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Lecture 31

Psychology 2020A/B Lecture Notes - Lecture 31: Detection Theory, Absolute Threshold, Teaspoon


Department
Psychology
Course Code
PSYCH 2020A/B
Professor
Dr.Mike
Lecture
31

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Chapter 5 Psychology
Synaesthesia: people with this condition experience sounds as colours or tastes as
touch sensations that have different shapes. It has been proposed that we are all born
synaesthetes, because the neural pathways of infants are fairly undifferentiated and
lead to cross modal perceptions.
Sensation: is the stimulus-detection process by which our sense organs respond to
and translate environmental stimuli into nerve impulses that are sent to the brain.
Perception: making “sense” of what our senses tell us- is the active process of
organizing this stimulus input and giving it meaning.
Sensory Perception
Psychophysics: studies the relations between the physical characteristics of stimuli
and sensory capabilities and is concerned with two kinds of sensitivity. The absolutes of
sensitivity, and the smallest detectable differences in sensitivity. The relation between
physical stimulus and psychological response.
- Fechner is the father of psychophysics, and said that you can measure exact intensity
but can measure “just noticeable difference”
- Threshold is the value of a stimulus characteristic required to produce some response.
-Fechner’s Law: sensation increases with the logarithm of the intensity.
- S= k log I
- Fechner’s law is more general and cognitively economic.
Absolute threshold (lower limit): the lowest intensity at which a stimulus can be
detected correctly 50 percent of the time. Thus, the lower the absolute threshold, the
greater the sensitivity.
- Vision: candle flame at 50km
- Hearing: tick of a watch at 6m
- Taste: tsp. of sugar in 8 L of water
- Smell: 1 drop of perfume in a 6 room apartment.
- Touch: wing of a fly falling on cheek from 1cm
Decision criterion: a standard of how certain they must be that a stimulus is present
before they will say they detect it. The decision criterion may change from time to time,
depending on such factors as fatigue, expectation, and the potential significance of
stimulus.
Signal detection theory: is concerned with the factors that influence sensory
judgements. For example, when the rewards or costs are manipulated, people become
more bold or conservative with their detections.
Subliminal stimulus: a stimulus that is received so weakly or briefly that it can not be
perceived by the senses.
- In general, no evidence that subliminal cuts influence consumer behaviour.
- But consider, (Bruce and Valentine, 1986): priming
- showed people pictures of famous faces
- the prime was shown just before of someone related or from the same historical
period for a very short time, way under the amount of detection

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- if the prime was related to the photo, the photo was identified 100 msec faster
Difference threshold: the smallest difference between two stimuli that people can
perceive 50 percent of the time.
Weber’s law (Ernst Weber): states that the difference threshold, or JND, is directly
proportional to the magnitude of the stimulus with which the comparison is being made,
and can be expressed as a Weber fraction.
I/I = CΔ
I = intensity
Weber’s Constant is 1/10th, meaning most of the time we can guess within 1/10 of the
difference of a stimulus.
- Value of JND is not constant, but the relative difference does. The smaller the Weber
constant, the better, the better the system.
Examples:
- Vision (brightness): 1/60
- Kinesthesis (weights): 1/50
- Pain (thermal): 1/30
- Audition (mid. pitch, moderate loudness): 1/10
- Pressure (skin): 1/7
- Smell (India rubber): 1/4
- Taste (salt): 1/3
Sensory adaptation: Sensory neurons are engineered to respond to a constant
stimulus by decreasing their activity, and the diminishing sensitivity to an unchanging
stimulus is called sensory adaptation. Ex. background noises, like the wind that are
unheard
Steven’s Power Law: S= k log I^N
- more predictive across a variety sensations
- ability to sense differences in electric shock, whereas the other two laws do not work
Sensory Systems
1. Accessory Structures: ex. outer ear
2. Transduction
a. receptors
3. Coding: ex. frequency
4. Interaction
a. physiological and psychological
Vision
- The normal stimulus for vision is electromagnetic energy, or light waves, which are
measured in nanometres.
- Our visual system is sensitive only to wavelengths extending from about 700nm down
to about 400nm.
The Eye:
- light waves enter through the cornea a transparent protective structure at the front of
the eye
- behind the cornea is the pupil, an adjustable opening that can dilate of constrict to
control the amount of light that enters

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- the pupil’s size is controlled by muscles in the coloured iris that surrounds the pupil
- The lens is located behind the pupil and is an elastic structure that becomes thinner to
focus on distant objects and thicker to focus on nearby objects.
- The retina is a multi-layered tissue at the rear of the fluid filled eyeball. The lens
reverses the image from right to left and top to bottom when it is projected upon the
retina, but the brain reconstructs the visual input into the image we perceive. The retina
is where transduction will take place.
- The vitreous humor is the jelly like fluid that the eye is filled with.
- People who are nearsighted suffer from myopia and people who are farsighted suffer
from hyperopia.
- The rods which function best in dim light, are primarily black and white brightness
receptors. They are about 500 times more sensitive to light that the cones are, but do
not give rise to colour sensations. There are no rods in the fovea.
- The cones are colour receptors, and function best in bright illumination.
- In humans, rods are found throughout the retina , except in the fovea, a small area in
the centre of the retina that contains only cones and is very sensitive. If you look directly
at something, you are looking at it with the fovea, and if it is a very small light source
you can not see it because there are no rods in the fovea.
- The fovea is a dark spot in the middle of the retina. The blind spot is a lighter
area, and is a spot where there is nothing to capture any light of the retina.
-Bipolar cells have synaptic connections with the rods and cones. The biopolar cells,
in turn, synapse with a layer of about one million ganglion cells, whose axons are
collected into a bundle to form the optic nerve. Thus input from more than 126 million
rods and cones is eventually funneled into only 1 million traffic lanes leading out of the
retina toward higher visual centres.
- In the fovea, however, the densely packed cones each have their own private line to a
single bipolar cell.
- Amacrine and horizontal cells are completely inhibitory and help sharpen and clean
the image up.
-Visual acuity is the ability to see fine detail, is greatest when the visual image projects
directly onto the fovea.
- The optic nerve formed by the axons of the ganglion cels exits through the back of the
eye not far from the fovea , producing a blind spot, where there are no photoreceptors.
Visual Transduction:
- The process whereby the characteristics of a stimulus are converted into nerve
impulses called transduction.
-Photopigments absorb light which produces a chemical reaction that changes the
rate of neurotransmitter release at the receptor’s synapse with the bipolar cells.
- In rods the chemical is called rhodopsin.
- In cones, the chemicals are chlorable (green), erythrolabe (red), cyanolabe
(blue). The three visual pigments suggest that these generate colour vision.
Brightness Vision and Dark Adaptation.
- Research has shown that rods have a much greater brightness sensitivity than cones
through the colour spectrum except at the red end, where rods are relatively intensive.
Cones are most sensitive to low illumination in the greenish-yellow range of the
spectrum.
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