Transcript Sensation and Perception

Sensation and
Perception
Prof David Irwin
Lecture for Psych 103
Introduction
The environment is loaded with
stimulation, but we are constructed in
such a way that we are sensitive to only
some of this stimulation
Definition
Sensation and perception involves the
study of how information is extracted and
interpreted from the environment
Perception is a biological process
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Sensory nerve fibers provide our only link
to the outside world
Our perceptual worlds are different from
our physical worlds because certain
environmental events cannot be detected
by our sensory channels
Perception is a psychological
process
Perception uses symbolic representations:
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Environmental stimuli trigger neural activity
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Neural patterns are symbols for environmental
events that occur in the physical world
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Perception is the interpretation of neural symbols
Psychological, Biological, and
Theoretical Approaches to Perception
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Psychological Approaches (behavior)
Biological Approaches (anatomy and
physiology)
Theoretical Approaches (hypothetical causes and
mechanisms)
Example: Spectral Sensitivity and
Photoreceptor Pigments
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Humans are differentially sensitive to different
wavelengths of light; this is due to the fact that
the chemicals (photopigments) inside our
photoreceptors (rods and cones) absorb some
wavelengths better than others
Spectral Sensitivity
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Absolute threshold (the dimmest light we can
just barely detect) varies with wavelength of
light
Spectral Sensitivity
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Absolute threshold (the dimmest light we can
just barely detect) varies with wavelength of
light
Different spectral sensitivity functions are found
for peripheral and central vision
Photopigments and
phototransduction
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Light is first registered by light-sensitive
molecules in the photoreceptors
Photopigments and
phototransduction
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Light is first registered by light-sensitive
molecules in the photoreceptors
When light strikes a photopigment molecule, the
molecule isomerizes (changes shape)
When the photopigment isomerizes, the
electrical current in the photoreceptor is
disrupted; this signals the presence of light
This process is called phototransduction
Fovea contains only cones, periphery contains mostly rods
Physiology of Spectral Sensitivity
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The photopigments in the rods and cones have
different pigment absorption spectra
An absorption spectrum is a plot of the amount
of light absorbed by a pigment vs. the
wavelength of the light
Pigment Absorption Measurement
Light passed out;
difference between
intensity in and intensity
out is amount absorbed
Light sent in
(vary wavelength
across spectrum)
Physiology of Spectral Sensitivity
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Wald and Brown (1958) chemically extracted
photopigment from donated eyes and measured
its absorption spectrum.
This was primarily rod photopigment because
there are many more rods than cones and each
rod has more photopigment than each cone
Physiology of Spectral Sensitivity
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They found that the pigment absorption
spectrum perfectly matched the rod spectral
sensitivity function
Physiology of Spectral Sensitivity
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Later they used microspectrophotometry to
measure the absorption spectra of the cones
In this procedure one shines a beam of light
through an individual receptor to measure the
pigment absorption spectrum
Three kinds of pigments were found (with
peaks at 435 nm, 545 nm, and 580 nm),
corresponding to the three kinds of cones
Physiology of Spectral Sensitivity
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So, the general cone spectral sensitivity function
reflects a mixture of these three cone types
Physiology of Spectral Sensitivity
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Color blindness is usually caused by a missing
cone pigment
There are individual differences in pigments
even when none are missing
Dartnall and Bowmaker found that individual
differences in spectral sensitivity functions are
accompanied by the same differences in pigment
composition
Sensation and Perception
Seeks to define sensory properties of organisms
Attempts to determine relationship between physical
variables and perceptual experience
Attempts to explain perceptual experience in terms of
underlying physiological mechanisms
Knowledge gained is important from a basic science
standpoint, and also has practical value in designing
and evaluating devices that we use in the world