Sensation and
Perception, Part I
BASIC PRINCIPLES
What are Sensation and Perception?
• Sensation: Earliest contact between environment and nervous
system
• Sensation involves translating physical stimulus properties (light,
sound, heat, pressure, chemical composition) into electrical
impulses
• Transduction: conversion of physical stimulus properties into
neural impulses
• Occurs within sensory organs (receptors)
What are Sensation and Perception?
• Perception: Further processing and interpretation of sensory
information
• Conscious experience of the world
Sensation and Perception: Basic Principles
• Need to signal quality and quantity of sensory experience
• Quality: determined by which sensory system is stimulated (e.g.,
stimulation of receptors in the eye signals that light is present)
• Quantity: signaled by the rate of neural impulses
• More intense stimulus elicits higher rate of action potentials
Sensation: Thresholds
• Basic question: How sensitive are our sensory systems?
• Absolute threshold: How intense does a stimulus need to be before
you can just detect it?
• Difference threshold: How much does the intensity of a stimulus
need to change before the change is noticeable?
Determining Absolute Threshold
Ideal Data
Typical Data
100
Percent Detection
Percent Detection
100
80
60
40
80
60
40
20
20
0
0
1
2
3
4
5
6
Stimulus Intensity
7
8
1
2
3
4
5
6
Stimulus Intensity
7
8
50%
Percent Detection
100
80
60
Absolute threshold: Stimulus
intensity that can be detected 50%
of the time.
40
20
0
1
2
3
4
5
6
Stimulus Intensity
7
8
Difference Threshold
• Amount of change that can be detected is a percentage of the
stimulus intensity
• Example: can detect a 7.9% change in brightness
• But a 1.3% change in shock intensity!
Sensation and Perception: Basic Principles
• Adaptation: sensory systems respond less over time to a constant
stimulus
• Can produce dramatic aftereffects
Sensation and
Perception, Part I
THE EYE
Vision: The Eye
• Pupil: opens (dilates) and
closes (contracts) to control
amount of light entering eye
• Iris: muscle that controls the
pupil’s size
Vision: The Eye
• Cornea
• Clear – lets light in
• Curved – focuses light as it
enters
• Lens
• Changes shape to bend light
more or less (accommodation)
• Adjustable focus
Vision: The Eye
• Retina
• Photosensitive layer at the back
of the eye
• Contains receptors: rods and
cones
Rods and Cones
Rods
Cones
• 120 million per eye
• 6 million per eye
• Very sensitive to light – work
best in dark conditions
• Not very sensitive to light –
work best in light conditions
• Not sensitive to color
• Sensitive to color
• Poor acuity
• Good acuity
The Fovea
• Area of highest acuity on the retina
• Contains only cones
• Center of vision falls on the fovea
• Note: rest of retina contains both rods and cones, but many more
rods than cones
The Eye: Transduction
• Rods and cones contain photopigment
• Breaks down in response to light
• Triggers action potentials in neurons that get input from rods and
cones
• These signals are carried to the brain by ganglion cells
• Axons form the optic nerve
• Blind spot where axons leave eye – no rods or cones
Sensation and
Perception, Part I
VISUAL PATHWAYS
Visual Pathways
• Optic chiasm: crossing over of
half of fibers
• Left visual field processed by
right hemisphere
• Right visual field processed by
left hemisphere
Visual Pathways
• Primary visual cortex:
• In occipital lobes
• Receives input from thalamus
• First area of cerebral cortex to
receive visual input
Visual Pathways
• Ventral Pathway: The “What”
Pathway
• Identifying objects and object
features
• Dorsal Pathway: The “Where”
Pathway
• Locating objects and perceiving
motion
Sensation and
Perception, Part I
COLOR VISION
Vision: Seeing Color
• Color is determined
primarily by wavelength
Vision: Seeing Color
• How do we see color?
• Trichromatic theory
• Opponent-process theory
Trichromatic Theory
• Three cone types (three types of photopigment)
• Sensitive to different wavelengths
Trichromatic Theory
• Pattern of activation across cone types determines perceived color
Opponent-Process Theory
• There are some color vision
phenomena that trichromatic
theory can’t explain
• Example: retinal afterimages
Opponent-Process Theory
• Color pairings in the visual system:
• Red with green
• Blue with yellow
• Theory:
• Cells that are sensitive to red and green
• Other cells sensitive to blue and yellow
Color Vision: Combining Theories
• Both theories are correct
• Trichromatic theory: cones in the retina
• Opponent-process theory: cells (e.g., in the thalamus) that get input
from the cones