Sensation and Perception
Synesthesia: rare and mysterious condition (“mixing of the senses”). They may
experience sounds as colours or tastes as touch sensations that have different shapes.
Transduction: You sensory receptors must translate all stimulus into the only language
your nervous system understand: the language of nerve impulses.
Feature detectors: specialized neurons that break down and analyze the specific features
of the stimuli.
Binding problem: How do we bind all our perceptions into one complete whole while
keeping its sensory elements separate?
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. It is the active process of
organizing this stimulus input and giving it meaning.
• Perception is influenced by context
Specialized sensory receptors transform energy forms (light waves, sound waves) into
the code language of nerve impulses.
• Sensory equipment of any species is an adaptation to its environment
• Nerve impulses is the only language the nervous system understands
Transduction: is the process whereby the characteristics of a stimulus are converted into
Psychophysics: studies the relations between the physical characteristics of stimuli and
sensory capabilities. Is concerned with two kinds of sensitivity:
• The absolute limits of sensitivity (e.g. the softest sounds or weakest salt solution
that humans can detect)
• The differences between stimuli (e.g. smallest difference in brightness that we
can detect or how much difference must there be in 2 tones before we can tell
they’re not the same)
Stimulus Detection: The Absolute Threshold
How intense must a stimulus be before we can detect its presence?
Absolute threshold: the lowest intensity at which a stimulus can be detected correctly
50% of the time. Thus, the lower the absolute threshold, the greater the sensitivity. Signal Detection Theory
A fixed absolute threshold is inaccurate because there is no single point on the intensity
scale that separates nondetection from detection of a stimulus.
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.
Focus on Neuroscience
The Neuroscience of Subliminal Perception and Prosopagnosia
Subliminal Stimulus: is a stimulus that is so weak or brief, although it is received by the
senses, it cannot be perceived consciously-the stimulus is well below the threshold.
• Subliminal stimulus can affect attitudes and behaviour without knowing it to a
• Prosopagnisia: condition where people cannot recognize familiar faces
o Cortical damage in areas that are involved with object perception
o Damage to the LOA (lateral occipital area). LOA has been associated
with object perception in the intact cortex.
o Fusiform gyrus: a second area associated with facial processing. Here we
find the Fusiform Facial Area (FFA), a brain region specifically
associated with facial perception.
• Higher-order facial recognition is a complex process involving several brain
regions, inclusing the LOA and FFA, in addition to the primary visual cortex.
The Difference Threshold
Difference threshold: the smallest difference between two stimuli that people can
perceive 50% of the time. This is sometimes called 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 by Weber’s fraction.
• e.g. the jnd value for weight is a Weber fraction of about 1/50. This means that is
you life a weight of 50 grams, a comparison weight must eight at least 51 grams
in order for you to be able to judge a difference.
• The smaller the fraction, the greater the sensitivity to differences
Sensory Adaptation: The diminishing sensitivity to an unchanging stimulus. Sensory
neurons are engineered to respond to a constant stimulus by decreasing their activity.
Adaptation or Habituation: monotonous background sounds are largely unheard.
• e.g. The feel of your wristwatch against your skin recedes from awareness The Sensory System
The normal stimulus for vision is electromagnetic energy, or light waves, which are
measured in nanometres.
• Human vision: 400-700nm
• Order of the spectrum: red, orange, yellow, green, blue, indigo, violet
• Electromagnetic spectrum includes:
o TV and radio signals
o Infrared and ultraviolet rays
The Human Eye
Cornea: Light waves enter the eye through the cornea, a transparent protective structure
at the front of the eye.
Pupil: Behind the cornea is the pupil, an adjustable opening that can dilate or constrict to
control the amount of light that enters the eye.
• Muscles in the iris control the pupil’s size
• 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: coloured part of the eye that surrounds the pupil.
Lens: an elastic structure that becomes thinner to focus on distant objects and thicker to
focus on nearby objects.
Retina: Light-sensitive multi-layered tissue at the rear of the fluid-filled eyeball.
Myopia (nearsightedness): Good vision for nearby objects but have difficulty seeing far
away objects. A visual defect in which the lens focuses distant images in front of the
retina rather than on it. This condition generally occurs because the eyeball is longer than
Hyperopia (farsightedness): Occurs when the lens does not thicken enough and the
image is therefore focused on a point behind the retina. The aging process typically
causes eyeball to become shorter over time.
Photoreceptors: The Rods and Cones
Rods (120 million is the human eye): Function best in dim light, are primarily black-and-
white brightness receptors. Rods are found throughout the retina except the fovea. The
periphery of the retina contains mainly rods.
Cones (6 million in the human eye): Colour receptors that function best in bright
illumination. Cones decrease in concentration as one moves away from the centre of the
Fovea: small area in the centre of the retina that contains only cones. Rods and cones send their messages to the brain via two additional layers of cells:
1. Bipolar cells: have synaptic connections with the rods and cones.
2. Ganglion cells: synapse with the bipolar cells, and their axons are collected into a
bundle to form the optic nerve.
Optic nerve: a bundle of ganglion cell axons in the retina that transmits visual
information to the brain.
Visual acuity: ability to see fine detail.
• Greatest when the visual image projects directly onto the fovea
Blind spot: where the ganglion cells exit through the back of the eye to form the optic
nerve. We are unaware of our blind spot because our perceptual system “fills in” the
missing part of the visual field.
Visual Transduction: From Light to Nerve Impulses
Transduction: process where the characteristics of a stimulus are converted into nerve
Photopigment: protein molecule within the rods and cones whose chemical reactions
when absorbing light result in nerve impulses being generated.
Brightness Vision and Dark Adaptation
Dark adaptation: is the progressive improvement in brightness sensitivity that occurs
over time under conditions of low illumination. During this process, the photopigment
molecules are regenerated after being exposed to conditions of high illumination (where
photoreceptors are depleted of its pigment molecules), and the receptor’s sensitivity
The trichromatic theory: Any colour in the visible spectrum can be produced by some
combination of blue, green, and red (aka additive colour mixture). According to this
theory, there are three types of colour receptors in the retina: blue, green, and red. If all
three receptors are activated equally, a pure white colour is perceived.
Opponent-process theory: Assumed that there are three types of cones, and 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.
Dual processes in colour transduction: Combines the trichromatic and opponent-
process theories to account for the colour transduction process. The cones do indeed
contain one of three different protein photopigments that are most sensitive to
wavelengths that are blue, red, and green. Different ratios of activity in the red, green,
and blue sensitive cones can produce a pattern of neural activity that corresponds to any
hue in the spectrum. Opponent process does not occur at the levels of the cones. Certain ganglion cells in the retina, and neurons in visual relay stations and visual cortex respond
in an opponent-process fashion by altering their rate of firing.
Colour-deficient vision: People with normal colour vision are referred to as trichromats
(sensitive to all 3: red-green, yellow-blue, and black-white). A dichromat is a person
who is colour-blind in only one of the systems (red-green or blue-yellow). A
monochromat is sensitive only to black-white system.
Feature detectors: sensory neurons that respond to particular features of a stimulus, such
as its shape, angle, or colour (they fire selectively in response to stimuli that have specific
Retina Optic Nerve Visual Relay Station (Thalamus) Primary Visual Cortex
Primary visual cortex: the area of the occipital lobe, which receives impulses generated
from the retina via the thalamus and analyzes visual input by using its feature detectors.
The fovea is represented by a disproportionately large area of the visual cortex.
Parallel processing: Our ability to use our senses to take in a variety of information
about an object and construct a unified image of its properties.
Visual association cortex: cortical areas in the occipital, parietal, and temporal lobes that
analyze visual stimuli sent to the primary visual cortex in relation to stored knowledge
and that establish the “meaning” of the stimuli.
Sounds waves: a form of mechanical energy. Sound waves have 2 characteristics:
• Frequency (pitch): measured in hertz (Hz), which is #cycles/second.
o Human hearing: 20-20,000 Hz
• Amplitude (loudness): measured in decibels (db), a measure of the physical
pressures that occur at the eardrum.
Auditory Transduction: From Pressure Waves to Nerve Impulses
1. Sound waves travel into the auditory canal at 1,200 kilometres an hour.
2. The sound waves reach the eardrum (a movable membrane that vibrates in
response to the sound waves).
3. Beyond the eardrum is the middle ear (a cavity housing 3 tiny bones-ossicles)
• Malleus (hammer)-attached to the eardrum
• Incus (anvil)
• Stapes (stirrup)-attached to the oval window
The vibrating activity of these bones amplifies the sound waves more than 30x.
4. The oval window forms the boundary between the middle ear and the inner ear.