sound pathway
- sound waves from outer ear down the auditory canal and cause the tympanic membrane (the eardrum) to vibrate.
- Vibrations transferred to the ossicles (malleus, incus, stapes) (middle ear)
- These triggers vibrations of the oval window, which in turn transfers the vibrations to the fluid of the snail-shaped cochlea (long, coiled tube with an internal receptor (organ of Corti) running almost to its tip). (inner ear)
- Each pressure change at the oval window travels along the organ of Corti.
- The auditory receptors, the hair cells, are mounted in the basilar membrane, and the tectorial membrane rests on the hair cells.
- Deflection of the organ of Corti at any point along its length produces a shearing force on the hair cells at the same point. This stimulates the hair cells, which in turn increase firing
in axons of the auditory nerve (branch of cranial nerve VIII). - The vibrations of the cochlear fluid are ultimately dissipated by the round window (elastic
membrane).
two models of sensory system organization
former: hierarchical, functionally homgeneous and serial. current model: hierarchical, functionally segregated and parallel
Physical and Perceptual Dimensions of Sound
amplitude (loudness), Frequency (pitch, fundamental frequencies) and Complexity (Timbre) –> Fourier analysis
fourier analysis
The breaking down of a sound—in
this case, the sound of a clarinet—
into its component sine waves by
Fourier analysis. When added
together, the component sine
waves produce the complex sound
wave.
From the Ear to the Primary Auditory Cortex
- Pathway (from inner ear to brain)
- Hair cells synapse on neurons
- Axons enter metencephalon
- Synapse in ipsilateral cochlear nucleus
- Travel to superior olives
- Travel to the inferior colliculus via the lateral
lemniscus - Fibers ascend to the medial geniculate nucleus of
the thalamus - Fibers ascend to the primary auditory cortex in the
lateral fissure - Projections from each ear are bilateral
Two streams of auditory cortex
- Anterior auditory pathway: it identifies sound (what).
- Posterior auditory pathway: it identifies where sound is.
ORGANIZATION OF PRIMATE AUDITORY CORTEX.
-Tonotopically organized.
-Contains functional columns.
-Poorly understood compared to vision.
Pitch perception
Anterior to primary auditory cortex. Contains many neurons that respond to pitch rather
than frequency. The same small area also contained neurons that responded to
frequency, and Bendor and Wang suggested that this area was likely the place where
frequencies of sound were converted to the perception of pitch. A comparable pitch area
has been identified by fMRI studies in a similar location in the human brain
Auditory cortex damage
- Bilateral lesions do not cause deafness
- Creates difficulty in localization
- Creates difficulty in recognizing rapid complex sequences of sound
deafness
- Total deafness is rare
- Two types
- Conductive (damage to ossicles)
- Nerve (damage to cochlea or nerve) (If only part of the cochlea is damaged, individuals may have nerve deafness for some
frequencies but not others.) (Some people with nerve deafness benefit from cochlear implants. Cochlear implants bypass damage
to the auditory hair cells by converting sounds picked up by a microphone on the patient’s ear to
electrical signals, which are then carried into the cochlea by a bundle of electrodes. These signals
excite the auditory nerve.
Although cochlear implants can provide major benefits, they do not restore normal hearing)
Age-related hearing loss
high frequencies helped by hearing aids or cochlear implant (hat is why elderly people often have difficulty distinguishing “s,” “f,” and “t” sounds.)
tinnitus
changes to the central auditory system that were caused by the deafness are the cause of tinnitus.
