Sensation and
Perception
Processing the World
Notice Anything Strange?
• “N is sort of…rubbery…smooth, L is sort of the
consistency of watery paint… Letters also have
vague personalities, but not as strongly as numbers
do.”
• “The letter A is blue, B is red, C is kind of a light gray,
D is orange…”
• I hear a note by one of the fellows in the band and
it’s one color. I hear the same note played by
someone else and it’s a different color. When I hear
sustained musical tones, I see just about the same
colors that you do, but I see them in textures.”
Synesthesia
• Synesthesia: the perceptual experience of one
sense that is evoked by another sense
• Some forms may be common: 1/100 people
experience synesthesia.
• Do you associate numbers or letters with colors,
tastes, unrelated sounds, etc.?
Synesthesia (continued)!
Sensation vs. Perception
• Synesthesia, among other phenomena, reveals a
difference between sensation and perception.
• Sensation: simple awareness due to the stimulation
of a sense organ
• Perception: the organization, identification, and
interpretation of a sensation in order to form a
mental representation.
Question!
• Hearing a noise: perception or sensation?
• Identifying a picture as a celebrity’s face:
perception or sensation?
• Halloween game: fools sensation into
misinterpreting perceived stimuli.
Perception!
• Transduction: occurs when sensors in the body
convert physical signals from the environment into
neural signals sent to the central nervous system.
• But where does perception end and sensation
begin?
o Memories and emotions intertwine with what you hear, see, and smell,
making your perception of an even UNIQUE TO YOU!
Psychophysics!
(Measuring Perception)
• Psychophysics: methods that measure the strength
of a stimulus and the observer’s sensitivity to that
stimulus.
o Operational Definition, anyone?
• Absolute threshold: the minimal intensity needed to
just barely detect a stimulus
o Boundaries between AWARENESS and UNAWARENESS
Absolute Threshold
Continued
Absolute Threshold
CONTINUED!
JND and Weber’s Law
• JND = “Just Noticeable Difference”: the minimal
change in a stimulus that can just barely be
detected
• Weber’s Law: the JND of a stimulus is a constant
proportion despite variation in intensity
Perception: Active or
Passive?
• Noise: all the other stimuli coming from the internal
and external environment
o Memories, moods, etc. intertwine with what you see, hear, etc. at any
given time
o Internal noise competes with ability to detect a stimulus with perfect focus
• Signal Detection Theory: response to a stimulus
depends both on a person’s sensitivity to the
stimulus in the presence of noise and on a person’s
response criterion
o Awareness Test
Selective Attention
• Selective Attention: perceiving only what’s currently
relevant to you
•
The reason it’s so hard to drive and talk on a cellphone at the same time
• Sensory Adaptation: sensitivity to prolonged
stimulation tends to decline over time as an
organism adapts to current conditions
•
Why can’t I smell my own house?
• Other examples of sensory adaptation?
Subliminal Messages!
Visual Perception
Rods, Cones, Fovea
• Rods: active only under low-light conditions, much
more sensitive photoreceptors than cones
• Cones: detect color, operate under noraml daylight
conditions, and allow us to focus on fine detail
o Dark adaptation
• Fovea: an area of the retina where vision is the
clearest and there are no rods at all
o Vision reduced in diminished light
Blind Spot
Lateral Inhibition
• “Turns off” rods (light sensing cells) located next to
the stimulated rods
• Creates the illusion of greater contrast
• System designed to notice changes
Hermann Grid
Color Opponent
Representation
Motion Aftereffect
Sensation!
• Area VI1: part of the occipital lobe that contains
the primary visual cortex
• Dorsal Stream: allows us to locate objects, to track
their movements, and to move in relation to them
• Ventral stream: represent an object’s shape and
identity
Visual-form Agnosia
• The inability to recognize objects by sight
How do we recognize
objects?
• Modular view: specialized brain areas or modules
detect and represent faces or houses or even body
parts
o Subregions in the temporal lobe respond selectively to faces compared
to just about any other object category while a nearby area responds
selectively to buildings and landscapes
• Distributed representation of object categories:
pattern of activity across multiple brain regions
identify viewed objects, including faces
Prosopagnosia
Perceptual Constancy
• Even as aspects of sensory signals change,
perception remains consistent
• But before an object can be identified…
• You must group the image regions that belong
together into a representation of an object
Grouping!
Finding Edges
Theories of Object
Recognition
• Image-based recognition theory: an object you
have seen before is stored in memory as a
template, a mental representation that can be
directly compared to a viewed shape in the retinal
image
o Shape templates are stored along with name, category, and other
associations to that object
Theories of Object
Recognition (part 2)
• Parts-based object recognition theories: the brain
deconstructs viewed objects into a collection of
parts
Depth and Size
• Monocular depth cues: aspects of a scene that
yield information about depth when viewed with
only one eye
o Relative Size
o Familiar Size
o Linear perspective
o Texture gradient
o Interposition
o Relative height in
the image
Binocular Depth Cues
• Motion Parallax: a depth cue based on the
movement of the head over time
• Stereographic Monsters!
Illusions!
Smell and the Brain
Pheromones
• Biochemical odorants emitted by members of a
species that can affect the animal’s behavior or
psychology