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Chapter 5-6

Psychology 1000 Chapter Notes - Chapter 5-6: Diffusion Mri, Absolute Threshold, Synesthesia

Course Code
PSYCH 1000
John Campbell

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Chapter Five: Sensation and Perception
“Mixing of the senses” May experience sounds as colours or tastes. Women are more likely to be
syntesthetes than men.
Sensory and Perceptual Processing
The brain receives nerve impulses, organizes and confers meaning into them and constructs a
perceptual experience
1. Stimulus is received by sensory receptors
2. Receptors translate stimulus properties into nerve impulses (transduction)
3. Feature detectors analyze stimulus features
4. Stimulus features are reconstructed into neural representation
5. Neural representation is compared with previously stored information in the brain
6. Matching processes results in recognition and interpretation of stimuli
Theories of
The pruning of neural connections that occurs in infancy has not yet occurred in people with
synaesthesia, some brain regions retain connections that are absent in most people. Diffusion
tensor imaging, which lights up white matter pathways in the brain, reveals increased
connectivity in patients with synesthesia
Sensory Processes
Transduction: The characteristics of a stimulus are converted into nerve impulses
Sensory capabilities researched:
1. Absolute limits of sensitivity
1. What is the softest sound that humans can detect?
2. Differences between stimuli
1. What is the smallest difference in brightness that humans can detect?
Stimulus Detection: The Absolute Threshold
How intense must a stimulus be before we can detect it?
The absolute threshold is the lowest intensity at which stimulus can be detected (50% of the
Absolute Thresholds of Senses
1. Vision
1. Candle flame seen at a approximately 50km on a clear, dark night
2. Hearing
1. Tick of a watch under quiet conditions at approximately 6 meters

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3. Taste
1. Single teaspoon of sugar in 7.5 litres of water
4. Smell
1. One drop of perfume diffused into the entire volume of a large apartment
5. Touch
1. Wing of a fly falling on you cheek from 1 cm
Signal Detection Theory : The factors that influence sensory judgements
Decision criteria: A participant must be some level of certain that the stimulus is present before
they say it is, which can change person to person and time to time. Some of the factors that
change the decision criteria are fatigue, expectation, significance of the stimulus
The Difference Threshold: The smallest difference that people can perceive between two stimuli
(50% of the time) (jnd)
Sensory Adaption: Sensory neurons are engineered to respond to a constant stimulus by
decreasing their activity and by diminishing sensitivity to an unchanging stimulus. Reduces
The Sensory Systems
Vision: Sensitive to wavelengths extending from 700 (red) nanometers to 400 nanometers (blue-
violet) ROY G BIV
The Human Eye:
-Light waves enter the cornea a transparent shield
-Behind the cornea is the pupil can control how much light enters the eye
-The coloured iris controls the pupil size. Low level lighting causes the pupil to widen
allowing more light in, high level lighting causes the pupil to constrict the amount of light
let in.
-Behind the pupil is the lens this becomes thinner to focus on distant objects and thicker to
focus on closer objects
-The image focuses on the retina reversing the image bottom to top and right to left
Myopia: Nearsightedness. The lens focuses the visual image in front of the retina resulting in a
blurred image for far away objects
Hyperopia: Farsightedness. Occurs when the lens does not thicken enough and the image is
therefor focused on a point behind the retina. This becomes more prevalent with age.
Photoreceptors: Rods and Cones:
Rods: Function best in dim lighting, are primarily black-and-white brightness receptors. 500
times more sensitive to light than cones. Do not give colour sensations. 120 million rods in a
human eye. There are no rods in the fovea

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Cones: Colour receptors, function best in bright lighting. The fovea contains only cones. Cones
decrease in concentration as you move away from the centre of the eye.
Rods and Cones send their messages to the brain via two additional layers of cells;
Bipolar cells: Synaptic connections with rods and cones. Synapse with a layer of one million,
Ganglion cells: whose axons are collected into a bundle to form the optic nerve
Visual acuity: Cones share bipolar cells on the peripherals of the retina but have their own direct
bipolar cell in the fovea, which is why the eye can focus better on something directly in front of
Visual Transduction: Characters of the lightwaves are converted into nerve impulses through
photopigments (protein molecules)
Brightness Vision and Dark Adaptation
-Rods have greater brightness sensitivity, except for at the red end, where they have no
-Cones are more sensitive to low illumination in the greenish-yellow range
Dark adaptation: Progressive improvement in brightness sensitivity that occurs over time under
conditions of low illumination. After absorbing light a photoreceptor is depleted of pigment for a
period of time. During this process the photopigment is regenerated and made stronger
Colour Vision
-The human eye can distinguish 7.5 million hue variations
Trichromatic theory:
-Three types of colour receptors in the retina
-Cones are most sensitive to either blue, green or red
-If all three cones are equally activated = white
-Yellow is produced by red and green receptors, however red-green colourblind people can see
-Colour afterimage; an image in a different colour appears after a colour stimulus has been
viewed steadily and then withdrawn
Opponent Process Theory
-Each of the three cones responds to two colour receptors
-Red or green
-Blue or yellow
-Black or white
-Explains afterimages
Dual Processes in Colour Transduction
-Combination of the trichromatic and opponent process theories to account for the colour
transduction process
-The cones contain one of three different photopigments that are most sensitive to red, blue and
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