Sensation & Perception
Synesthesia
Mixing of senses
May experience sounds as colours or tastes as touch sensations that
have different shapes
Women are more likely to be synaesthetes than men
Cross-wiring
One activity in one part of the brain evokes responses in another part
of the brain dedicated to another sensory modality
Word-colour linkages – hearing certain words is associated with
neural activity in parts of the visual cortex – doesn’t occur
Theory
o One
Pruning of neural connections that occurs in infancy
has not occurred
Brain regions retain connections that are absent in
most people
o Two
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
Introduction to the Process
Transduction
o Sensory receptors must translate this information into the language of
nerve impulses
Feature Detectors
o Specialized neurons called feature detectors break down and analyze the
specific features of the stimuli
Numerous stimulus ―pieces‖ are reconstructed into a neural representation that is
then compared with previously stored information, such as our knowledge of
what particular objects look, smell, or feel like
This matching of a new stimulus with our internal storehouse of knowledge
allows us to recognize the stimulus and give it meaning. We then consciously
experience a perception
Sensation vs. Perception
Sensation
o Stimulus-detection process by which our sense organs respond to and
translate environmental stimuli into nerve impulses that are sent to the
brain
Perception
o Making ―sense‖ of what our senses tell us—is the active process of
organizing this stimulus input and giving it meaning – decision making
1 Sensory Processes
Contact with outer world only possible through neurons that are specialized
sensory receptors – transform energy forms into language of nerve impulses
o Energy Forms – Senses
Taste – gustation
Sight – vision
Hearing – audition
Touch - pain, pressure, temperature
Smell – olfaction
Sense magnetic field
Electric currents
Infrared radiation
Balance and body position
Chemical composition of blood
Immune system– detection of foreign invaders
Stimuli to which different animals are sensitive vary
Sensory equipment adaptation to the environment
Transduction
o Process whereby the characteristics of a stimulus are converted into
nerve impulses
o Range of stimuli and the manner in which sense organs carry out
Sensory systems designed to extract information form the environment to survive
Psychophysics
o Studies relations between the physical characteristics of stimuli and
sensory capabilities
o Concerned with two kinds of sensitivity
Absolute limits of sensitivity
Weakest salt solution
Softest sound
Differences between stimuli
Smallest difference in brightness
Weight differences
Stimulus Detection: The Absolute Threshold
Absolute Threshold
o Lowest intensity at which a stimulus can be detected correctly 50 percent
of the time
o Lower absolute threshold, the greater the sensitivity
2 Signal Detection Theory
Individuals apparent sensitivity can fluctuate
Concept of fixed absolute threshold is inaccurate – no single point on the
intensity scale that separates nondetection from detection
Range of uncertainty – people set their own decision criterion
o Standard of how certain they must be that a stimulus is present before
they will say they detect it
o Change from time to time – depends on fatigue, expectation and potential
significance of stimuli
Signal Detection Theory
o Concerned with the factors that influence sensory judgments
o Experiment example
Participants are told that after a warning light appears, a barely
perceptible tone may or may not be heard
Tell the experimenter whether they heard the tone
Four possible outcomes
Tone - ―Yes‖ (a hit) or ―No‖ (a miss)
No tone - ―Yes‖ (a false alarm) or ―No‖ (a correct rejection)
At low stimulus intensities, both the participant's and the situation's
characteristics influence the decision criterion
Can be influenced to become bolder or more conservative by manipulating the
rewards and costs for giving correct or incorrect responses.
Increasing the rewards for hits or the costs for misses results in lower detection
thresholds
o Ex.
Navy radar operator more likely to notice a faint blip during
wartime than during peacetime (disastrous consequence for miss)
Doctors who will not perform a risky medical procedure without
strong evidence to support their diagnosis – patients careful about
―Yes‖ responses as costs are higher - higher detection thresholds
Difference Threshold
Subtle differences important
Difference Threshold (jnd – just noticeable difference)
o Smallest difference between stimuli that can be perceived50% of the time
Weber’s Law
o Difference threshold is directly proportional to the
magnitude of the stimulus with which the
comparison is being made, and can be expressed
as a Weber fraction
o Breaks down at incredibly high and low intensities
o Smaller the fraction – greater sensitivity to
differences
o Ex.
jnd value for weights is a Weber fraction
of approximately 1/50
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
3 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
Sensory Adaptation
Sensory systems are finely attuned to changes in stimulation
Sensory Adaptation
o Sensory neurons are engineered to respond to a constant stimulus by
decreasing their activity and diminishing sensitivity to unchanging
stimulus
o Part of everyday experience
o Reduce overall sensitivity – frees senses from constant and mundane
informative changes in environment
Sensory Systems
Vision
Normal stimulus: electromagnetic energy, or light waves
Measured in nanometres (or one billionth of a metre)
Electromagnetic Spectrum
o X-rays
o TV & Radio signals
o Infrared
o Ultraviolet
o Visual spectrum
Wavelengths from 700 (red) to 400
(blue-violet) nanometers
Human Eye
Cornea
o Light waves enter the eye through
o Transparent protective structure at the front of the eye
Pupil
o Behind cornea
o Adjustable opening that can dilate or constrict to control the amount of
light that enters the eye
o Size is controlled by muscles in the coloured iris that surrounds the pupil
o Low levels of illumination cause the pupil to dilate, letting more light into
the eye to improve optical clarity
o Bright light triggers constriction of the pupil
Lens
o Elastic structure that becomes thinner to focus on distant objects and
thicker to focus on nearby objects
o Lens of the eye focuses on the visual image on the light-sensitive retina
Retina
o Multi-layered tissue at the rear of the fluid-filled eyeball
o Lens reverses the image from right to left and top to bottom when
projected on the retina
o Brain reconstructs the visual input into the image that we perceive
o Ability to see clearly depends on the len’s ability to focus the image
4 directly onto the retina
Myopia
o Good vision for nearby objects but difficultly seeing faraway objects
(nearsightedness)
o Lens focuses the visual image in front of the retina (too near the lens),
resulting in a blurred image for faraway objects
o Eyeball is longer (front to back) than normal
Hyperopia
o Excellent distance vision but have difficulty seeing close-up objects
clearly
o Farsightedness
o Lens does not thicken enough and the image is therefore focused on a
point behind the retina (too far from the lens)
o Eyeball becomes shorter overtime
o Middle-aged people acquire reading glasses
Astigmatism
o Refractive errors due to a curvature of the cornea
Photoreceptors: Rods & Cones
Retina
o Multi-layered screen that lines the back surface of the eyeball and
contains specialized sensory neurons – an extension of the brain
o Contains two types of light-sensitive receptor cells, called rods and cones
because of their shapes
o 120 million rods, 6 million cones in human eye
Rods
o Function best in dim light
o Primarily black-and-white brightness receptors
o 500 times more sensitive to light than are the cones – no not give rise to
colour sensations
o Retinas of night creatures – contain only rods
o Humans – rods are found throughout retina except in the fovea (small
area in the centre of the retina)
o Periphery of the retina contains mainly rods
Cones
o Colour receptors
o Function best in bright illumination
o Active during the day – pigeon and chipmunk
o Humans – decrease in concentration as one moves away from the centre
of the retina
Send messages to the brain via two additional layers of cells
Bipolar Cells
o Synaptic connections with the rods and cones
o Synapse with a layer of about one million ganglion cells, whose axons are
collected into a bundle to form the optic nerve
Process
o Input from more than 126 million rods and cones is eventually funneled
into only one million traffic lanes leading out of the retina toward higher
visual centres
5 o Rods and cones not only form the rear layer of the retina – light-sensitive
end actually point away from the direction of the entering light so that they
receive only a fraction of the light energy entering
o Many rods are connected to the same bipolar cell
Combine or ―funnel‖ their individual electrical messages to the
bipolar cell – additive effect of many signals may be enough to fire
it
We can more easily detect a faint stimulus
Ex. Dim star - If we look slightly to one side so that its image falls
not on the fovea but on the peripheral portion of the retina – rods
are packed most densely
o Cones that lie in the periphery of the retina
Share bipolar cells
In fovea, the densely packed cones each have their own private
lane to a single bipolar cell
Visual acuity (fine detail) is greatest when the visual image
projects directly onto fovea
Focusing results in the firing of large number of cones and their
private-line bipolar cells
o 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 (no photoreceptors)
Unaware of blind spot because our perceptual system ―fills in‖
missing part of field
Visual Transduction: From Light to Nerve Impulses
Transduction
o Process whereby the characteristics of a stimulus are converted into
nerve impulses
Photopigments
o Rods and cones translate light waves into nerve impulses through the
action of protein molecules
o Absorption of light by molecules produces a chemical reaction – changes
the rate of neurotransmitter release at the receptor’s synapse with the
bipolar cells
o Greater the change in transmitter release – stronger the signal passed to
bipolar cells and in turn to the ganglion cells whose axon forms the optic
nerve
Nerve Responses
6 o Triggered at three levels
Rod or cone
Bipolar cell
Ganglion cell
o Message is instantaneously on its way to the visual relay station in the
thalamus – then onto visual cortex of brain
Brightness Vision & Dark Adaptation
Rods are far more sensitive than cones under conditions of low illumination
Brightness sensitivity of both the rods and the cones depends in part on the
wavelength of the light
Rods have much greater brightness sensitivity than cones throughout the colour
spectrum except at the red end, where rods are relatively insensitive
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
After absorbing light, a photoreceptor is depleted of its pigment molecules for a
period of time
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
Cones gradually become sensitive to fainter lights as time passes, but after about
5 to 10 minutes in the dark, their sensitivity has reached its maximum
Rods - whose photopigments regenerate more slowly, do not reach their
maximum sensitivity for about half an hour
Colour Vision
Two theories of colour vision
Trichromatic Theory
Additive Colour Mixture
o Three types of colour receptors in the retina
o A beam of light of a specific wavelength directed onto a white surface is
perceived as the colour that corresponds to that wavelength on the visible
spectrum
o Beams of light that fall at certain points within the red, green, or blue
colour range are directed together onto the surface in the correct
proportions
o Combined or additive mixture of wavelengths will result and any colour in
the visible spectrum can be produced
o White at the point where all three colours intersect
o Assumes that colour perception results from the additive mixture of
impulses from cones that are sensitive to red, blue, and green
o Problems
Theory: yellow is produced by activity of red and green receptors
People with red-green colour blindness are able to
experience yellow
Colour Afterimage
7 An image in a different colour appears after a colour
stimulus has been viewed steadily and then withdrawn
Subtractive Colour Mixture
o Mixing pigments or paints produces new colours by subtraction—that is,
by removing (i.e., absorbing) other wavelengths
o Paints absorb (subtract) colours different from
themselves while reflecting their own colour
o Ex. blue paint mainly absorbs wavelengths that
correspond to nonblue hues. Mixing blue paint with
yellow paint (which absorbs wavelengths other than
yellow) will produce a subtractive mixture that emits
wavelengths between yellow and blue (i.e., green).
o Theoretically, certain wavelengths of the three
primary colours of red, yellow (not green, as in
additive mixture), and blue can produce the whole
spectrum of colours by subtractive mixture
Opponent-Process Theory
Assumed that there are three types of cones
Proposed that 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
For example, a red-green cone responds with one chemical
reaction to a green stimulus and with its other chemical reaction (opponent
process) to a red stimulus
Ex. Staring at a coloured-image
o Stared at the black and green colours, the neural processes that register
these colours became fatigued
o Cast gaze on the white surface, which reflects all wavelengths, a
―rebound‖ opponent reaction occurred
o Each receptor responded with its opposing white or red reactions
Dual Process – Colour Transduction
Combines the theories to account for the colour transduction process
Trichromatic theorists were right that cones do indeed 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 in the spectrum
Opponent processes do not occur at the level of the cones, as he maintained
Certain ganglion cells in the retina, as well as some neurons in visual relay
stations and the visual cortex, respond in an opponent-process fashion by
altering their rate of firing
Colour-Deficient Vision
Trichromats
o Normal colour vision
o Sensitive to all three systems
Red-green
Yellow-blue
8 Black-white
o Dichromat
Person who is colour-blind in only one systems (red-green or blue-
yellow)
o Monochromat
Sensitive to only black-white system
Analysis & Reconstruction of Visual Scenes
Once the transformation of light energy to nerve impulses occurs, the process of
combining the messages received from the photoreceptors into the perception of
a visual scene begins
Feature Detectors
From the retina, the optic nerve sends nerve impulses to a visual relay station in
the thalamus, the brain's sensory switchboard
Input is then routed to various parts of the cortex, particularly the primary visual
cortex in the occipital lobe
Point-to-point correspondence between tiny regions of the retina and groups of
neurons in the visual cortex
Fovea
o One-to-one synapses of cones with bipolar cells produces high visual
acuity, is represented by a disproportionately large area of the visual
cortex
o More than one cortical ―map‖ of the retina; there are at least 10 duplicate
mappings
Groups of neurons within the primary visual cortex are organized to receive and
integrate sensory nerve impulses originating in specific regions of the retina
Feature Detectors
o Fire selectively in response to stimuli that have specific characteristics
o Certain neurons fired most frequently when lines of certain orientations
were presented
o One neuron might fire most frequently when a horizontal line was
presented; another neuron would fire most frequently to a line of a slightly
different orientation
o Some cells respond to bars, slits, edges in certain positions
o Respond to colour, movement, depth
Parallel processing
o Subdivide a visual picture into components and process them at the same
time
o Construct a unified image of its properties
Visual Association Cortex
o Final stages in the process occur
o Information is analyzed and recombined by the primary visual cortex and
is routed to the VAC
o Complex features of visual image are combined and interpreted in light of
memories and knowledge
Audition
Stimuli for sense of hearing – sound waves – mechanical energy
Sound
9 o Pressure waves in air, water or conducting medium
o Vibrations of sound caused by successive waves of compression and
expansion among the air molecules surround source
o Two Characteristics
Frequency
Number of sound waves, or cycles per second
Hertz (Hz) - technical measure of cycles per second
Related to the pitch (higher pitch: higher frequency)
Human Range: 20 Hz – 20,000 Hz
Amplitude
Vertical size of sound waves
Amount of compression and expansion of molecules in
conducting medium
Primary determinant of the perceived ―loudness‖
Differences expressed as decibels (db) – measure of
physical pressures that occur at eardrum
Increase tenfold – decibel
Auditory Transduction: From Pressure Waves to Nerve Impulses
Transduction System
o Made up of tiny bones, membranes, and liquid-filled tubes designed to
translate pressure waves into nerve impulses
o Sound waves travel into an auditory canal leading to the eardrum –
movable membrane that vibrates in response to waves (1,200 km/h)
Middle Ear
o Beyond eardrum
o Cavity housing three smallest bones
Hammer (malleus)
Attached to eardrum
Anvil (incus)
Stirrup (stapes)
Attached to the oval window (membrane) – boundary
between middle and inner ear
o Amplifies sound waves 30x
Inner Ear
o Contains cochlea
Coiled, snail-shaped tube
Filled with fluid
Contains basilar membrane
o Basilar Membrane
Sheet of tissue
Runs its length
o Origin of Corti
Resting on basilar membrane
Contains 16,000 tiny hair cells (sound receptors)
o Hair Cells
Tips are attached to tectorial membrane that overhands the
basilar membrane along the entire length of cochlea
10 Synapse with neurons of the auditory nerve – sends impulses via
an auditory relay station in thalamus to auditory cortex – temporal
lobe
Process
o Sound waves strike eardrum
o Pressure created at the oval window by hammer, anvil and stirrup
o Sets fluid inside cochlea into motion
o Fluid waves vibrate the basilar membrane and the membrane above it
o Bending of hair cells
o Triggers a release of neurotransmitter substance into synaptic space
between hair cells and neurons of auditory nerve
o Nerve impulses result and sent to brain
o Auditory cortex – feature detector neurons that respond to specific kinds
of input
Coding of Pitch & Loudness
Transforms sensory qualities of loudness and pitch into nerve impulses
Loudness
o High-amplitude sound waves cause hair cells to bend more
o Release more neurotransmitter substance
o Resulting in a higher rate of
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