LAW 122 Chapter Notes - Chapter 5: Detection Theory, Subliminal Stimuli, Stimulus Modality
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Psychology Chapter 5 Notes
Sensation And Perception
Sensory and perceptual processes precede form the reception and translation of physical energies into nerve
impulses to the active process by which the brain receives the nerve impulses, organizes and confers
meaning on them, and constructs a perceptual experience.
(1)Stimulus is received by sensory receptors
(2)Receptors translate stimulus properties into nerve impulses (transduction)
(3)Feature detectors analyze stimulus features
(4)Stimulus features are reconstructed into neural representation
(5) Neural representation is compared with previously stored information in brain
(6) Matching process results in recognition and interpretation of stimuli
Synesthesia- “mixing of the senses” Patients may experience sounds as colors or tastes as touch
sensations that have different shapes. Women are more likely than men to have synesthesia.
Sensory impaired people such as those who experience synesthesia provide glimpses into different
aspects of how we “sense” and understand our world. refer to the above chart!
Transduction= the process of translating information into the only language the nervous system
understands which is the language of nerve impulses. Once this translation occurs specialized
neurons called feature detectors break down and analyze the specific features of the stimuli. At
the next stage these pieces are reconstructed into a neural representation that is then compared
with previously stored information, such as our knowledge of what particular objects look, smell
or feel like. This matching of a new stimulus with our internal storehouse of knowledge allows us
t recognize the stimulus and give it meaning. We then consciously experience a perception.
FMRI studies have shown that for people with synesthesia with word- color linkages, hearing
certain words is associated with neural activity in parts of the visual cortex.
Several explanations have been offered for the sensory mixing
o One is the pruning of neural connections that occurs in infancy has not occurred in people
with synesthesia, so that brain regions retain connections that are absent in most people.
Diffusion tensor imaging has revealed increased connectivity in patients with synesthesia.
o Another theory is, with synesthesia there is a deficit in neural inhibitory process in the
brain that ordinarily keep input from one sensory modality from overflowing into the
other sensory areas and stimulating them
Binding Problem- how do we bind all our perception into one complete whole while keeping its
sensory elements separate?
Sensation= the stimulus detection process by which our sense organs respond to and translate
environmental stimuli into nerve impulses that are sent to the brain.
Perception= making sense of what our senses tell us.- is the active process of organizing this
stimulus input and giving it meaning.
Your interpretation, or perception of the characters is influenced by their context
Transduction= the process whereby the characteristics of a stimulus are converted into nerve
5 classical senses:
o audition, hearing
o gustation, taste
o olfaction (smell)
Touch can be subdivided into separate senses:
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Receptors deep within the brain monitor the chemical composition of our blood. The immune
system also has sensory functions that allow it to detect foreign invaders and to receive stimulation
from the brain
Psychophysics= studies relation between the physical characteristics of stimuli and sensory
capabilities, is concerned with 2 kinds of sensitivity
o First, the absolute limits of sensitivity.
o Second, the differences between stimuli
How intense must a stimulus be before we can detect its presence?
Absolute threshold is the lowest intensity at which a stimulus can be detected correctly 50% of the
The lower the absolute threshold the greater the sensitivity
Signal Detection Theory
People’s apparent sensitivity can fluctuate quite a bit. They conclude that the concept of a fixed
absolute threshold is inaccurate because there is n o single point on the intensity scale that separate
moderation from detection of a stimuli.
There is a range of uncertainty and people set their own decision criteria- a standard of how
certain they must be that a stimulus is present before they will say they detect it.
Signal detection theory- is concerned with the factors that influence sensory judgments.
At low stimulus intensities, both the participant’s and the situation’s characteristics influence the
Subliminal Stimulus- one that is so weak or brief that although it is received by the senses it
cannot be perceived consciously- the stimulus is well below the absolute threshold.
People can also be influenced to become bolder or more conservative by manipulating the reward
and costs for giving correct or incorrect responses. Increasing the reward s for hits or the costs for
misses results in lower detection threshold.
Signal detection research shows us that perception is, in part, a decision.
The Difference Threshold
Defined as the smallest difference between 2 stimuli that people can perceive 50% of the time.
This is also sometimes called the just noticeable difference (JND).
Ernst Weber, a German physiologist discovered in the 1830’s there is some degree of lawfulness
in the range of sensitivities within our sensory systems.
Weber’s Law= the difference threshold is directly proportional to the magnitude of the stimulus
with which the comparison is being made and can be expressed as Weber Fraction. Example- the
jnd value for weight is a weber fraction of approximately 1/50. This number means that if a weight
of 50 grams, a comparison weight must weigh at least 51 grams in order for you to be able to
judge it as heavier. If the weight is more than 500 grams, a second weight would have to weigh at
least 510 grams (1/50= 10g/ 500g) for you to discriminate between them.
The smaller the fraction, the greater the sensitivity in their visual sense than they do in.
Weber’s fraction also show how humans are highly sensitive to loudness of differences
Sensory neurons are engineered to respond to a constant stimulus by decreasing their activity and
the diminishing sensitivity to an unchanging stimulus is called sensory adaptation
Adaptation (sometimes called inhabitation) is a part of everyday experience. After a while
monotonous background sounds are largely unheard.
Adaptation occurs in all sensory modalities including vision.
Although sensory adaptation may reduce our overall sensitivity it is adaptive because it frees our
senses from the constant and the mundane to pick up informative changes in the environment
Sensation refers to the activity by which our sense organs receive and transmit information,
whereas perception involves the brain’s processing and interpretation of the information.
Psychophysics is the scientific study of how the physical properties of stimuli are related to
sensory experiences. Sensory sensitivity is concerned in part with the limits of stimulus
dectectability (absolute threshold) and the ability to discriminate between stimuli (difference
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threshold). The absolute threshold is the intensity at which a stimuli is detected 50% of the time.
Signal detection theory is concerned with factors that influence decisions about whether or not a
stimuli is present
Research indicates that subliminal stimuli, which aare not consciously perceived, can influence
perception and behavior in subtle ways, but not strongly enough to justify concerns about the
subconscious control of behavior through subliminal messages
The difference threshold or just noticeable difference (jnd) is the amount by which two stimuli
must differ for them to be perceived as different 50% of the time. Studies of the jnd led to weber’s
law which states that the jnd proportional to the intensity of the original stimulus and is constant
within a given sense modality
Sensory systems are particularly responsive to changes in stimulation and adaptation occurs in
response to unchanging stimuli.
The Sensory Systems
Electromagnetic energy, or light waves which are measured in nanometers.
Our visual system is only sensitive to wavelengths extending from about 700 nanometers (red)
down to about 400 nanometers (blue-violet).
Remember: ROY G BIV (red, orange, yellow) (Green) (Blue, Indigo and violet)
The Human Eye
Light waves enter the eye through the cornea. Behind the cornea is the pupil. The pupils size is
controlled by muscles in the colored Iris that surrounds the pupil.
Behind pupil is lens
Retina- a multi layered tissue at the rear of the fluid filled eyeball
If you have good vision for nearby objects but have difficulty seeing far away objects then you
probably suffer from MYOPIA
Hyperopia (farsightedness) occurs when the lens does not thicken enough and the image is
therefore focused on a point behind the retina (too far from the lens). The aging process typically
causes the eyeball to become shorter over time, contributing to the development of hyperopia and
the need for many middle-aged people to acquire reading glasses.
Photoreceptors: The Rods and Cones
The retina is an extension of the brain
Retina contains 2 types of light sensitive receptor cells called rods and cones, because of their
shapes. These are about 120 million rods and 6 million cones in the human eye.
Rods function best in dim light = are primarily black and white brightness receptors. They are
about 500 times more sensitive to light than are the cones, but they do not give rise to color
Cones are color receptors, function best in bright illumination.
In humans rods are found throughout the retina except in the FOVA- a small area in the centre of
the retina that contains only cones.
Cones decrease in concentration as one moves away from the centre of the retina, and the
periphery of the retina contains mainly rods.
Bipolar cells have synaptic connections with the rods and cones. The bipolar cells in turn synapse
with a layer of about one million GAGLION CELLS- whose axons are collected into a bundle to
form the OPTIC NERVES. Thus, input from more than 126 million rods and cones is eventually
funneled into only one million traffic lanes leading out of the retina towards the higher visual
Rods and cones not only not only form the rear layer of the retina, but their light- sensitive ends
actually point away from the direction of the entering light so that they receive only a fraction of
the light energy that enters the eye. Furthermore, the manner in which the rods and cones are
connected to the bipolar cells accounts for both the greater importance of rods in dim light and our
greater ability to see fine detail in bright illumination, when the cones are most active.
Many rods are connected to the same bipolar cell.
They can combine or funnel their individual electrical messages to the bipolar cell, where the
additive effect of the many signals may be enough to fire it.
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