Psychology 1000 Lecture Notes - Lecture 5: Detection Theory, Subliminal Stimuli, Absolute Threshold
13 May 2018
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Sensation and Perception
•synesthesia - “mixing of the senses”
• They may experience sounds as colours or tastes as touch sensations that have
different shapes. Women are more likely to be synaesthetes than men
• Maurer and Mondloch (2006) have suggested that we are all born synaesthetic: The
neural pathways of infants are fairly undifferentiated and lead to cross-modal
perceptions.
• Whether the stimulus is light, sound waves, a chemical molecule, or pressure, your sensory
receptors must translate this information into the only language your nervous system
understands: the language of nerve impulses. This process is called transduction. Once this
translation occurs, specialized neurons called feature detectors break down and analyze the
specific features of the stimuli. At the next stage, these numerous stimulus “pieces” are
reconstructed into a neural representation that is then compared with previously stored
information
• This matching of a new stimulus with our internal storehouse of knowledge allows us
to recognize the stimulus and give it meaning.
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• Sensory and perceptual processes proceed from 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.
• In people with synesthesia, there is some sort of cross-wiring, so that activity in one part of
the brain evokes responses in another part of the brain dedicated to another sensory
modality
• Functional MRI studies have shown that for people with synesthesia with word-colour
linkages, hearing certain words is associated with neural activity in parts of the visual
cortex.
• Several explanations offered for the sensory mixing
1. One theory is that 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 increased connectivity in patients with synesthesia
2. Another theory is that with synesthesia, there is a deficit in neural inhibitory
processes in the brain that ordinarily keep input from one sensory modality from
“overflowing” into other sensory areas and stimulating them.
•Sensation is 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
• Because perception is an active and creative process, the same sensory input may
be perceived in different ways at different times
SENSORY PROCESSES
• your brain cannot “understand” light waves, sound waves, or the other forms of energy that
make up the language of the environment.
• Contact with the outer world is possible only because certain neurons have
developed into specialized sensory receptors that can transform these energy forms
into the code language of nerve impulses.
•Transduction is the process whereby the characteristics of a stimulus are converted into
nerve impulses. the conversion of one form of energy into another; in sensation
• Five classical senses - vision, audition (hearing), touch, gustation (taste), and olfaction
(smell)
• There are senses that provide information about balance and body position
• the sense of touch can be subdivided into separate senses of pressure, pain, and
temperature.
• The immune system has sensory functions that allow it to detect foreign invaders
and to receive stimulation from the brain
• Psychophysics studies relations between the physical characteristics of stimuli and sensory
capabilities, is concerned with two kinds of sensitivity. The first concerns the absolute limits
of sensitivity. For example, what is the softest sound or the weakest salt solution that
humans can detect? The second kind of sensitivity has to do with differences between
stimuli. What is the smallest difference in brightness that we can detect?
Stimulus Detection: The Absolute Threshold
• researchers designate the absolute threshold as the lowest intensity at which a stimulus
can be detected correctly 50 percent of the time.
• Thus, the lower the absolute threshold, the greater the sensitivity.
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Signal Detection Theory
• The concept of a fixed absolute threshold is inaccurate because there is no single point on
the intensity scale that separates nondetection from detection of a stimulus.
• There is instead a range of uncertainty, and people set their own decision criterion,
a standard of how certain they must be that a stimulus is present before they will say
they detect it.
• The decision criterion can also change from time to time, depending on such
factors as fatigue, expectation, and the potential significance of the stimulus.
• In signal detection theory, the potentially changing standard of how certain a
person must be that a stimulus is present in order to report its presence
•Signal detection theory is concerned with the factors that influence sensory
judgments.
• A theory that assumes that stimulus detection is not based on a fixed
absolute threshold but rather is affected by rewards, punishments,
expectations, and motivational factors
• Experience also plays a role in signal detection—experienced drivers respond more quickly
to signs of danger partly because they have a lower threshold for detecting and identifying
hazardous situations than do novice drivers
The Difference Threshold
•The difference threshold is defined as the smallest difference between two stimuli that
people can perceive 50 percent of the time. The difference threshold is sometimes called the
just noticeable difference
•Weber's law states that the difference threshold, or jnd, is directly proportional to the
magnitude of the stimulus with which the comparison is being made, and can be expressed
as a Weber fraction. the principle that to perceive a difference between two stimuli, the
stimuli must differ by a constant percentage or ratio
• Weber's law breaks down at extremely high and low intensities of stimulation
• Weber fractions for the various senses. The smaller the fraction, the greater the
sensitivity to differences. As highly visual creatures, humans show greater sensitivity
in their visual sense than they do in
• for example, their sense of smell
• Weber fractions also show that humans are highly sensitive to differences in the
pitch of sounds but far less sensitive to loudness differences
THE NEUROSCIENCE OF SUBLIMINAL PERCEPTION AND
PROSOPAGNOSIA
•A subliminal stimulus is 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.
• Jon Krosnick (1992) showed participants nine slides of a particular person and then
measured their attitudes toward the target person. For half of the participants, each
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