Sensation and Perception

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Sensation and Perception
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Sensation and Perception
The Eye
1. Light waves pass through the lens and reach
receptor cells
2. Receptor cells are activated and send signal
to bipolar cells
3. Bipolar cells then send signal to retinal
ganglion cells
4. Retinal ganglion cell axons form the optic
nerve, which transmits signals to the visual
cortex ( 1,000,000 fibers)
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Sensation and Perception
The Eye
Rods and Cones
There are two types of receptor cell in the
retina, rods and cones
Cones (colour vision) are concentrated in the
fovea (area of central focus)
Rods (black/white vision) are concentrated in the
periphery (none in the fovea)
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Sensation and Perception
Visual Sensitivity
Cone cells each synapse on to a single bipolar
cell, while several rod cells may synapse onto a
single bipolar cell
While the cone cells in the fovea provide the
most visual detail, rod cells in the peripheral
visual field are better motion detectors
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Sensation and Perception
Visual Processing
Information is processed at increasingly abstract
levels as it passes through the visual system
Some processing occurs at the retinal level
e.g., some retinal ganglion cells may fire only
when they receive a certain pattern of input from
bipolar/receptor cells (bug detector
cells in the frog eye)
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Sensation and Perception
Visual Processing
Signals from the right side of each retina are
sent to the right visual cortex
Signals from the left side of each retina are
sent to the left visual cortex
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Sensation and Perception
Feature Detection
Some cells in the visual cortex respond to
particular features of the input (edges, lines,
orientation, direction of movement, etc). These
are called feature detectors
The individual detected features are then
assembled at another level of the visual cortex
and interpreted as a visual object
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Sensation and Perception
Feature Detection
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Sensation and Perception
Feature Detection
Sometimes the activation of certain feature
detectors can result in the perception of visual
objects that are not really there
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Sensation and Perception
Feature Detection
Other times, ambiguous features can cause our
perceptions to fluctuate
The Necker Cube Which is the front surface of
the cube?
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Sensation and Perception
Feature Detection
Other times, ambiguous features can cause our
perceptions to fluctuate
All the necessary cues are present to interpret
this surface as the front
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Sensation and Perception
Feature Detection
Other times, ambiguous features can cause our
perceptions to fluctuate
but the same is true of this interpretation
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Sensation and Perception
Parallel Processing in the Visual System
Some examples of visual processing failures
Inability to perceive colour
Inability to perceive motion
Inability to perceive complex shapes
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Sensation and Perception
Parallel Processing in the Visual System
Unlike computers, which process information
serially (one piece at a time, but at a high
speed), the visual system processes information
in several parallel streams at the same time (at
a much slower speed)
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Sensation and Perception
Parallel Processing in the Visual System
A failure or interruption of one processing
stream (e.g., colour processing) may not disrupt
other streams
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Sensation and Perception
Parallel Processing in the Visual System
so motion, form, depth, etc are still
percieved, but all visual images are in black and
white
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Sensation and Perception
Parallel Processing in the Visual System
If visual processing was serial, a problem
anywhere in the system would probably result in
complete blindness
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Sensation and Perception
Colour Vision
Trichromatic colour theory
Cone cells perceive 1 of 3 primary colours of
light (red, green, and blue)
This colour info is combined through additive
mixing to represent all other colours
There is strong evidence that this is true at the
retinal level
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Sensation and Perception
Subtractive colour mixing (e.g., paint)
Mixing all colours results in black
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Sensation and Perception
Additive colour mixing (light)
Mixing all colours results in white
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Sensation and Perception
Colour Vision
Opponent-Process Theory
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Sensation and Perception
Colour Vision
Opponent-Process Theory
People who are red-green colourblind lack either
red or green cone cells
Yellow is a combination of red and green light
People who are red-green colourblind can still
see yellow as a colour (????)
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Sensation and Perception
Colour Vision
Opponent-Process Theory
3 (or 6) types of opponent-process
cells Red-Green Green-Red Blue-Yellow Yellow-Bl
ue Black-White White-Black
One colour turns cell on, the other colour
turns cell off
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Sensation and Perception
Colour Vision
Evidence for the Opponent-Process Theory
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Sensation and Perception
Colour Vision
If we tire out the green-perceiving cells, we
will get the opponent colour afterimage when
the green goes away
While processing is clearly trichromatic at the
retinal level, there certainly seems to be some
opponent-process involved en route to the visual
cortex
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Sensation and Perception
Colour Constancy
The perception of colour depends on the context
(colour of surroundings), as well as the actual
colour of the object
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Sensation and Perception
Audition
The intensity of an auditory stimulus is measured
in decibels (dB)
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Sensation and Perception
Audition
Sound, like light, is a waveform
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Sensation and Perception
Audition
Frequency pitch Amplitude volume
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Sensation and Perception
Audition
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Sensation and Perception
Audition
1. Sound waves enter the ear and cause the
eardrum to vibrate (sound to mechanical energy)
2. Vibrating eardrum moves a chain of 3 small
bones in the middle ear (hammer, anvil, and
stirrup)
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Sensation and Perception
Audition
3. Stirrup transmits vibrations through the oval
window into the fluid filled cochlea (mechanical
energy to waves)
4. Waves in cochlear fluid cause a particular
portion of the basilar membrane to
resonate/vibrate (which portion depends on the
frequency of the sound)
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Sensation and Perception
Audition
5. The cilia of hair cells in the part of the
basilar membrane that is affected will be
deflected, causing that cell to fire
6. Action potentials generated by the hair cells
are transmitted to the auditory cortex via the
auditory nerve (combined hair cell axons)
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Sensation and Perception
Audition
Pitch Perception How do we determine the pitch
of a sound?
Two different answers for high and low frequency
sounds
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Sensation and Perception
Audition
Place Theory (high-pitched sounds)
In the higher range of audible stimuli, a
particular frequency will cause a particular
point on the basilar membrane to resonate
Therefore, we can identify a pitch by determining
which hair cells are firing
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Sensation and Perception
Audition
Frequency Theory (low-pitched sounds)
Low frequency sounds will cause the entire
basilar membrane to vibrate, so place theory will
not work here
Hair cells along the entire membrane will fire,
but will do so at the same frequency as the
incoming stimulus, allowing us to determine the
pitch
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Sensation and Perception
Selective Attention
Which features you attend to will influence your
perception
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Sensation and Perception
Selective Attention
Unattended stimuli can also influence perception
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Sensation and Perception
Selective Attention
Unattended stimuli can also influence perception
Unattended message
river
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Sensation and Perception
Selective Attention
Unattended stimuli can also influence perception
Unattended message
Bank
money
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Sensation and Perception
Perceptual Illusions
Muller-Lyer Illusion
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Sensation and Perception
Perceptual Illusions
Muller-Lyer Illusion
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Sensation and Perception
Perceptual Illusions
Blue Worm Illusion
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Sensation and Perception
Perceptual Illusions
Why are visual illusions so common?
Visual Capture Humans are primarily
visually-oriented organisms, and in any situation
where visual information conflicts with other
sensory information, we tend to interpret the
situation as if the visual information is correct
(even when this is clearly not the case, as in
ventriloquism)
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Sensation and Perception
Perceptual Organization
Gestalt an overall form or whole
the whole is more than the sum of the parts
How do we organize separate elemental sensations
into a perceived whole?
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Sensation and Perception
Perception of Form
Figure vs. Ground
Figure and ground may flip, but we always make
the distinction between the two
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Sensation and Perception
Perception of Form
Grouping
There are 5 basic principles for grouping
objects Proximity Similarity Continuity Connected
ness Closure
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Sensation and Perception
Perception of Form
Proximity grouping things that are close
together in space
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Sensation and Perception
Perception of Form
Similarity grouping similar things together
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Sensation and Perception
Perception of Form
Continuity we will see a smooth, continuous
pattern, rather than an interrupted,
discontinuous pattern
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Sensation and Perception
Perception of Form
Connectedness areas / objects that are uniform
and linked are perceived as a single object
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Sensation and Perception
Perception of Form
Closure we will fill in gaps to create the
appropriate objects
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Sensation and Perception
Perception of Depth / Distance
Binocular Cues
Retinal Disparity
Convergence
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Sensation and Perception
Perception of Depth / Distance
Monocular Cues
Interposition
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Sensation and Perception
Perception of Depth / Distance
Monocular Cues
Relative Size
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Sensation and Perception
Perception of Depth / Distance
Monocular Cues
Relative Clarity
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Sensation and Perception
Perception of Depth / Distance
Monocular Cues
Texture Gradient
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Sensation and Perception
Perception of Depth / Distance
Monocular Cues
Relative Height
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Sensation and Perception
Perception of Depth / Distance
Monocular Cues
Motion Parallax
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Sensation and Perception
Perception of Depth / Distance
Monocular Cues
Linear Perspective
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Sensation and Perception
Perception of Depth / Distance
Monocular Cues
Light and Shadow
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The Questions
1. We have evolved perceptual processes that are
designed to alert us to informative changes in
the environment
2. Higher-level organizational structures being
used to determine what sensory info is relevant
to the task at hand is referred to as top-down
processing