1) Psychophysics: basic concepts and issues. Sensation is the
stimulation of sense organs. Perception is the selection, organization, and
interpretation of sensory input. Sensation involves the absorption of energy,
such as light or sound waves, by sensory organs, such as the ears and eyes.
Perception involves organizing and translating sensory input into something
meaningful, such as your best friend’s face or other environmental stimuli.
Psychophysics: the study of how physical stimuli are translated into
a. Thresholds: looking for limits
i. Sensation begins with a stimulus, any detectable input from the
ii. Fechner: Threshold is a dividing point between energy levels that
do and do not have a detectable effect.
iii. Absolute threshold for a specific type of sensory input is the
minimum amount of stimulation that an organism can detect. It
defines the boundaries of an organism’s sensory capabilities. It is
anything but absolute. Researchers had to arbitrarily define the
absolute threshold as the stimulus intensity detected 50% of the
iv. Vision: a candle flame seen at 50 km on a dark clear night. Hearing:
the tick of a watch under quiet conditions at 6 meters. Taste: one
teaspoon (5ml) of sugar in 7.5 litters of water. Smell: one drop of
perfume diffused into an entire volume of a six-room apartment.
Touch: the wing of a fly falling on your cheek from a distance of 1
b. Weighing the differences: the JND
i. Just noticeable difference (JND) is the smallest difference in the
amount of stimulation that a specific sense can detect. Absolute
threshold is simply the JND from nothing (no stimulus input).
ii. Weber’s law states that the size of a JND is a constant proportion of
the size of the initial stimulus. This constant proportion is called
the weber fraction. JND vary by sense, and the smallest detectable
difference is a fairly stable proportion of the size of the original
stimulus. His law not only apply to weight perception but to all of
the senses. As stimuli increase in magnitude, the JND becomes
c. Psychophysical scaling
i. Fechner’s law: states that the magnitude of a sensory experience is
proportional to the number of JNDs that the stimulus causing the
experience is above the absolute threshold.
1. An important ramification of Fechner’s law is that constant
increments in stimulus intensity produce smaller and
smaller increases in the perceived magnitude of sensation.
2. Perceptions can’t be measured on absolute scales. In the
domain of sensory experience virtually everything is
relative. ii. Signal-detection theory: proposes that the detection of stimuli
involves decision processes as well as sensory processes, which
are both influenced by a variety of factors besides stimulus
1. According to signal-detection theory, your performance will
also depend on the level of “noise” in the system. Noise
comes from all of the irrelevant stimuli in the environment
and the neural activity they elicit. Variation in noise
provides another reason why sensory thresholds depend
on more than just the intensity of stimuli.
iii. Subliminal perception: the registration of sensory input without
1. Using diverse methodological and conceptual approaches,
researchers examining a variety of phenomena, such as
unconscious semantic priming, subliminal affective
conditioning, and subliminal mere exposure effects. And
subliminal psychodynamic activation, have found evidence
that perception without awareness can take place.
2. Researchers have recently begun to use brain-imaging
technology to study how the brain processes subliminal
iv. Sensory adaptation: a gradual decline in sensitivity due to
1. In reality, the stimulus intensity of the odor is stable but
with continued exposure, your sensitivity to it decreases.
Sensory adaptation is a pervasive aspect of everyday life.
2. Sensory adaptation is an automatic, built-in process that
keeps people tuned in to the changes rather than the
constants in their sensory input. It allows people to ignore
the obvious and focus on changes in their environment that
may signal threats to safety.
3. Sensory adaptation also shows once again that there is no
one to one correspondence between sensory input and
2) Our sense of sight: the visual system
a. The stimulus: light
i. Light: a form of electromagnetic radiation that travels as a wave,
moving, naturally enough, at the speed of light.
ii. Light waves vary in amplitude (height) and in wavelength (the
distance between peaks).
iii. Amplitude affects mainly the perception of brightness, while
wavelength affects mainly the perception of color.
iv. Light can also vary in its purity (how varied the mix is). Purity
influences perception of the saturation, or richness, of color.
v. Many insects can see shorter wavelengths than humans can see, in
the ultraviolet spectrum, whereas many fish and reptiles can see
longer wavelengths in the infrared spectrum.
b. The eye: a living optical instrument i. Two main purposes: they channel light to the neural tissue that
receives it, called the retina, and they house that tissue.
1. Light enters the eye through a transparent “window” at the
front, the cornea. The cornea and the crystalline lens,
located behind it, form an upside down image of objects on
the retina. But the brain knows the rule for relating position
on the retina to the corresponding positions in the world.
2. Lens is the transparent eye structure that focus the light
rays falling on the retina.
a. Accommodation: the lens is made up of relatively
soft tissue, capable of adjustments tat facilitate a
process. It occurs when the curvature of the lens
adjusts to alter visual focus. Close object, the lens
gets fatter (rounder); distant objects, flattens out.
b. Nearsightedness: close objects are seen clearly but
distant objects appear blurry because the focus of
light from distant objects falls a little short of the
retina. This focusing problem occurs when the
cornea or lens bends the light too much, or when the
eyeball is too long.
c. Farsightedness: distant objects are seen clearly but
close objects appear blurry because the focus of light
from close objects falls behind the retina. This
focusing problem typically occurs when the eyeball
is too short.
3. Iris: the colored ring of muscle surrounding the pupil, or
black center of the eye.
4. Pupil: the opening in the center of the iris that helps
regulate the amount of light passing into the rear chamber
of the eye.
a. When the pupil constricts, it lets less light into the
eye but it sharpens the image falling on the retina.
When the pupil dilates (opens), it lets more light in
but the image is less sharp. In bright light, the pupils
constrict to take advantage of the sharpened image.
But in dim light, the pupils dilate; image sharpness is
sacrificed to allow more light to fall on the retina so
that more remains visible.
b. The eye itself is constantly in motion, moving in
ways that are typically imperceptible to us. When we
are looking at something, our eyes are scanning the
visual environment and making brief fixation at
various parts of the stimuli. Eye movements are
referred to as saccades.
c. The retina: the brain’s envoy in the eye.
i. Retina: the neural tissue lining the inside back surface of the eye; it
absorbs light, processes images, and sends visual information to
the brain. ii. Optic disk: a hole in the retina where the optic nerve fibers exit the
eye. Blind spot: the optic disk is a hole in the retina; you cannot see
the part of an image that falls on it.
iii. Visual receptors: rods and cones. The retina contains millions of
receptor cells that are sensitive to light. Light must pass through
several layers of cells before it gets to the receptors that actually
detect it. Only about 10% of the light arriving at the cornea reaches
these receptors. The retina contains two types of receptors, rods
and cones. Their names are based on their shapes, as rods are
elongated and cones are stubbier. Rods outnumber cones by a
huge margin, as humans have 100 million to 125 million rods, but
only 5 million to 6.4 million cones.
1. Cones are specialized visual receptors that play a key role
in daylight vision and color vision. However, cones do not
respond well to dim light, which is why you don’t’ see color
very well in low illumination. Cones provide better visual
acuity, that is sharpness and precise detail than rods. Cones
are concentrated most heavily in the center of the retina
and quickly fall off in density toward its periphery.
a. Fovea: a tiny spot in the center of the retina that
contains only cones; visual acuity is greatest at this
2. Rods are specialized visual receptors that play a key role in
night vision and peripheral vision. Rods handle night vision
because they are more sensitive than cones to dim light.
They handle the lion’s share of peripheral vision because
they greatly outnumber cones in the periphery of the retina.
The density of the rods is greatest just outside the fovea and
gradually decreases toward the periphery of the retina.
When you want to see a faintly illuminated object in the
dark, it’s best to look slightly above or below the place
where the object should be. Averting your gaze this way
moves the image from the cone-filled fovea, which requires
more light, to the rod-dominated area just outside the fovea,
which requires less light.
iv. Dark and light adaptation
1. Dark adaptation: the process in which the eyes become
more sensitive to light in low illumination. The declining
absolute thresholds over time indicate that you require less
and less light to see. Dark adaptation is virtually complete
in about 30 minutes, with considerable progress occurring
in the first 10 minutes.
2. Light adaptation: the process whereby the eyes become less
sensitive to light in high illumination. Light adaptation
improves your visual acuity under the prevailing
3. Both types of adaptation are due in large part to chemical
changes in the rods and cones, but neural changes in the
receptors and elsewhere in the retina also contribute. 4. The declining thresholds over time indicate that your visual
sensitivity is improving, as less and less light is required to
see. Visual sensitivity improves markedly during the first 5
to 10 minutes after entering a dark room, as the eye’s
bright-light receptors (the cones) rapidly adapt to low light
levels. However, the cones’ adaptation, which is plotted in
purple, soon reaches its limit, and further improvement
comes from the rods’ adaptation, which is plotted in red.
The rods adapt more slowly than the cones, but they are
capable of far greater visual sensitivity in low levels of light.
v. Information processing in the retina
1. Optic nerve: a collection of axons that connect the eye with
the brain. These axons, which depart from the eye through
the optic disk, carry visual information, encoded as a
stream of neural impulses, to the brain.
2. Receptive field of a visual cell: the retinal area that, when
stimulated, affects the firing of that cell.
a. Particularly common shapes and sizes of receptive
fields are circular fields with a center-surround
arrangement. In these receptive fields, light falling in
the center has the opposite effect of light falling in
the surrounding area. The rate of firing of a visual
cell might be increased by light in the center of its
receptive field and decreased by light in the
surrounding area. Other visual cells may work in just
the opposite way. Either way, when receptive fields
are stimulated, retinal cells send signals both toward
the brain and laterally (sideways) toward nearby
visual cells. These lateral signals allow visual cells in
the retina to have interactive effects on each other.
3. Lateral antagonism (lateral inhibition): the most basic of
these interactive effects. Occurs when neural activity in a
cell opposes activity in surrounding cells. It is responsible
for the opposite effects that occur when light falls on the
inner versus outer portions of center-surround receptive
fields. It allows the retina to compare the light falling in a
specific area against general lighting. This means that the
visual system can compute the relative amount of light at a
point instead of reacting to absolute levels of light.
d. Vision and the brain
i. Visual pathways to the brain
1. 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. This arrangement ensures that
signals from both eyes go to both hemispheres of the brain.
2. After reaching the optic chiasm, the optic nerves fibers
diverge along two pathways. The main pathway projects
into the thalamus, the brain’s major relay station. Here,
about 90% of the axons from the retinas synapse in the lateral geniculate nucleus (LGN). Visual signals are
processed in the LGN and then distributed to areas in the
occipital lobe that make up the primary visual cortex. The
second visual pathway leaving the optic chiasm branches
off to an area in the midbrain called the superior colliculus
before travelling through the thalamus and on to the
occipital lobe. The principal function of the second pathway
appears to be the coordination of visual input with other
3. Lightrods and conesneural signalsbipolar
cellsganglion cellsoptic nerveoptic chiasmopposite
a. Main pathway: optic chiasmlateral geniculate
nucleus (thalamus)primary visual cortex (occipital
i. Magnocellular: where
ii. Parvocellular: what
b. Second pathway: superior colliculus
thalamusprimary visual cortex
4. The main visual pathway is subdivided into two more
specialized pathways called the magnocellular and
parvocellular channels (based on the layers of the LGN they
synapse in). These channels engage in parallel processing,
which involves simultaneously extracting different kinds of
information from the same input.
ii. Information processing in the visual cortex
1. Individual cells in the primary visual cortex don’t really
response much to little spots—they are much more
sensitive to lines, edges, and other more complicated
2. Simple cells respond best to a line of the correct width,
oriented at the correct angle, and located in the correct
position in its receptive field.
3. Complex cells also care about width and orientation, but
they respond to any position in their receptive fields. Some
complex cells are most responsive if a line sweeps across
their receptive field—but only if it’s moving in the “right”
4. The key point of all of this is that the cells in the visual
cortex seem to be highly specialized. They have been
characterized as feature detectors, neurons that respond
selectively to very specific features of more complex stimuli.
5. After visual input is processed in the primary visual cortex,
it is often routed to other cortical areas for additional
processing. These signals travel through two streams:
a. Ventral stream: processes the details of what objects
are out there (the perception of form and color)
b. Dorsal stream: processes where the object are (the
perception of motion and depth) 6. As signals move farther along in the visual processing
system, neurons become even more specialized or fussy
about what turns them on, and the stimuli that activate
them become more and more complex. Ex: cells in the
temporal lobe of monkeys and humans that are especially
sensitive to pictures of face. These neurons respond even to
pictures that merely suggest the form of faces.
7. Another dramatic finding in this area of research is that the
neurons in the ventral stream pathway that are involved in
perceiving faces can learn from experience.
8. Cortical processing of visual input is begun in the primary
visual cortex. From there, signals are shuttled through the
secondary visual cortex and onward to a variety of other
areas in the cortex along a number of pathways. Two
prominent pathways are highlighted here. The
magnocellular, or “where pathway,” which processes
information about motion and depth, moves on to areas of
the parietal lobe. The parvocellular, or “what pathways”,
which processes information about color, form, and texture,
moves on to areas of the temporal lobe.
iii. Multiple methods in vision research
1. fMRI, microelectrodes, observations of the performance.
2. Early 1960’s: Hubel and Wiesel
a. Microelectrode recording of axons in primary visual
cortex of animals.
b. Discovered feature detector: neurons that respond
selectively to lines, edges, etc.
c. Groundbreaking research: Nobel Prize in 1981.
3. McCullough effect: a well-known afterimage phenomenon
that differs from other color afterimage effects because it is
contingent on both color and pattern/form.
4. Visual agnosia: an inability to recognize familiar objects.
The area of the brain known as V1, or the primary visual
cortex mediates the effect, and that it does not depend on
conscious form perception.
e. Viewing the world in color
i. The stimulus for color
1. Light with the longest wavelengths appears red, whereas
those with the shortest appear violet.
2. Although wavelength wields the greatest influence,
perception of color depends on complex blends of all three
properties of light. Wavelength is most closely related to
hue, amplitude to brightness, and purity to saturation.
3. There are two kinds of color mixture:
a. Subtractive color: mixing works by removing some
wavelengths of light, leaving less light than was
originally there. Paints yield subtractive mixing
because pigments absorb most wavelengths,
selectively reflecting specific wavelengths that give rise to particular colors. Subtractive color mixing can
also be demonstrated by stacking color filters.
b. Additive color: mixing works by superimposing
lights, putting more light in the mixture than exists
in any one light by itself.
4. Human processes of color perception parallel additive color
mixing much more closely than subtractive mixing.
ii. Trichromatic theory of color vision
1. Trichromatic theory of color vision holds that the human
eye has three types of receptors with differing sensitivities
to different light wavelengths. They are red, green, and blue.
2. Color-blindness encompasses a variety of deficiencies in the
ability to distinguish among colors. More in males than
females. Most people who are color-blind are dichromate:
they make do with only two color channels.
iii. Opponent process theory of color vision
1. Complementary colors are pairs of colors that produce grey
tones when mixed together.
2. Afterimage: a visual image that persists after a stimulus is
3. Trichromatic theory cannot account for the appearance of
4. Opponent process theory of color vision holds that color
perception depends on receptors that make antagonistic
responses to three pairs of color. Red versus green, yellow
versus blue, and black versus white.
5. Grapheme-color synesthesia: when individuals perceive a
letter or digit, they concurrently and unintentionally
experience the perception of an associated color.
iv. Reconciling theories of color vision
1. George Wald demonstrated that the eye has three types of
cones, with each type being most sensitive to a different
band of wavelengths. The three types of cones represent
the three different color receptors predicted by
2. Cells in the retina, the LGN, and the visual cortex that
respond in opposite ways to red versus green and blue
versus yellow. There are ganglion cells in the retina that
are excited by green and inhibited by red. Other ganglion
cells in the retina work in just the opposite way, as
predicted in opponent process theory.
3. Mapmakers have long known that a minimum of four colors
is needed to create maps in which no two adjacent
countries are the same color. Purves, Lotto and Polger
argue that the human visual system evolved to solve a
similar problem—ensuring that no two areas separated by
a common boundary will look the same if they actually are
different. f. Perceiving forms, patterns, and objects
1. Reversible figure: a drawing that is compatible with two
interpretations that can shift back and orth.
2. The key point is simply this: the same visual input can
result in radically different perceptions. No one-to-one
correspondence exists between sensory input and what you
perceive. This is a principal reason that people’s experience
of the world is subjective.
3. Perceptual set: a readiness to perceive a stimulus in a
4. Inattentional blindness: the failure to see fully visible
objects or events in a visual display.
ii. Feature analysis: assembling forms
1. Feature analysis: the process of detecting specific elements
in visual input and assembling them into a more complex
2. Feature analysis assumes that form perception involves:
a. Bottom-up processing, a progression from individual
element to the whole. (Feature detection theory)
i. Detect specific features of stimuluscombine
specific features into more complex
b. Top-down processing: a progression from the whole
to the elements. (Form perception)
i. Formulate perceptual hypothesis about the
nature of the stimulus as a wholeselect and
examine features to check
c. Subjective contours: the perception of contours
where none actually exist.
iii. Looking at the whole picture: gestalt principles
1. Gestalt psychologists: the whole is more than the sum of its
a. Reversible figures and perceptual sets demonstrate
that the same visual stimulus can result in very
2. Phi phenomenon: the illusion of movement created by
presenting visual stimuli in rapid succession.
3. Gestalt principles of form perception:
a. Figure and ground: the figure is the thing being
looked at, and the ground is the background against
which it stands.
b. Proximity: things that are close to one another seem
to belong together.
c. Closure: people often group elements to create a
sense of closure, or completeness.
d. Similarity: people also tend to group stimuli that are
similar e. Simplicity: the Gestaltist’s most general principle
was the law of Pragnanz. The idea is that people tend
to group elements that combine to form a good