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Chapter 7

PSY 220 Chapter Notes - Chapter 7: Sound, Aspirin, Calcium Carbonate

Course Code
PSY 220
Hurwitz Barry

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PSY220- Psychobiology
Main Ideas:
Our senses have evolved to give us information we can use rather than complete
information about the world.
Each sensory system has receptors tuned to specific types of energy.
As a rule, the activity in a single sensory neuron is ambiguous by itself. The
meaning depends on the pattern across a population of neurons.
Different species are sensitive to different kinds of information (ex. the ears of the green
tree frog, Hyla cinera, are highly sensitive to sounds at two frequencies- 900 and 3000
hertz [Hz, cycles per second], which are prominent in the adult male’s mating call).
Human sense of taste alerts us to the bitterness of poisons, but does not respond to
substances such as cellulose that neither help nor harm us.
Our olfactory systems are unresponsive to gases that we do not need to detect (ex. carbon
dioxide) but highly responsive to the smell of rotting meat.
7.1- Audition
Sound waves- periodic compressions of air, water, or other media; vary in amplitude and
The amplitude of a sound wave is its intensity (a lightning bolt produces sound waves of
great amplitude).
Loudness is a sensation related to amplitude but not identical to it.
The frequency of a sound is the number of compressions per second, measured in Hz.
Pitch is the related aspect of perception; higher frequency sounds are higher in pitch.
The height of each sound wave corresponds to amplitude and the number of waves per
second corresponds to frequency.
Most adult humans hear sounds ranging from about 15 Hz to somewhat less than 20,000
Children hear higher frequencies than adults, because the ability to perceive high
frequencies decreases with age and exposure to loud noises.
Anatomists distinguish the outer ear, the middle ear, and the inner ear:
1. Outer ear

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o Pinna- the familiar structure of flesh and cartilage attached to each side of the
head; by altering the reflections of sound waves, the pinna helps us locate the
source of a sound
o External auditory canal- sound waves pass through here on their way to the
tympanic membrane
2. Middle ear
o Tympanic membrane (eardrum)- vibrates at the same frequency as the sound
waves that strike it; connects to 3 tiny bones that transmit the vibrations to the
oval window
o Oval window- a membrane of the inner ear
o Hammer
o Anvil these 3 bones are called the ossicles
o Stirrup
3. Inner ear
o Cochlea- snail-shaped structure containing 3 long fluid-filled tunnels:
- Scala vestibuli
- Scala media
- Scala tympani
o Hair cells- auditory receptors that lie between the basilar membrane of the
cochlea on one side and the tectorial membrane on the other
*When sound waves strike the tympanic membrane, they vibrate three tiny bones (the
ossicles), which convert the sound waves into stronger vibrations in the fluid-filled
cochlea. Those vibrations displace the hair cells along the basilar membrane in the
cochlea. A hair cell responds within microseconds to displacements as small as 10-10
meter, thereby opening ion channels in its membrane. The hair cells excite the cells of the
auditory nerve, which is part of the eighth cranial nerve.
Place theory- the basilar membrane resembles the strings of a piano in that each area
along the membrane is tuned to a specific frequency
-according to this theory, each frequency activates the hair cells at only one place along
the basilar membrane, and the nervous system distinguishes among frequencies based on
which neurons respond
-the downfall of this theory is that the various parts of the basilar membrane are bound
together to tightly for any part to resonate like a piano string.
Frequency theory- the basilar membrane vibrates in synchrony with a sound, causing
auditory nerve axons to produce action potentials at the same frequency (ex. a sound at
50 Hz would cause 50 action potentials per second in the auditory nerve)
-the downfall of this theory in its simplest form is that the refractory period of a neuron,
though variable, is typically about 1/1000 second, so the maximum firing rate of a neuron
is about 1000 Hz, far short of the highest frequencies we hear
The current theory is a modification of both theories.

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For low-frequency sounds, the basilar membrane vibrates in synchrony with the sound
waves, in accordance with the frequency theory, and auditory nerve axons generate one
action potential per wave.
Soft sounds activate fewer neurons, stronger sounds activate more. Thus, at low
frequencies, the frequency of impulses identifies the pitch, and the number of firing cells
identifies loudness.
Because of the refractory period of the axon, as sounds exceed 100 Hz, it becomes harder
for a neuron to continue firing in synchrony with the sound waves.
Volley principle of pitch discrimination- the auditory nerve as a whole produces volleys
of impulses for sounds up to about 4,000 per second, even though no individual axon
approaches that frequency
Most human hearing takes place below 4000 Hz, the approximate limit of the volley
An estimated 4% of people have amusia, impaired detection of frequency changes
(commonly called “tone-deafness”).
-associated with a thicker than average auditory cortex in the right hemisphere but fewer
than average connections from the auditory cortex to the frontal cortex
Q: Through which mechanism do we perceive low-frequency sounds (up to about 100
A: At low frequencies, the basilar membrane vibrates in synchrony with the sound waves,
and each responding axon in the auditory nerve sends one action potential per sound
Q: How do we perceive middle-frequency sounds (100 4000 Hz)?
A: At intermediate frequencies, no single axon fires an action potential for each sound
wave, but different axons fire for different waves, and so a volley (group) of axons fires
for each wave.
Q: How do we perceive high-frequency sounds (above 4000 Hz)?
A: At high frequencies, the sound causes maximum vibration for the hair cells at one
location along the basilar membrane.
As information from the auditory system passes through subcortical areas, axons cross
over in the midbrain to enable each hemisphere of the forebrain to get most of its input
from the opposite ear.
The information ultimately reaches the primary auditory cortex (area A1) in the superior
temporal cortex.
The organization of the auditory cortex mirrors that of the visual cortex in that it has a
“what pathway” sensitive to patterns of sound in the anterior temporal cortex, and a
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