Speech and Hearing Science Final

Central Auditory System anatomy

• Cochlear nerve

• Ascending auditory pathway: brainstem, thalamus

• Auditory cortex

Cochlear nerve is also known as…

… acoustic nerve, auditory nerve

The cochlear nerve is part of which cranial nerve?

VIII: Vestibulocochlear

Vestibulocochlear nerve

• is entirely sensory and consists of two parts; vestibular and cochlear nerves

• vestibular nerve: it transmits vestibular impulses from the vestibular system

• cochlear nerve: transmits auditory impulses from the cochlea

Cochlear nerve function

• after it is stimulated, it sends an electrically coded sound to the brain

• sound is coded for frequency (pitch), intensity (loudness), duration, timing patterns (rhythm, rate)

• includes ascending auditory pathway: connection between the cochlear and the auditory cortex

Ascending auditory pathway

• ascending= sensory, traveling from periphery (ear) to the central nervous system (auditory cortex)

Ascending auditory pathway anatomy

cochlea→ 3 brainstem nuclei→thalamus→primary auditory cortex

Cochlear nuclei

• located in the medulla oblongata

• first brain structure of the central auditory pathway

• receives coded sound from cochlea→ leaves cochlea and the cochlear nerve enters the brainstem and synapses with the cochlear nuclei

Superior Olivary complex

• located in the pons

• analyzes intensity and time-of-arrival differences between the ears: beginning of sound localization

Inferior colliculus

• located in the midbrain

• end point of brainstem nuclei outputs

• vertical and horizontal sound location

• generates acoustic startle response, orients the body toward relevant stimuli, and discriminates pitch and rhythm

Acoustic startle response

rapid contraction of facial muscles evoked by sudden loud noises

Medial geniculate nucleus

• located in the nucleus

• relay point between the brainstem nuclei and the auditory cortex

Auditory cortex

• areas of auditory perception and reception in the temporal lobe in both hemispheres

• superior temporal gyrus (Heschel’s area)

Auditory cortex tonotopic organization

orderly representation of frequency created in the cochlea is preserved all the way to the auditory cortex

Right ear advantage

• the auditory system is not symmetrical

• when two different speech stimuli are presented simultaneously to both ears, listeners report stimuli more correctly from the right ear because of speech processing happening in the left hemisphere

Dichotic listening of speech sounds

psychological test used to study left-hemispheric dominance for speech processing; lateralization of brain function within the auditory system

Psychoacoustics

1. Physical world vs. psychological world

2. Psychophysics

3. Psychoacoustics

4. Psychophysical methods: threshold, limen

Physical world vs. psychological world

• the senses do not exactly let us know what is going on in the world, but they help the mind (consciousness) construct a “virtual world” in our heads

• the virtual world resembles the real world, but it is not its exact projection

Phys vs. Psych world sensory misinterpretation

• sensory misinterpretations are typically easily resolved with more sensory inputs

• Illusions, and auditory illusions

Illusions

sensory misinterpretation that are not resolved even with additional input

Auditory illusions

sensory misinterpretations in the auditory sphere

McGurk effect

• psychoacoustic effect that occurs when the auditory component of speech conflicts with the visual component leading to an altered perception of speech sounds

• what we see influences what we hear; non-auditory input affects what we hear

The Shepard Tone

• three sine waves (high, middle, and low pitched) layered on top of each other and separated by octaves

• continually ascending or descending in pitch

• never reaching the end

The Shepard tone is often used in…

movies, music to build anxiety and panic

Psychophysics

• branch of psychology that studies the relationship between the physical world (physical stimuli) and the psychological world (sensations and perception stimuli cause)

• psychological world and psychoacoustics

Psychophysics quantitively…

… investigates how much of a stimulus we can detect and how we detect differences between stimuli in the environment with our sensory systems (vision, hearing…)

Psychological world

the world created by our minds based on sensory input, and to a lesser extent, past experiences

Psychoacoustics

• branch of psychophysics

• studies the mental representation generated from nerve impulses that represent the acoustic input

Psychoacoustics: percept

mental representation of a phenomenon perceived in the real world (the thing we perceive)

Sound percept

Affected by three factors:

1. the physical characteristics of the conductive mechanism

2. inner ear processing

3. brain interpretation of the received input and its presentation to the conscious mind

Psychophysical methods

Research focuses focuses on:

1. the characteristics of the physical sound

2. the characteristics of the sound percept

3. how the physical sound and the sound percept compare

Physical sound is measurable, while percepts…

… are subjective and difficult to quantify

Threshold of hearing/absolute threshold

• dividing line between hearing and not hearing

• 50% rule

• hearing threshold changes with the sound frequency: our hearing is more/less sensitive at different frequencies

Equal loudness contours (Fletcher Munson)

graph that shows the intensity in dB(SPL) at which a frequency (pure tone) must be to be perceived as equally loud as a tone at another frequency

Phon

unit of perceived loudness (subjective)

Fletcher Munson equal loudness contours

• 1000 Hz: reference frequency

• it shows the amount of sound pressure required for a frequency to be perceived: the lower the line is, the less pressure/intensity is required to generate the perception

Hearing sensitivity for speech (“Speech banana”)

• the contours drop to their lowest point at around 4000 Hz

• the hearing mechanism is most sensitive to the 1000-6000 Hz range, which loosely corresponds to the speech frequencies

Speech banana

the area where most sounds of average conversational speech occur (250-8000Hz)

Threshold of pain

• the threshold at which a very loud sound changes to being very loud and accompanied by pain

• the pain threshold is not the same at each frequency: at some frequencies, less acoustic energy will cause pain than at other frequencies

• 120 dB: pain threshold at 1000 Hz

Limen types

• difference limen

• precise difference limen

• just noticeable difference

Difference limen

the amount by which one stimulus must be different from another for a person to notice that the two stimuli are different

Precise difference limen

the difference at which the listener detects that the sounds are different 50% of the time

Just noticeable difference limen

the difference at which the listener detects that the sounds are different 100% of the time