Lecture 1: Sensory and Perceptual Processes
Sensation and Perception
The process by which our sense organs respond to and translate stimuli into
nerve impulses sent to the brain
Organizing the stimulus input and giving it meaning
Stages of Sensation and Perception
1. Stimuli activate sensory receptors
2. Sensory receptors translate information into nerve impulses
3. Specialized neurons and analyze stimuli features
4. Stimulus pieces are reconstructed and compared to stimuli in memory
5. Perception is then consciously experienced
Psychophysics: Sensitivity to stimuli
Absolute limits of sensitivity
o Dimmest light in which we can see objects
o Softest sound we can hear
Recognizing differences between stimuli, measure the difference between
what you can and can’t sense
o Smallest difference in brightness detectable
o Recognizing differences between tones
The lowest intensity at which a stimulus can be detected 50% of the time
However, the environment contains a background level of stimulation for
each sense and this level (the ADAPTION level) must be overcome if a
stimuli is to be detected The Absolute Threshold
Going to plot the amount of positive responses at
Generic chart for psychophysics.
At: 50% say it is there and 50% say its not – that is
our Absolute Threshold
Measured in units of physical energy depending
on the type of sensation
Below 50% we don’t count because it is below
guessing, everything over that is better than
You have to take the absolute limit because it is
the limit between below and above guessing.
all your senses
However, the environment contains a background level of stimulation for each
sense and this level (the Adaption level) must be overcome if stimuli is to be
Background contains a level of stimulation for each sense and this level
needs to be overcome
Sometimes this signal will be able to overcome
The amount of energy required to overcome the Adaption level
Is subject to variation with changes in circumstances
Materials: 3 buckets/large bowls
Method: fill 1 with very hot water, 1 with cold water, 1 with tepid water. Procedure: subject closes eyes – place one hand in hot and the other in cold
for approx 3 minutes. Then place both hands in tepid water
Ask subject to report sensation for each hand separately
Instruct subject to open eyes
Although both hands are in the same water the sensations differ to the prior
Adaptation, they adapted to the hot and cold water and it exceeded
Signal Detection Theory
Detection Criterion: A personal standard of certainty before a person will
say that they detect a stimulus, brought into play when they report into you.
o Affected by:
Conservativeness (require more info) or Boldness (require
Increasing rewards for hits or costs for misses
If you say yes and it is there, if you say no and it is not
there = hit
If you say yes and it is not there, if you say no and it is
there = miss
o Bold subjects show high hits and high false alarms
o Conservative subjects show high misses and correct rejections
Conservative high in
Bold high in these cells
o However, Criterions can be manipulated by changing the payoff for each
cell of the response matrix: going to create a bolder subject
o This shows perception is, to some extent, a decision
A subliminal stimulus cannot be perceived consciously but do register in the
nervous system o “Subliminal” advertising during a movie.
o Exp. the watch now becomes associated with that guy. Consciously
you are not aware of it.
o OR Exp. that is why apples light up on Macbooks when you turn them
on, they are present but you never actually think about it.
Stimuli about threshold influence behavior much more than subliminal
Subliminal stimuli have stronger effects on attitudes
Effects may be die to placebo effects – think something is going on and might
lead towards it.
The Difference Threshold
The difference threshold (just noticeable difference or JND) is the smallest
difference between two stimuli that people can perceive 50% of the time.
o Weber’s Law: the JND is directly proportional to the magnitude of
the stimulus with which the comparison is made, can’t sense the
chance (e.g. 1/50for weight)
o (won’t be able to tell the difference between 200 – 203 pound but on
the 204 pound you can tell the difference.
The smaller the fraction the less change is necessary to produce a JND.
Thus only a 2% change is necessary to detect a
difference in lifted weights
Compared to a 33% change necessary for a
change in the taste of salt to be notices.
**The more of it you use the more you will need to use in the future to sense a
You need to have ________ to sense a difference Sensory Adaption (Habituation)
Sensory neurons respond to a constant stimulus by decreasing their activity,
(you will become habituated to a certain level of salt in your food), the
stimulus that is used in the senses will decrease their activity. You can
reverse this by not using those stimulations.
Sensory Systems: Vision
Retina: multilayered tissue at back of the eyeball
Optic Disk – a small blind spot with no receptors where the optic nerve exits
Fovea – small, central area of the retina where acuity is greatest,
Lens: becomes thinner to focus on distant objects and thicker to focus on closer
Iris: Regulates the size of the pupil
Pupil: adjustable opening controlling the amount of light entering the eye
Cornea: transparent protective covering
o Black and white
o Sensitive to dim light
o Located in Fovea
o Color receptors ¼ mm Vision: Transduction
Absorption of light by photopigments produces a chemical reaction changing
the rate of neurotransmitter release at the receptor’s synapse
The greater the change in release, the stronger the signal passed into the
Vision: Visual Pathways
At the level of the retina one the rod & Cone receptors have transduced the
photonic energy into electrical impulses the information is then passed to
bipolar cells and from there to ganglion cells
This process condenses signals from 130 million receptors to a few million
bipolar cells to less than a million ganglion cells.
Lateral Summation: A major source of this condensing is due to the fact that
as many as sex Rods may be connected from one bipolar cell and several
bipolar cells may converge on a single ganglion cell.
This means that signals which originate with Rods have poorer resolution
than signals from Cones which are connect 1 to 1 or 2 to 1 with bipolar cells.
Factors which Influence Vision
Although we have sent he system reduced input from approximately 130
million to just under 1