audition functions: adequate stimuli: class # 12&13: audition, p. 1

AUDITION

FunctionsFunctions::

Adequate stimuli:Adequate stimuli:

Class # 12&13: Audition, p. 1

Receptor apparatus

AUDITION


Sound pressure waves are funneled through the ext. canal eardrum bones of middle ear OVAL

window of cochleaThe tympanic membrane is 16 X the size of the oval

window!!

Thus, the middle ear transforms low energy sound pressure waves to small amplitude, but much more forceful displacements of the stapes against the oval window so that it can move fluid in the cochlea. (For example: picture a 300 pound person walking in stiletto high heel shoes.)

MOTION OF THE BASILAR MEMBRANE

AUDITION


Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed., 1991, McGraw Hill, New York, p. 595.

APEX: floppy (low frequencies)

BASE: stiff (high frequencies)

Receptors: “hair cells” anchored into basilar membrane, with their tips extending into the tectorial membrane. The entire structure is called the ORGAN OF CORTI.

AUDITION


Main CNS routes: (note the extensive crossing of information from one side of the brain to the other)

Auditory (8th) nerve Cochlear nucleus (in medulla/pons) Superior olive (in pons) Inferior colliculus (in midbrain) Medial geniculate nucleus (in thalamus) Temporal lobe of neocortex (“auditory cortex”)

AUDITION


Submodality coding: There are three main features of auditory perception:

LOUDNESS

PITCH

SOUND LOCALIZATION

AUDITION


Audiogram - A record of a person’s pure-tone hearing threshold levels

Threshold – A level of sound that a person can detect 50% of the time or more

Audiometric Zero – threshold of normal, young adults at 1000Hz

LOUDNESS

(1) Loudness is related to the amplitude of the sound wave, BUT also depends on the frequency of the sound pressure waves.

(2) Loudness is coded mainly by the number of neural fibers activated (& a little by their frequency of firing).

(3) Measures of loudness (dB) are used to assess an individual’s ability to hear.

Audiometric Zero Audiometric Zero 0 dB Hearing Level at 1000 Hz = 7 dB SPL


AUDIOGRAM

A hearing test is given in which pure sound frequencies are played into each ear, and the intensity necessary for the person being tested to detect the sound is measured. The results are recorded on an audiogram which allows the audiologist to determine an individual’s hearing loss. The audiogram above shows different sounds and where they would be represented on an audiogram. The yellow banana shaped figure represents all the sounds that make up the human voice when speaking at normal conversational levels

Website: http://www.pacificaudiology.com/audiogram/uya.html


The audiogram at right depicts the hearing of a person whose left ear shows normal hearing at low frequen-cies (~250-500 Hz) but slopes to a severe hearing loss at high frequen-cies (2000-8000 Hz). This person’s right ear shows overall moderate to severe hearing loss at all frequencies.

X=LEFT EAR O=RIGHT EAR

In the left ear, this person is able to hear all the low and mid speech sounds but is not able to hear the high pitch speech sounds (i.e. F, S, TH) . In the right ear, this person is unable to hear any of speech sounds.

X=LEFT EAR O=RIGHT EAR

HEARING LOSS

AGING: As we age, the basilar membrane loses its stiffness. Thus, we lose the ability to hear HIGHER frequencies; i.e., we lose 160 Hz per yr after the age of 40—(!). These are the frequencies that allow consonants such as K, T, TH, S, SH, F, etc to be distinguished.

HELPING: YELLING DOES NOT HELP: What does help is to ENUNCIATE and to allow the person to LIP READ. Thus:

(1) make sure the person can see your face(2) exaggerate your facial movements and lips as you talk(3) differentially amplify the consonants


PITCH

(1) Pitch is related to the frequency of the sound pressure waves.

(2) Pitch is thought to be coded by both frequency of firing of neurons and their place along the basilar membrane (i.e., which neurons are firing).

(a) Frequency theory (rate coding): The firing frequency of the fibers matches the frequency of the sound pressure waves. Rate coding works only for sound pressure waves of 15 Hz to 100 Hz.

(b) Volley theory (modified rate coding): The firing frequency of groups of fibers overall matches the frequency of sound pressure waves. Such coding can work up from 100 Hz to 4,000 Hz.

(c) Place theory (place coding): NEXT PAGE…


PITCH (cont’d)

(c) Place theory (place coding): Different frequencies can cause different parts of the basilar membrane to vibrate (flex). Therefore, different frequencies are coded by different neurons. Although place coding is thought to be most relevant to higher frequencies (i.e., >4,000 Hz), evidence in support of the conclusion that is it also important for lower frequencies in the speech range (200-4,000 Hz) comes from work with cochlear implants.


http://www.cadinfo.net/editorial/cochlear-3.jpg

COCHLEAR IMPLANT


From Garrett, B. Brain and Behavior. Wadsworth, Belmont, CA, 2003, p.229.

SOUND LOCALIZATION

Two aspects: (a) the direction from which the sound is

coming (b) how far away it is.

** Note: Recognition and appreciation of spoken language and music is another issue, not to be considered today (If you are curious about language issues, read module 14.2 – you won’t be tested on this module.) Unfortunately, there is very little in your book about music, although we do know that auditory cortex is involved in recognition of complex sounds and melodies (see p. 200-201, & Fig. 7.6).

**

At frequencies of sound greater than 1,500 Hz, we use amplitude differences to estimate the direction of the sound.

At frequencies of sound less than 1,500 Hz, phase differences provide the cues.

*

*


Don’t forget!

THE VESTIBULAR SYSTEMTHE VESTIBULAR SYSTEM(p. 205 and Fig. 7.10 on p. 206)


audition functions: adequate stimuli: class # 12&13: audition, p. 1

Documents

persons right ear

left ear o

normal hearing

level of sound

sound wave

pure sound frequencies

severe hearing loss

pound person