Chapter 4 Notes.docx

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21 Mar 2012
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Chapter 4 Sensation and Perception
- Sensation: the process where our sensory organs gather information from the outside
world (the stimulation of the sensory organs)
- Perception: the process where the brain organizes and interprets these sensations so that
they have meaning
Psychophysics: Basic Concepts and Issues
- Psychophysics: the study of how physical stimuli are translated into psychological
experience
- Fechner’s question – How much of a stimulus is necessary for you to detect it?
o Threshold: a dividing point between energy levels that do and do not have a
detectable effect
o Absolute threshold: for a specific type of sensory input, the minimum amount of
stimulation that an organism can detect50% of the time (see next point)
As stimulus intensity increases, subjects’ probability of responding to
stimuli gradually increases (so it’s not really absolute)
- Just noticeable difference (JND): the smallest difference in the amount of stimulation that
a specific sense can detect
o Weber’s law: people are able to perceive very small changes if the magnitude of
the reference stimulus is small, but as the magnitude of that reference stimulus
increases, a greater change is necessary before a difference is perceived
- Fechner’s law: the magnitude of a sensory experience is proportional to the number of
JNDs that the stimulus causing the experience is above the absolute threshold
o Ex: three equal increases in stimulus intensity produce progressively smaller
differences in the magnitude of sensation (think 3 light bulbs pg. 134)
Perceptions can’t be measured on absolute scales but they can be
measured on psychophysical scales
- Signal-detection theory: the detection of a stimuli involves decision processes as well as
sensory processes, which are both influence by a variety of factors besides stimulus
intensity
o 4 possibilities when trying to detect signals: hits, misses, false alarms, correct
rejections
Your decision depends on:
The criterion how sure you must feel before you react
The “noise” in the system irrelevant stimuli
The intensity of the stimulus
- Subliminal perception: the registration of sensory input without conscious awareness
o Evidence has supported that this can take place
- Sensory adaptation: a gradual decline in sensitivity due to prolonged stimulation
Our Sense of Sight: The Visual System
- Light form of electromagnetic radiation that travels as a wave moving at the speed of
light
o Vary in amplitude (height, affects perception of brightness), wavelength (the
distance between peaks, effects perception of colour) and purity (the mix of the 2)
o Humans can only see a fraction of the total range of wavelengths
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o To see, visual input must be converted into neural impulses that are sent to the
brain
The Eye
- The eyes serve two main purposes:
o Channel light to the retina
o To house the retina
- Cornea = the transparent window at the front of the eye
o With the lens, forms an upside-down image of objects on the retina (righted by
the brain)
- Lens: transparent eye structure that focuses the light rays falling on the retina
o Allows for accommodation (change in the curvature of the lens to alter visual
focus)
o Focussing problems: near- and farsightedness
- Pupil: helps regulate the amount of light passing into the rear chamber of the eye
o Dilate in the dark, constrict when there is lots of light
- Saccades: constant motion of the eyes, scanning the visual environment and making brief
fixations at various parts of the stimuli
o Essential to good vision
The Retina: the neural tissue lining the inside back surface of the eye that absorbs light,
processes images and sends visual information to the brain
- Optic disk: hole in the retina where the optic nerve fibres exit the eye
o Where the axons that run from the retina to the brain meet
o A hole = a blind spot in each eye… goes unnoticed because one usually
compensates for the other
- Contains 2 typed of receptors:
o Cones specialized visual receptors that play a key role in daylight and colour
vision
Fovea: tiny spot in the centre of the retina that only contains cones
Provide visual acuity (sharpness and precise detail)… best in the fovea
o Rods specialized visual receptors that play a key role in night and peripheral
vision
More sensitive than cones to dim light
Far outnumber the cones (especially in the periphery of the eye)
- Dark adaptation: process where the eyes become more sensitive to light in low
illumination (can’t see when you enter a dark theatre)
o Cone adapt quicker than rods
- Light adaptation: process where the eyes become less sensitive to light in high
illumination (squint when you exit a dark theatre on a sunny day)
- The retina transforms light into neural impulses that are sent to the brain via the optic
nerve
- Receptive fields: affect the firing of a visual cell
o Circular fields with a centre-surround arrangement (light falling in the centre has
the opposite effect of light falling in the surrounding area: lateral antagonism)
Ex: rate of firing increases in one and decreases in the other, or vice versa
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