Introduction to Sensory
Systems
Fall 2002
Everything you learn is
the result of input from sensory systems
(e.g., vision, audition, etc.).
Peripheral nerves carry information about the environment to the
brain where perception
occurs. Everyone's nervous system is a little
bit different so everyone's perceptions are
different. Moreover,
our perceptions don't accurately reflect the real world.
There are two reasons
why our perceptions are flawed:
1) Detection--We can't detect many forms
of energy (e.g. magnetic fields; ultraviolet light).
2) Translation--Sensory systems must convert
one form of energy into electrochemical messages that the brain can interpret.
Some meaning is always lost during translations.
The conversion of physical energy into neural activity
is called stimulus transduction.
Stimulus transduction is carried out by specialized
neurons called sensory receptors (e.g. pain receptors in your finger,
photoreceptors in your eye).
The nervous system codes four aspects of a stimulus:
modality
(quality), intensity, duration, and location.
A. Modality/quality--describes the type
of stimulus (e.g., a red light)
The nervous system codes this information in
two ways:
1) labeled line code--specialized sensory
receptors that only respond to one type of stimulus (e.g., auditory neurons
only carry information about sounds).
2) pattern code--a pattern of activity
in a variety of sensory receptors (e.g., different colors are seen by comparing
the activity in three types of photoreceptors).
B. Intensity--describes how strong a stimulus
is (e.g., a bright light).
The nervous system codes this information in
two ways:
1) Frequency code--intensity is coded
by a neuron's firing rate (AP/sec)
2) Population code--intensity is coded
by the number of active neurons
C. Duration--describes how long a stimulus
lasts.
Duration is coded by ongoing neuronal
activity. However, the nervous system responds best to changes in stimuli.
A constant stimulus often causes neurons to adapt (i.e., they stop firing
action potentials).
D. Location--Describes where a stimulus is.
The neural mechanism is different for each sensory
system.
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Psych 372 Syllabus