Transcript CHAPTER 3

FEM 4100
Topic 5
Perception Mechanism,
Awareness & Attention
Overview
1.The Process of Sensation: Brain waves & perception
2.Vision
3.Hearing
4.Smell and Taste
5.The Skin Senses
6.The Spatial Orientation Senses
7.Influences on Perception
8.Principles of Perception & Unusual Perceptual Experiences
Questions:
 The Skin Senses
 How does the skin provide sensory information?
 What is the function of pain, and how is pain influenced by psychological
factors, culture, and endorphins?
 The Spatial Orientation Senses
 What kinds of information do the kinesthetic and vestibular senses
provide?
 Influences on Perception
 What is gained and what is lost in the process of attention?
 How does prior knowledge influence perception?
 How does information from multiple sources aid perception?
The Process of Sensation:
Question address
How is sensory information
transmitted to the brain?
1: The Process of Sensation
 Sensation
 is the process through which the
senses pick up visual, auditory, and
other sensory stimuli and transmit
them to the
.
 Perception
 is the process by which sensory
information is actively organized and
interpreted by the brain.
 Information → organised
 Interpretation → brain
Just Noticeable Difference (JND)
 The smallest increase or
decrease in a physical
stimulus that is required to
produce the “just noticeable
difference (JND).” The JND is
the smallest change in
sensation that a person is able
to detect 50% of the time.
Which one weighs more?
Weber’s Law states the JND is based on a percentage or
proportion of stimulus change rather than a fixed amount of change.
 A weight must increase or decrease by 1/50th or 2% for JND
2 lbs difference needed in 100 lb weight (2% of 100 lb)
 a tone must be .33% higher or lower .
Sensory Receptors
 Are highly specialized cells in
the sense organs
 detect and respond to one
type of sensory stimuli
 converts the stimuli into
nerve impulses (neural)
through transduction
process.
Sensory Adaptation
The process in which sensory receptors grow accustomed to
constant, unchanging levels of stimuli over time.
Smokers grow accustomed to smell of cigarettes
Vision: Question to be addressed
How does each part of the eye function
in vision?
What path does visual information take
from the retina to the primary visual
cortex?
How do we detect the difference
between one color and another?
What two major theories attempt to
explain color vision?
2. Vision
The most studied sense
Sensory receptor
 A specialized neuron that detects a particular category of physical
events/stimulus.
Sensory transduction
 The process by which sensory stimuli are transduced into slow,
graded receptor potentials.
Receptor potential
 A slow, graded electrical potential produced by a receptor cell in
response to a physical stimulus.
What are the visual stimulus?
 Feature detectors:
 Neurons in the brain that respond only to specific visual patterns.
1. Perceived color of light is determined by
 Hue (the specific color perceived)
 Determined by wavelength.
2. Brightness
 Determined by the intensity of the electromagnetic radiation or light
energy that is perceived.
3. Saturation
 Determined by the purity of the light wave or color
Vision: What we can/can’t see
 Visible Spectrum the band of electromagnetic waves visible to the human eye.
 Wavelength – the distance from the peak of a light wave to the peak of the next wave.
 Saturation (purity) vs Brightness (intensity of light energy)
The Anatomy of the Visual System
The Eye
 Sclera
 The white tissue of the eye
 Conjunctiva
 Mucous membranes that line
the eyelid and protect the
eye.
 Cornea
 Tough, transparent,
protective layer
 Covers front of eye
 Bends light rays inward
through the pupil
 Lens
 Consists of a series of
transparent, disk-shaped,
onion-like layers behind the
iris & pupil
 Its shape can be changed by
contraction of ciliary muscles.
 Changes shape as focusing
on objects
 Pupil
 Adjustable opening in the iris
that regulates the amount of
light that enters the eye.
 Iris
 Pigmented ring of muscles
situated behind the cornea.
The Eye
Accommodation
Changes in the thickness of the
lens, accomplished by the ciliary
muscles, that focus images of near
or distant objects on the retina
 Retina
 The neural tissue and photoreceptive
cells located on the inner surface of
the posterior portion of the eye
 Contains visual sensory receptors
 Rods
 Photoreceptor cells in the retina,
 sensitive to the light of low intensity
 Look like slender cylinders
 Allow eye to respond to low light
 Cones
 Photoreceptor cells in the retina;
 maximally sensitive to one of three
different wavelengths of light and
hence encodes color vision
 Enable humans to see color and fine
detail
 Do not function in very dim light
The Eye
 Fovea
 A small center area of retina that
mediates the most acute vision.
 Contains only color-sensitive cones.
 Has largest concentration of cones
 Provides clearest and sharpest vision
 Optic Disk
 Location on the retina where fibers of
ganglion cells exit the eye;
responsible for the blind spot (point in
each retina with no rods or cones)
 Optic Nerve
 Caries visual information from retina
to both sides of the brain
 Primary Visual Cortex
 Part of brain which processes visual
information
The Eye
From Retinal Image
to Meaningful Information
Major Structures of the Visual System
Structure
 Cornea
 Iris
Their Functions
 Translucent covering on front of eye that bends light rays inward
towards pupil.
 Colored part of the eye that adjusts so constant amount of light
enters through the pupil.
 Pupil
 Opening in the center of the iris through which light enters the eye.
 Lens
 Transparent disk-shaped structure behind pupil that adjusts its
shape to allow focusing on objects at varying distances.
 Retina
 Layer of tissue on inner surface of the eye. Contains sensory
receptors for vision.
 Rods
 Cones
 Specialized receptor cells in retina that are sensitive to light
changes
 Specialized receptor cells in retina that enable humans to see fine
detail and color in adequate light.
 Fovea
 Small area at center of retina, packed with cones, on which objects
viewed directly are clearly and sharply focused.
 Optic Nerve
 Nerve that carries visual information from the retina to the brain.
 Blind Spot
 Area in each eye where the optic nerve joins the retinal wall and no
vision is possible.
Major Structures of the Visual System… Continue

Bipolar cell
• A bipolar neuron located in the middle layer of the retina, conveying
information from the photoreceptors (rod & cones) to the ganglion cells.

Ganglion cell
• A neuron that receives visual information from bipolar cells; its axons
give rise to the optic nerve

Horizontal cell
• A neuron in the retina that interconnects adjacent photoreceptors and
the outer processes of the bipolar cells.

Amacrine cell
• A neuron in the retina that interconnects adjacent ganglion cells and the
inner processes of the bipolar cells.
Theories of Color Vision
 Trichromatic Theory
 Three types of cones in the retina each make a maximal
chemical response to one of three colors.
 BLUE, GREEN, or RED.
 Each cone is sensitive to one of the colors.
 These three colors can then be combined to form
any visible color in the spectrum.
 EG: when the red and blue cones are simulated in a certain way
you will see the colour purple.
 Application on colourblindness:
 Protanopia (blindness to red):
 ‘Red’ cones filled with ‘green’ cones → only see blue and green →
red & green -- yellowish
 Deuteranopia (blindness to green)
 ‘Green’ cones filled with ‘red’ cones → cant see green
 Tritanopia (blindness to blue)
 Blue cones are damaged or a lack of them → only see in greens
and reds → cant differentiate green and yellow (pink).
Photoreceptors: Trichromatic coding
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Protanopia (blindness to red)
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Deuteranopia (blindness to green)
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An inherited form of defective color vision in which red and green hues are
confused but acute vision is normal;
“Red” cones are filled with “Green” cone cells (absence of L-Cones).
Red appears dark - only two types of cone pigments are present (Green & Blue).
They see the world in shades of yellow and blue; both red and green look
yellowish to them.
An inherited form of defective color vision in which red and green hues are
confused but acute vision is normal;
“Green” cones are filled with “Red” cone cells (all M-cones are absent).
Only two types of cone pigments are present: Red and Blue.
Cant see the green part but visual acuity is normal.
Tritanopia (blindness to blue)
 An inherited form of defective color vision in which “Blue” cones are either
lacking or faulty (total absence of S-cones); but acute vision is normal;
 See the world in greens and reds. Blue looks green and yellow looks pink.
(Cant distinguish blue and yellow)
 Resulting in blindness to the blue end of the spectrum.
WHICH PICTURE IS MATCH WITH WHICH TYPE OF COLOUR DEFICIENCY?
 “Color” is determined by the wavelength of a stream of light, by detecting
the wavelength of incoming light, the eye can determine what color it is
looking at. The (normal) eye contains 3 types of cone cells, each containing
a different pigment:
 The L-cone detecting long wavelength light (peaking in the yellows – but
also responsible for reds).
 The M-cone detecting medium wavelength light (peaking in the greens).
 The S-cone which detects short wavelength light (peaking with blue).
 Your brain determines what color it is seeing by observing the ratio between
the signals it receives from each of the three types of cones. Color
blindness occurs when one or more types of cones are either totally absent,
or has a limited spectral sensitivity.
Theories of Color Vision
 Opponent-Process Theory
 Three kinds of cells respond by increasing or decreasing their
rate of firing when different colors are present.
 The opponent color process works through a process of
excitatory and inhibitory responses, with the two components of
each mechanism opposing each other.
 For example, red creates a positive (or excitatory) response
while green creates a negative (or inhibitory) response.
 These responses are controlled by opponent neurons, which
are neurons that have an excitatory response to some
wavelengths and an inhibitory response to wavelengths in the
opponent part of the spectrum.
Types of cells:
Red/green – firing increases when red present;
green decreases firing (R+G-)
Yellow/blue – firing increases when yellow
present; blue decreases firing (Y+B-)
White/black – firing increases when white
present, black decreases firing (W+Bl-)
The opponent-process theory explains how we
see yellow though there is no yellow cone.
It results from the excitatory and inhibitory
connections between the three cone types.
Specifically, the simultaneous stimulation of red (
L cones) and green (M cones) is summed and in
turn inhibits B+Y-, which results in the
perception of yellow.
However, when blue light is present, the S cone
is activated, the B+Y- cell receives excitatory
input and blue is perceived.
COLOR MIXING (Left) vs PIGMENT MIXING (Right)
Color mixing: Addition of 2 or more light source.
Pigment mixing: Combine 2 types of pigment (such as mixing paint)
Thus, OUTCOME IS different
CM: Red light + Green light (on white screen) = yellow
PM: Yellow + Blue = green
Analysis of Visual Information:
The Striate Cortex (Primary Visual Cortex or V1)
 David Hubel and Torsten Wiesel
• 1960s at Harvard University
• Discovered that neurons in the visual cortex did not simply
respond to light; they selectively responded to specific
features of the visual world.
• Specific features:
• Orientation and movement: Neuron in visual cortex - Simple cell
(orientation), complex cell (movement) and hypercomplex cell (orientation).
 Spatial frequency: Sine-wave grating and Spatial frequency
 Retinal disparity
 Color
• Two system of visual: Dorsal stream and ventral
stream.
Analysis of Visual Information:
The Visual Association Cortex
 Extrastriate cortex
• A region of the visual association cortex; receives fibers from the
striate cortex and projects to the inferior temporal cortex.
• Regions respond to particular features of visual information such as
orientation, movement, spatial frequency, retinal disparity, or color.
 Dorsal stream
• A system of interconnected regions of the visual cortex involved in
the perception of spatial location, beginning with the striate cortex
and ending with the posterior parietal cortex.
 Ventral stream
• A system of interconnected regions of visual cortex involved in the
perception of form,
• beginning with the striate cortex and ending with the inferior
temporal cortex.
Analysis of Visual Information:
The Striate Cortex
 Retinal Disparity
• The fact that points on objects located at different distances from
the observer will fall on slightly different locations on the two
retinas; provides the basis for depth perception
 Color
 Cytochrome oxidase (CO) blob
• The central region of a module of the primary visual cortex,
revealed by a stain for cytochrome oxidase; contains
wavelength-sensitive neurons.
• Ocular dominance
• The extent to which a particular neuron receives more input from
one eye than from the other.
 Cortical blindness
• Blindness caused by damage to the optic radiations or primary
visual cortex.
Analysis of Visual Information:
The Visual Association Cortex
 Studies with humans
 Achromatopsia
• Inability to discriminate among different hues/shades; caused
by damage to the visual association cortex.
 Inferior temporal cortex
• In primates, the highest level of the ventral stream of the visual
association cortex ("What Pathway", is associated with form
recognition and object representation); located on the inferior
portion of the temporal lobe.
 Agnosia
• Inability to perceive or identify a stimulus by means of a
particular sensory modality.
 Visual agnosia
• Deficits in visual perception in the absence of blindness;
caused by brain damage.
 Apperceptive visual agnosia
• Failure to perceive objects even though visual acuity is
relatively normal.
Analysis of Visual Information:
The Visual Association Cortex
 Analysis of form (what do you see?)
 Prosopagnosia
• Failure to recognize particular people by the sight of
their faces.
 Associative visual agnosia
• Inability to identify objects that are perceived visually,
even though the form of the perceived object can be
drawn or matched with similar objects.
 Perception of movement (how thing move?)
 Fusiform face area
• A region of the extrastriate cortex located at the base
of the brain; involved in perception of faces and other
objects that require expertise to recognize.
 Akinetopsia
• Inability to perceive movement, caused by damage to
area V5 of the visual association cortex.
Neural Circuitry in the Retina
Neural Circuitry in the Retina
 The photoreceptors → contain light-absorbing pigment
molecules.
 In the dark: Photoreceptors constantly release
neurotransmitter (negatively charged) –
hyperpolarised dendrite of bipolar cell
 Thus, when light strike → it does not cause the neuron to
depolarize → instead, the photoreceptor does exactly
the opposite.
 When pigment molecules in a photoreceptor cell absorb
a photon of light → sodium gates (neg. charged) in the
membrane of the cell close, and the neuron
becomes hyperpolarized.
 This means → the photoreceptor is no longer generating
an action potential → so it is not delivering inhibitory
neurotransmitters to the bipolar cell(s) it synapses with.
 Since the bipolar cells are no longer receiving inhibitory
neurotransmitters from the photoreceptors → they
depolarize and generate action potentials.
 The neurotransmitters released by the bipolar cells
are excitatory → cause the ganglion cells they synapse
with to depolarize and generate action potentials of their
own.
 And since the optic nerve is just the axons of the
ganglion cells, the impulses are relayed to the brain.
Pathway:
Stimulus from eye to brain
DARK → photoreceptor (release inhibitory NT)
→ LIGHT → photoreceptor (stop inhibitory NT)
→ bipolar cell (excitatory NT) → ganglion cell
(increase firing) → Axon of ganglion cell (optic
nerve) → BRAIN
Brain → Thalamus → Primary visual cortex
COLOR MIXING (Left) vs PIGMENT MIXING (Right)
Color mixing: Addition of 2 or more light source.
Pigment mixing: Combine 2 types of pigment (such as mixing paint)
Thus, OUTCOME IS different
CM: Red light + Green light (on white screen) = yellow
PM: Yellow + Blue = green
WHICH PICTURE IS MATCH WITH WHICH TYPE OF COLOUR DEFICIENCY?