Transcript Sensation and Perception

Sensation and Perception
Ch. 4 (pp. 92-119)
Let’s begin with some basic
definitions…



Sense – physical system that receives physical
stimulation from surrounding environment and
translates that stimulation into an electrochemical
message
Sensation – detecting physical energy in
environment and encoding it as neural signals.
Neurons transmit the information from the sense
organ to the brain.
Perception – processing of information done by
the brain – mental processes that organize and
interpret sensory information that has been
transmitted to the brain.
Six stages in sensation &
perception:






Stimulus is received by sensory receptors
Receptors translate stimulus properties into nerve
impulses
Feature detectors analyze stimulus features
Stimulus features are reconstructed into neural
representation
Neural representation is compared with previously
stored info in brain
Matching process results in recognition and
interpretation of stimuli
Transduction


Communication between the brain & the rest of the
body (& between different regions of the brain)
occurs via neuron. We recently learned how
communication between neurons occurs
electrochemically (within neurons: electrical;
between neurons: chemical). So the brain’s
“language” is electrochemical!
All senses involve something called receptor cells.
Their job is to transduce (transform or even
“translate”) physical stimulation/physical energy
from the environment into electrochemical
messages that can be understood by the brain.
So where do vision and hearing
(& the other senses) happen?
 The

Brain!
The physical energy in
the environment is
detected by the eyes,
ears, etc. but we can’t
see, hear, etc. until the
brain interprets them—
i.e., makes sense of
them. So in a way, we
see, hear, smell, etc. in
our brains!
The Visual System

Stimulus = Light
 Electromagnetic
energy in the form
of a lightwave
 Amplitude-hue
 Wavelengthbrightness
 Purity-saturation
Learning about the eye…
Cornea – transparent membrane covering the front of the eye
(protective) – the light enters the eye through the cornea
Pupil – small adjustable opening – regulates the amount of light
entering the eye
Iris – muscle which surrounds the pupil and controls the size
of the pupil
Lens – focuses incoming rays on back surface of the eye (retina) - elastic,
muscles stretch or thicken (thus changing the curvature of the lens) to bend
the light at the right angle (so light is focused on retina, depending upon
distance of object) this is called accommodation.
Common visual problems


Myopia
(nearsightedness)
 Focus of light falls
short of retina
Hyperopia
(farsightedness)
 Focus of light falls
behind retina
The Retina
Network of neurons
on rear surface
inside the eye
 Functions:
 Absorb light
 Process images
 Send visual info.
to brain

Sensory Receptor Cells

Two types of sensory receptor cells in the
retina
 Rods enable us to see black and white,
are more sensitive to light, and there are
100 million in each eye

Play key role in night vision and
peripheral vision
• p. 95 fig. 4.5 gradually adapt, but
improve vision the best
Sensory Receptor Cells

Cones enable us to see color, they work
best in bright light, there are 6.5 million
in each eye and they are located mostly at
the center of the eye
Play key role in daylight vision and color
vision
 Rapidly adapt

More on Sensory Receptor Cells



These cells transform light energy into neural
impulses that are sent to various areas in the brain
through the bundle of neurons called the optic
nerve.
What is the place called in which the optic nerve
leaves the eye?
 The blind spot
Why is it called “the blind spot”?
 Because there are no rods & cones on this area
(there are neurons instead), images that fall on
this area of the retina are not “seen”!
Fovea – center of retina with densely packed cones; visual acquity (picture
sharpness) is the greatest here
Optic Nerve – carries neural impulses to the brain
Occipital Lobe – neural impulses are than sent to the primary visual cortex
within the occipital lobe
Pathways from the Eyes to the
Visual Cortex
The mystery of feature
detectors…



Feature detectors are cortical neurons that respond
to specific features of a scene (e.g., particular
edges, lines, angles and movements)
The brain then needs to assemble the whole image
from these specific features.
“…perceptions arise from the interaction of many
neuron systems, each performing a simple task.”
How we see color…
Based on the wavelength of light the object
reflects rather than absorbs.
 Thus if an object absorbs every color of
light except red, then the object reflects red
and therefore we see red.
 Color is also based on interpretation of
context.
 Is the red I see the same red you see?

Color Perception Theories

Trichromatic (three color) Theory
 Young and Helmholtz
 three different retinal color receptors
respond in varying degrees to different
colored stimuli
 red
 green
 blue
Opponent-Process Theory
Opposing retinal processes
enable color vision
 signals are processed
by red-green, blueyellow, black-white
opponent process cells
 stimulated by one
wavelength and
inhibited by its
opponent




Each of the 3 cone types responds to two different
wavelengths
Activity in one set of cells in hypothalamus inhibit
activity of “opposite” cells
 (Green : Red; Blue : Yellow; Black : White)
Active Green cells inhibit Red cells.
Thus, no such colors (to us) as “greenish-red” or
“bluish-yellow”
Opponent ProcessAfterimage Effect
It takes both theories to explain
color perception…
Trichromatic
 The eye has three cones that respond to color
 Opponent Process
 Cells in the retina, thalamus, and visual
cortex respond in opposite ways to red-vsgreen, blue-vs-yellow, etc…
 Thus-both theories are needed in order to
explain the perception of color!

Perception goes beyond vision.
It involves what we see.


Perception goes “right”
more times than when it
goes “wrong.”
The times it “goes wrong”
however, can provide us
with important info on
how perceptions works!


Top-down processing
 Sensory info is
interpreted in light of
existing knowledge,
concepts, ideas, and
expectations
Bottom-up processing
 System takes in
individual elements of
a stimulus and then
combines them into a
unified perception
What shapes our perceptual sets?




What things do you
think might affect our
perceptual sets (i.e.,
our tendency to
perceive things in
certain ways)?
Context effects
Past experiences
Effects of our culture
A perceptual set is…


“a mental predisposition
to perceive one thing and
not another”
This is based on
experiences, assumptions
and expectations.
Gestalt Principles
Grouping
 The perceptual tendency to organize stimuli
into coherent groups
 Grouping Principles







Phi phenomenon-illusion of movement created by presenting
stimuli in rapid succession
proximity--group nearby figures together
similarity--group figures that are similar
continuity--perceive continuous patterns
closure--fill in gaps
connectedness--spots, lines, and areas are seen as unit when
connected
simplicity--organize elements in simplest way
Perceptual Organization:
Grouping Principles
Perceptual Organization:
Gestalt


Visual Capture
 tendency for vision to dominate
the other senses
Gestalt (Necker Cube) p. 107
 an organized whole
 tendency to integrate pieces of
information into meaningful
wholes
Perceptual Organization:
Closure

Gestalt
grouping
principles
are at work
here.
Perceptual Organization:
Grouping Principles

Gestalt
grouping
principles
are at work
here
Perceptual Organization: Depth
Perception
Depth perception chalk drawings
Perceptual Organization: Depth
Perception


Depth Perception
 ability to see objects in three dimensions
 allows us to judge distance
Binocular Cues – Depth Cues
 retinal disparity
 images from the two eyes differ
 closer the object, the larger the disparity
 convergence
 neuromuscular cue
 two eyes move inward for near objects
Perceptual Organization: Depth
Perception
Visual Cliff
Perceptual Organization: Depth Perception

Monocular Cues – Distance Cues
 relative size
 smaller image is more distant
 interposition
 closer object blocks distant object
 relative clarity
 hazy object seen as more distant
 texture coarse --> close
fine --> distant
Schemas



Another word used to
describe perceptual set is
schemas.
Schemas can influence our
perception because we
tend to see things that we
expect to see.
We interpret things based
on similar things we have
experienced in the past –
this is called assimilation.
Schemas…..



Now what do you think
would happen if we
receive new information
that doesn’t not fit our
schemas?
When we receive new
information we can ignore
it (!)
or we can change our
schemas to fit the new
information. This is
called accommodation.
Schemas are almost like a box that we put information into. The way we organize compartments
in the box is based on things like our past experiences, contexts, etc. and this influences our perceptions!