Chapter 4 “Sensation and Perception”
-Ordinary people rely on three types of cues to determine which way is up: visual, gravity,
and body direction
-Sensation is the stimulation of sense organs.
-Perception is the selection, organization, and interpretation of sensory input.
Psychophysics: Basic concepts and issues
-Psychophysics- the study of how physical stimuli are translated into psychological
-a particularly important contributor to psychophysics was Gustav Fechner, who
published a seminal work on the subject in 1860
Thresholds: Looking for limits (What is the weakest detectable stimulus?)
-This question can be explained with the concept central to psychophysics: the threshold.
-A threshold is a dividing point between energy levels that do and do not have a
-An absolute threshold for a specific type of sensory input is the minimum amount of
stimulation that an organism can detect. (Define the boundaries of an organism’s sensory
-Absolute threshold is not really absolute (if it’s truly absolute the probability of
detecting a stimulus is from 0 to 100%. In reality, as stimulus intensity increases, subjects’
probability of responding to stimuli gradually increases)
-An “absolute” threshold is defined as the intensity level at which the probability of
detection is 50 percent.
Weighing the differences: The JND
-A just noticeable difference (JND) is the smallest difference in the amount of stimulation
that a specific sense can detect.
-Weber’s law states that the size of a just noticeable difference is a constant proportion of
the size of the initial stimulus. This constant proportion is called the Weber fraction. (ex:
you would be able to detect the different of $1 with 50 cent discount because 50 cent of 1
dollar has a huge JND, while you wouldn’t be able to detect $900 with 50 cent off,
because the difference is too small compared to the initial proportion of the stimulus.)
- Fechner’s law- the magnitude of a sensory experience is proportional to the number of
JNDs that the stimulus causing the experience is above the absolute threshold.
-This simply means that equal increases in stimulus intensity produce progressively
smaller differences in the magnitude of sensation.
-Larger stimulus intensity are required to produce JND in the magnitude to sensation.
(Ex: A lamp has three light bulbs, when one turns on in a dark room, the difference is
striking. Turn again, the second light bulb comes on and there double amount of light, but
the room will not seem to be twice as brighter. As you turn on the third one, you wouldn’t
even notice the difference.) Signal-detection theory
-Signal-detection theory proposes that the detection of stimuli involves decision process
as well as sensory processes which are both influenced by a variety of factors beside
(Ex: During a task where it provide different available outcomes, it involves higher
mental process rather than raw sensation)
-Signal-detection theory replaces Fechner’s sharp threshold with the concept of
“detestability”. Detestability is measured in term of probability and depends on decision-
making processes as well as sensory process.
Perception without awareness (Can sensory stimuli that fall beneath the threshold of
awareness still influence behaviour?)
-Subliminal perception – the registration of sensory input without conscious awareness.
-Generally produces weak effects, and can only be detected by very precise measurement
under carefully controlled laboratory conditions.
-Sensory adaptation is a gradual decline in sensitivity due to prolonged stimulation.
-Sensory adaptation is an automatic, built-in process that keeps people turned in to the
changes rather than the constants in their sensory input.
-reduction in sensitivity to constant stimulation
-it’s a type of behavioural adaptation sculpted by natural selection
Our sense of sight: The visual system
- Amplitude affects mainly the affects mainly the perception of brightness, while
wavelength affects mainly the perception of colour.
The eye: A living optical instrument
-The eye serve two main purposes: They channel light to the neural tissue that receives it,
called the retina, and they house that tissue.
-The lens is the transparent eye structure that focuses the light rays falling on the retina.
-The lens is made up of relatively soft tissue, capable of adjustments that facilitate a
process called accommodation.
-close object: lens gets fatter (rounder), far object: lens gets flatten out
-Nearsightedness – close objects are seen clearly but distant object appear blurry
-Usually caused by the lens bending the light too much, or when the eyeball is too long.
-Farsightedness – distant object are seen clearly but close object appear blurry
-Typically occurs when the eyeball is too short
-Iris is the coloured ring of muscles surrounding the pupil, or black centre of the eye
-The pupil is the opening in the centre of the iris that helps regulate the amount of light
passing into the rear chamber of the eye.
-when pupil constricts (under bright light), it lets less light into the eye but sharpens the
-when the pupil dilates (under darkness), it lets more light in but the image is less sharp.
-Eye movement are referred to as saccades. The retina: the brain’s envoy in the eye
-The retina is the neural tissue lining the inside black surface of the eye; it absorb light,
process images and send visual information to the brain
-Optic disk –a hole in the retina where the optic nerve fiber exits the eye
-Optic disk is a hole in the retina, so you cannot see the image falling on it; therefore
known as the blind spot (Normally your other eye will compensates the blind spot of
Visual receptors: rods and cones
-The retina contains two types of receptors, rods and cones
-Rods outnumber cones by a huge margin as human have 100 million to 125 million rods,
but only 5 million to 6.4 million cones
-cones are specialized visual receptors that play a key role in daylight vision and colour
-The fovea is a tiny spot in the centre of the retina that contains only cones; visual acuity
is greatest at this spot
-rods are specialized visual receptors that play a key role in night vision and peripheral
vision (The density of the rods is greatest just outside the fovea and gradually decreases
toward the periphery of the retina)
Dark and Light adaptation
-Dark adaptation – the process in which the eyes become more sensitive to light in low
-Light adaptation is the process whereby the eyes become less sensitive to light in high
Information processing in the retina
-Optic nerve – a collection of axons that connect the eye with the brain
-The collection of rod and cones receptors funnel signals to particular visual cell in the
retina make up that cell’s receptive field
-The receptive field of a visual cell is the retinal area that, when stimulated affects the
firing of that cell
-Lateral antagonism occurs when neural activity in a cell opposes activity in surrounding
cells (when light hits the center of the receptive field it increase the firing rate, when the
light hits the surrounding of the field it decreases the firing rate as compared to the
Visual pathways to the brain
-Optic chiasm – the point at which the optic nerves from the inside half of each eye cross
over and then project to the opposite half of the brain
-After reaching the optic chiasms, the optic nerve fiber diverges along two pathways. The
main pathway projects into the thalamus, here the visual signals are processed in the later
geniculation nucleus and then distributed to areas in the occipital lobe that make up the
primary visual cortex. The second visual pathway leaving the optic chiasm braches off to
an area in the midbrain called the superior colliculus before travelling through the
thalamus and on to the occipital lobe -after reaching the optic chiasm, the major visual pathway projects through the lateral
geniculation nucleus in the thalamus and onto the primary visual cortex. A second
pathway detours through the superior colliculus and then projects through the thalamus
and onto the primary visual cortex
-The main visual pathway is subdivided into two more specialized pathways called the
magnocellular and parvocellular channels. These channels engage in parallel processing,
which involves simultaneously extracting different kinds of information from the same
-Cells in the visual cortex seems to be high specialized and have been characterized as
feature detectors, neurons that respond selectively to very specific feature of more
-After visual input is process in the primary visual cortex, it is often routed to other areas
for additional process- signals travels through two stream: the ventral stream, which
process the details of what objects are out here (form and colour), and the dorsal stream
which process where the object are (motion and depth).
-Visual agnosia – an inability to recognize objects
Prosopagnosia- inability to recognize familiar faces
Viewing the world in colour
The stimulus for colour
There are two kinds of colour mixture: subtractive and additive
-subtractive colour mixing works by removing some wavelengths of light, leaving less
light than was originally there (Paints)
-Additive colour mixing woks by superimposing lights, putting more light In the mixture
than exists in any one light by itself
-Human colour perception depends on processes that resemble additive colour mixing
Trichromatic theory of colour vision
-The trichromatic theory of colour vision holds that human eye has three types of
receptors with differing sensitivities to different light wavelengths.
-Trichromatic theory holds that people have three types of receptors that are sensitive to
wavelengths associated with red, green, and blue.
Opponent process theory of colour vision
-Complementary colours are pairs of colours that produce grey tones when mixed
-Afterimage – a visual image that persist after a stimulus is removed (starting at a strong
colour then look at a white background, you’ll then see the afterimage)
-The colour of the after image will be the complement of the colour originally stared at
-opponent process theory holds that colour perception depends on receptors that make
antagonistic responses to three pairs of colours
Reconciling theories of colour vision
-it takes both trichromatic and opponent process theory to explain colour vision
-Reconciling theory of colour vision – eye has three types of cones, with each type being
most sensitive to a different band of wavelengths (representing different colour receptors predicted by trichromatic theory. Some of these cells respond in antagonistic ways to blue
versus yellow red versus green, and black versus white (Opponent process theory)
Effect of colour on behaviour
-According to Elliot et al, colours can have automatic, unconscious effects on behaviour.
They assert that these effects are probably rooted in two basic sources.
1. People learn to associate based on certain colour pair with certain experience (red ink
on a test paper…etc)
2. Over the course of human evolution, certain colour may have adaptive significant for
survival (Fire, blood)
Perceiving forms, patterns, and object
-Reversible figure – a drawing that is compatible with two interpretations that can shift
back and fourth
-Perceptual set –a readiness to perceive a stimulus in a particular way (if you were told
that the picture is about something, you are more likely to see it in that particular way)
-Inattentional blindness- the failure to see fully visible objects or events in visual display
Feature analysis: assembling forms
-feature analysis is the process of detecting specific elements in visual input and
assembling them into a more complex form
-Feature perception involves bottom-up processing –a prog