Psychology Chapter 5
Sensation and Perception
Synesthesia: “mixing of the senses” people who experience sounds as colours or tastes as touch
sensations that have different shapes. Women are more likely to be synaesthetes.
The five stages that constitute the process of sensory processing and perception of information:
Stimulus is received by sensory receptors (sensation)
Receptors translate stimulus properties into nerve impulses (transduction)
Feature detectors analyze stimulus features
Stimulus features are reconstructed into neural representation and is compared with previously
stored information in brain
Matching process results in recognition and interpretation of stimuli (perception)
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, the active process of organizing this
stimulus input and giving it meaning
Transduction: is the process whereby the characteristics of a stimulus are converted into nerve
• There are more than the 5 classical senses (touch, taste, sight, auditory, smell) such as
sense of balance, detection within the immune system, etc.
Psychophysics: studies relations between the physical characteristics of stimuli and sensory
capabilities. Concerned with:
• Absolute limits of sensitivity. What is the softest sound or the weakest salt solution that
humans can detect?
• Differences between stimuli. What is the smallest difference in brightness that we can
Absolute threshold: the lowest level of stimulation that a person can detect correctly 50 percent
of the time. The lower the absolute threshold, the greater the sensitivity.
Decision criterion: 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. Signal
detection research shows us that perception is, in
part, a decision.
Subliminal stimulus: stimulus 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.
• Higher-order facial recognition is a
complex process involving several brain
regions, including the LOA(lateral
occipital area) and the FFA(Fusiform FacialArea) in addition to the primary visual
• Individuals with prosopagnosia are unable to recognize familiar faces.
• We may not be consciously aware of stimuli, but aspects of the stimuli could be
processed at a different level and available for us to use in subsequent decisions.
Difference threshold: the smallest difference between two stimuli that people can perceive 50
percent of the time a.k.a. the “just noticeable difference” (jnd)
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. For example, the jnd value for weights is a Weber fraction of approximately
1/50. This number means that if you lift 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 were 500 grams,
a second weight would have to weigh at least 510 grams (i.e., 1/50 = 10 grams/500 grams) for
you to discriminate between them. The smaller the fraction, the greater the sensitivity to
Sensory adaptation: the diminishing sensitivity to an unchanging stimulus. In Review:
• Sensation refers to the activities by which our sense organs receive and transmit
information, whereas perception involves the brain's processing and interpretation of the
• 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
detectability (absolute threshold) and the ability to discriminate between stimuli
(difference threshold). The absolute threshold is the intensity at which a stimulus is
detected 50 percent of the time. Signal detection theory is concerned with factors that
influence decisions about whether or not a stimulus is present.
• Research indicates that subliminal stimuli, which are not consciously perceived, can
influence perceptions and behaviour in subtle ways, but not strongly enough to justify
concerns about the subconscious control of behaviour 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 percent of the time. Studies of
the jnd led to Weber's law, which states that the jnd is 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
• Cornea: a transparent protective structure at the front of the eye (where light waves
• Pupil: an adjustable opening that can dilate or constrict to control the amount of light that
enters the eye. Low levels of illumination cause the pupil to dilate, letting more light into
the eye to improve optical clarity; bright light triggers constriction of the pupil.
• Iris: regulates the size of the pupil.
• Ciliary muscles: regulate the shape of the lens. • Lens: (behind pupil) an elastic structure that becomes thinner to focus on distant objects
and thicker to focus on nearby objects
• Retina: a multi-layered, light-sensitive tissue at the rear of the eyeball. Contains
specialized sensory neurons and is actually an extension of the brain. The lens reverses
the image from right to left and top to bottom when it is projected on the retina, but the
brain reconstructs the visual input into the image that we perceive. The retina contains
two types of light-sensitive receptor cells, called rods and cones because of their shapes
• The ability to see clearly depends on the lens's ability to focus the image directly onto the
• Myopia (nearsightedness). In nearsighted people, the lens focuses the visual image in
front of the retina (too near the lens), resulting in a blurred image for faraway objects.
This condition generally occurs because the eyeball is longer (front to back) than normal.
• 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 use glasses.
• Rods: black-and-white brightness receptors. More sensitive to light than the cones,
function best in dim light.
• Cones: colour receptors, function best in bright illumination.
• In humans, rods are found throughout the retina except in the fovea, 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
• Fovea: a small area in the centre of the retina that contains only cones.
• Bipolar cells have synaptic connections with the rods and cones. The bipolar cells, in
turn, synapse with a layer of about one million ganglion cells, whose axons are collected
into a bundle to form the optic nerve. Typically, many rods are connected to the same
bipolar cell. They therefore can combine or “funnel” their individual electrical messages.
• In the fovea, however, the densely packed cones each have their own “private line” to a
single bipolar cell.As a result, our visual acuity, or ability to see fine detail, is greatest
when the visual image projects directly onto the fovea. • The optic nerve formed by the axons of the ganglion cells exits through the back of the
eye not far from the fovea, producing a blind spot. Our perceptual system “fills in” the
missing part of the visual field.
• Transduction: the process whereby the characteristics of a stimulus are converted into
nerve impulses. Rods and cones translate light waves into nerve impulses through the
action of protein molecules called photopigments.
• Brightness Vision and Dark Adaptation
• Rods have much greater brightness sensitivity than cones throughout the colour spectrum
except at the red end.
• Cones are most sensitive to low illumination in the greenish-yellow range of the spectrum
• Dark adaptation: is the progressive improvement in brightness sensitivity that occurs
over time under conditions of low illumination. If the eye has been exposed to conditions
of high illumination, such as bright sunlight, a substantial amount of photopigment will
be depleted. During the process of dark adaptation, the photopigment molecules are
regenerated, and the receptor's sensitivity increases greatly.
• The trichromatic (three-colour) theory: (1800s) three types of colour receptors in the
retina. All cones can be stimulated by most wavelengths to varying degrees; individual
cones are most sensitive to wavelengths that correspond to blue, green, or red. The visual
system combines the signals to recreate the original hue. If all three cones are equally
activated, a pure white colour is perceived.
• Opponent-process theory: also assumed that there are three types of cones. Each of the
three cone types responds to two different wavelengths. One type responds to red or
green, another to blue or yellow, and a third to black or white.
• The dual-process theory combines the trichromatic and opponent-process theories to
account for the colour transduction process.
o The cones contain one of three different protein photopigments that are most
sensitive to wavelengths roughly corresponding to the colours blue, red, and
green. Different ratios of activity in the red, blue, and green-sensitive cones can
produce a pattern of neural activity that corresponds to any hue.
o Certain ganglion cells respond in an opponent-process fashion by altering their
rate of firing. If red light is shone on the retina, an opponent-process ganglion cell
may respond with a high rate of firing, but a green light will cause the same cell to
fire at a very low rate. • Trichromats: people with normal colour vision
• Dichromat: person who is colour-blind in only one of the systems (red-green or
• Monochromat is sensitive only to the black-white system and is totally colour-
• Primary visual cortex in the occipital lobe at the rear of the brain.
• Feature detectors: fire selectively in response to stimuli that have specific
characteristics. Viewing the letter “A” comes from feature detectors seeing: –, /, \
• Parallel processing: separate but overlapping modules within the brain
simultaneously analyze colours, shape, distance, and movement and constructing a
unified image of its properties
• Visual association cortex: complex features of the visual scene are combined and
interpreted in light of our memories and knowledge
• Sound waves: form of mechanical energy. Successive waves of compression and
expansion among the air molecules surrounding the source of the sound.
• Frequency is the number of sound waves, or cycles, per second. The hertz (Hz) is the
technical measure of cycles per second; 1 hertz equals one cycle per second. Frequency =
• Amplitude refers to the vertical size of the sound waves—that is, to the amount of
compression and expansion of the molecules in the conducting medium. Decibels (db), a
measure of the physical pressures that occur at the eardrum.Amplitude = loudness