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Chapter 4

PSYC 1010 Chapter Notes - Chapter 4: Detection Theory, Absolute Threshold, Neural Adaptation

Course Code
PSYC 1010

of 7
Nov/1/2003 CHANAPS
Notes From Reading
I. Psychophysics: Basic Concepts and Issues
A. Introduction
1. Virtual Agnosia – inability to recognize objects through sight.
2. Sensation – the stimulation of sensory organs.
a. Involves the absorption of energy (i.e. light or sound waves) by sensory
organs (i.e. eyes, ears)
3. Perception – the selection, organization, and interpretation of the sensory
a. Organizing and translating sensory input into something meaningful.
4. Psychophysics – the study of how physical stimuli are translated into
psychological experience.
a. Important psychologist – Fechner (Germany).
B. Thresholds – Looking for Limits
1. Stimulus – any detectable input from the environment.
2. Threshold – a dividing point between energy levels that do and do not have a
detectable effect.
3. Absolute Threshold – minimum amount of stimulus that an organism can
detect. Defines the boundaries of an organism’s sensory capabilities.
a. As Stimulus intensity increases, probability of responding to a stimuli
gradually increases.
b. As such, absolute threshold is the stimulus intensity that is detected 50%
of the time.
C. Weighing the Differences: The JND
1. Just Noticeable Difference (JND) – smallest difference in the amount of
stimulation that a specific sense can detect.
2. JND is greater for heavier objects than lighter ones.
3. Smallest detectable difference is a constant proportion of the weight of the
original object.
4. Webber’s law – the size of a just noticeable difference is the constants
proportion of the size of the initial stimulus, known as the “Webber Fraction”.
a. Applies not only to weight but to all senses.
5. As the stimulus increase in magnitude, the JND becomes larger.
D. Psychophysical Scaling
1. Fechner’s Law – the magnitude of a sensory experience is proportional to the
number of JND’s that the stimulus causing the experience is above absolute
a. Constant increments in stimulus intensity produce smaller and smaller
increases in the perceived magnitude of sensation (i.e. Light bulbs in a
2. Questioned by Stevens, who came up with idea of Magnitude estimation –
asking subjects to assign numbers to stimuli on the basis of how intense they
appeared to be.
a. Magnitude estimation is the best way to map the relations between
stimulus intensity and sensory experience.
E. Signal Detection Theory
Nov/1/2003 CHANAPS
Notes From Reading
1. Signal detection theory proposes that the detection of stimuli involves
decision processes as well as sensory processes which are both influenced by a
variety of factors beside stimulus intensity.
a. Your Performance will depend on the criterion you set for how sure you
must feel before you react, the level of noise in the system.
2. Detectability is measured in probability and depends on decision making
processes as well as sensory processes.
F. Perception without Awareness
1. Subliminal Perception – the registration of sensory input without conscious
2. Subliminal stimulation generates weak effects.
G. Sensory Adaptation
1. Sensory adaptation is a gradual decline in sensitivity to prolonged stimulation.
2. Automatic, built in process that keeps people tuned in to the changes rather
than the constants – ignore the obvious, and watch for threats.
II. Our Sense of Sight: The Visual System
A. The Stimulus: Light
1. Light is a form of electromagnetic radiation that travels as a wave.
2. Vary in amplitude (height), wavelength, purity.
a. Amplitude – perception of brightness
b. Wavelengths – perception of color
c. Purity – perception of richness/saturation
B. The Eye: A Living Optical Instrument
1. Two Purposes:
a. Channel light to the neural tissue that receives it (retina)
b. House the retina
2. Light enters the eye through the cornea (transparent window). Cornea and
Lens form an upside down image of the object on the retina.
3. Lens – transparent eyes structure that focuses the light rays falling on the
a. Made up of relatively soft tissue, capable of adjustments and
b. Accommodation – when the curve of the lens adjusts to alter visual
4. Nearsightedness – close objects are seen clearly, but distant objects blurry.
a. Due to the focus of light from distant objects falling short of the retina.
5. Farsightedness – farther objects are seen clearly but close seem blurry.
a. Due to the focus of light from near objects falling behind the retina
6. Iris – colored ring surrounding pupil (black center of the eye)
7. Pupil – the opening in the center of the iris that helps regulate the amount of
light passing into the rear chamber of the eye.
C. The Retina: The Brain’s Envoy in the Eye
1. Retina- the neural tissue lining the inside back surface of the eye. Absorbs
light, processes images, and sends visual info to the brain.
2. Optic Disk - a hole in the retina where the optic nerve fibers exit the eye.
3. Visual Receptors – Rods and Cones
Nov/1/2003 CHANAPS
Notes From Reading
a. Cones – specialized visual receptors in the retina that play a key role in
daylight vision and color vision.
i. Do not respond well to dim light
ii. Cones provide visual acuity (sharpness and detail)
iii. Concentrated in center of retina
iv. Fovea – tiny spot in center of the retina which only contains cones,
greatest visual acuity.
b. Rods – specialized visual receptors in the retina that play a key role in
night vision and peripheral vision.
i. More sensitive than cones to dim light
ii. Handle peripheral vision
iii. Density greatest outside the fovea, and gradually decreases
4. Dark and Light Adaptation
a. Dark Adaptation – the process in which the eyes become more sensitive
to light in low illumination.
b. Light Adaptation – the process where eyes become less sensitive to light
in high illumination.
5. Info Processing in the Retina
a. Light hitting cones and rods triggers neural signals that pass into the
intricate network of cells in the retina.
b. Signals move from receptors to bipolar cells to ganglion cells, sending
impulses to optic nerve (axons that connect the eye to brain).
c. Axons carry visual info encoded as a stream of neural impulses to the
d. The Receptive field of a visual cell – retinal area that when stimulated
effects the firing of that cell.
D. Visual and The Brain
1. Visual Pathways to the Brain
a. Axons from the eye form the optic nerve, travel to the optic chiasm
b. Optic Chiasm - The point at which the optic nerves from the inside half
of each eye cross over and then project to the opposite half of the brain.
Ensures that each half of the brain gets info from both eyes.
c. After reaching the optic chiasm, the optic nerve splits between 2
pathways – main and secondary.
i. Main – hypothalamus
ii. Secondary pathway function – coordination of visual info with other
sensory info.
d. Main visual pathway is divided in 2 more parts – magnocellular (where)
and parvocellular (what)
i. Engage in parallel processing – simultaneously extracting different
kinds of info from the same input.
E. Info Processing in the Visual Cortex
1. Cells in the primary cortex don’t respond to little spots, instead they respond
to lines, edges, and other stimuli.
2. Feature Detectors – neurons that respond selectively to very specific features
of complex stimuli.