherein the auditory system integrates energy for sounds shorter than about $200\,\text{ms}$, requiring approximately a $10\,\text{dB}$ increase in intensity for every 10-fold decrease in duration to remain equally audible.

For sounds < 200 ms:

·         Shorter duration → perceived softer

·         Longer duration → perceived louder

Just Noticeable Difference (JND)

The smallest detectable difference between two stimuli, denoted by difference limens such as $\Delta I$ for intensity difference limen and $\Delta F$ for frequency difference limen

Example

40 dB → 43 dB just noticeable

ΔI = 3 dB

Weber Fraction

Formula

or

·         Considers both the difference and starting point.

·         Measures relative sensitivity.

A measure of relative sensitivity that considers both the absolute difference and the starting baseline level ($\frac{\Delta I}{I}$). Frequency discrimination is best between 600-2000Hz, and is important because much speech information occurs in this range

JND = absolute difference

Weber Fraction = relative difference

Loudness

The psychological, perceptual correlate of physical sound intensity, which also depends upon frequency and bandwidth.

Critical Bandwidth (CBW)

The specific bandwidth threshold at which a perceptual change in loudness begins to occur; it is narrower/smaller at low frequencies and wider/larger at high frequencies.

Pitch

The psychological perception of frequency, requiring sound durations of at least $\sim 10\,\text{ms}$ at high frequencies and up to $\sim 60\,\text{ms}$ at low frequencies to be heard tonally rather than as clicks. Lowest tonal pitch ≈ 20Hz

Missing Fundamental

The perceptual phenomenon in which listeners perceive a pitch corresponding to a fundamental frequency even when that specific fundamental frequency is absent from the harmonic series.

Example:

·         Harmonics: 1800, 2000, 2400 Hz

·         Listener perceives a 200 Hz pitch.

Periodicity Pitch

Pitch derived directly from temporal waveform patterns calculated by the reciprocal of the interval period ($f = \frac{1}{T}$).

Pulse every 2 ms

Perceived pitch = 500 Hz.

Distortion Product Otoacoustic Emissions (DPOAEs)

Example:

·         f₁ = 1000 Hz

·         f₂ = 1200 Hz

Acoustic signals emitted by the inner ear that reflect healthy outer hair cell function, commonly tested using within audiology non-linear combination formulas like $2f_1 - f_2$.

Beats

Perceptual loudness fluctuations that occur when two tones of close frequencies are presented simultaneously, occurring at a rate equal to the frequency difference ($|f_1 - f_2|$).

Formula

Example:

·         1005 Hz

·         1000 Hz

Beat frequency = 5 Hz

Auditory Nerve Overview

• Inner hair cells transduce basilar membrane motion.

·         Type I auditory nerve fibers carry information to the brain.

·         About 20 auditory nerve fibers synapse with each inner hair cell.

Action Potentials

·         All-or-none neural discharges.

·         Continue firing even without stimulation.

Spontaneous Firing Rate

The continuous rate of action potentials fired by auditory nerve fibers in the complete absence of acoustic sound stimulation.

Post-Stimulus Time (PST) Histogram

A graphic display plotting the firing rate of an auditory nerve fiber over time during presentation of a sound stimulus, illustrating onset, adaptation, steady-state, offset, and recovery.

Displays auditory nerve firing over time during a sound stimulus.

Response Pattern

1.      Spontaneous activity

2.      Onset response

3.      Adaptation

4.      Steady-state response

5.      Offset response

6.      Recovery

Dynamic Range (Auditory Nerve)

The range in decibels between the neural threshold (lowest level eliciting a neural discharge) and neural saturation (point where discharge rate plateaus and no longer increases).

Dynamic Range

Example:

·         Threshold = 20 dB

·         Saturation = 60 dB

DR = 40 dB.

Place Coding

A mechanism of frequency coding where pitch is represented by the specific location of maximum vibration along the basilar membrane, operating across all audible frequencies.

Temporal Coding (Phase Locking)

A frequency coding mechanism where neurons discharge preferentially at a specific phase of the stimulus waveform, functioning up to approximately $3\text{--}4\,\text{kHz}$.

Major Structures (Know Order)

1.      Auditory Nerve

2.      Cochlear Nucleus

3.      Superior Olivary Complex (SOC)

4.      Inferior Colliculus

5.      Medial Geniculate Body

6.      Auditory Cortex

·         Predominantly crossed pathways

·         Binaural processing begins at SOC

·         Tonotopic organization maintained throughout pathway 

Cerebral Lobes

Temporal: Auditory processing, Memory functions

Frontal lobe: Thought, emotion

Parietal lobe: somatosensory processing

Occipital lobe: Vision

Superior Olivary Complex (SOC)

The third major auditory structure along the ascending central auditory pathway, and the primary site where binaural auditory processing begins.

Vestibular System Overview

Main Function

Maintains balance and spatial orientation.

Balance depends on integration of:

1.      Vestibular information

2.      Vision

3.      Proprioception/Somatosensory information

Semicircular Canals

Vestibular structures that detect angular acceleration and rotational movement of the head.

Utricle and Saccule

Otolith organs of the vestibular system that detect linear acceleration and head tilt/position relative to gravity.

Vestibulo-Ocular Reflex (VOR)

A physiological reflex that stabilizes vision/retinal images on a target during head movement by driving eye movements in the exact opposite direction of head motion. This allows fixation on a target

Clinical Importance

Many vestibular tests measure eye movements because of the VOR.

Vertigo

A false sensation of movement or spinning, distinct from general lightheadedness, which typically indicates vestibular dysfunction.

Clinical Importance

Many vestibular tests measure eye movements because of the VOR.

Motion sickness

One theory:

·         Vision indicates movement.

·         Vestibular/proprioceptive systems indicate no movement.

·         Sensory mismatch leads to nausea.