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Chapter 5

Psychology 1000 Chapter Notes - Chapter 5: Interposition, Sound Localization, Binocular Disparity

Course Code
PSYCH 1000
John Campbell

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Chapter 5 – Sensation and Perception
Sensation – 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 – active process of organizing the stimulus input and giving it meaning
Transduction- is the process whereby the characteristics of a stimulus are converted into nerve impulses.
Psychophysics- studies relations between the physical characteristics of stimuli and sensory capabilities
Sensory Processes
Stimulus detection – absolute threshold designated as the lowest intensity at which a stimulus can be detected 50%
of the time
Signal detection theory – concerned with the factors that influence sensory judgments
Decision criterion – standard of how certain a person must be that a stimulus is present before they will say they
detect it
Increased rewards for noticing stimuli often results in lower detection thresholds
Increased danger/punishment for noticing stimuli often raises detection threshold
Difference threshold – smallest difference between two stimuli that can be perceived 50% of the time (just
noticeable difference – jnd)
Weber’s Law – to perceive a difference between two stimuli, one must differ by a constant ratio
Value for weights = 1/50, therefore if 50 lbs. is lifted, increased weight will only be detected at 51 lbs.
Smaller fraction = higher sensitivity
Doesn’t apply to extremely high or low stimulation intensities
Sensory adaptation – the diminishing sensitivity to an unchanging stimulus
Perception of stimuli will decrease if constantly present
The Sensory Systems
The Human Eye
Light enters eye through cornea (transparent protective structure)
Pupil – adjustable opening that dilates or constricts to control amount of light entering
Iris – controls the pupil
Lens – elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects
Image flipped and reversed onto retina
Ability to see clearly depends on lens’ ability to focus image onto retina
Myopia (nearsightedness) – lens focuses image in front of retina
Hyperopia (farsightedness) – lens focuses image behind retina
Retina – multi-layered tissue at rear of eyeball
Photoreceptors: Rods and Cones
Retina covered in light-sensitive receptor cells
Rods – black and white receptors
Function best in dim light
Cones – color receptors
Function best in bright light
In humans, rods are everywhere except fovea (direct center of retina)
Cones decrease in concentration distant from the fovea
Rods and cones send message to brain via two additional layers of cells
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Bipolar cells have synaptic connections with rods and cones
Bipolar cells synapse with ganglion cells, whose axons form into optic nerve
Cones in the fovea each have private line to a single bipolar cell (unlike others, which have many rods/cones for
each bipolar cell)
Visual acuity (ability to see fine detail) increases with image directly on fovea
Blind spot exists at point where ganglion cells exit to form optic nerve
Transduction - process where characteristics of a stimulus are converted into nerve impulses
Rods and cones accomplish transduction through photopigments
Absorption of light be photopigments increases release of neurotransmitters
Brightness Vision and Dark Adaptation
Dark adaptation – the progressive improvement in brightness sensitivity that occurs over time in low illumination
Cones adapt completely in 10 minutes
Rods continue adapting for 30 minutes, allowing extreme sensitivity to light
Color vision
Trichromatic theory – three types of color receptors in retina (blue, green, red)
All colors produced by combination of wavelengths between these three colors
Flaws in theory:
Yellow produced by red and green, yet people with red-green color blindness can see yellow
Color afterimage (image in different color appears after stimulus shown for a while then withdrawn)
Opponent-process theory – three color receptors, each responding to two different wavelengths (red-green, blue-
yellow, black-white)
Explains color afterimage issue
Dual processes in color transduction
Modern dual-process theory combines both theories to account for color transduction process
Cones contain one of three different photopigments that are sensitive to blue, green, and red
Different combinations of intensities will produce different colors
Opponent processes occur, but not in cones
Ganglion cells respond in opponent-process by altering firing rate
Color-deficient vision
Dichromat – color blind to only one system (red-green or yellow-blue)
Monochromat – completely colorblind (only sees black-white)
Analysis and Reconstruction of Visual Scenes
Feature detectors
Optic nerve sends nerve impulses to brain (thalamus, then primary visual cortex)
Groups of neurons in the cortex are organized to receive and integrate sensory nerve impulses from specific regions
of retina
Feature detector cells fire selectively to stimuli that have specific characteristics
Certain cells fire when horizontal line present, others when other angles present
Parallel processing – different cells analyze stimuli and construct unified image of its properties
Visual association processes
Information analyzed and reconstructed in primary visual cortex is routed to other regions known as visual
association cortex
Frequency – number of sound waves or cycles per second (Hz = one cycle per second)
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