Sensation and Perception - Chapter 4
Sensation: stimulation of sense organs. Involves absorption of energy.
Perception: selection, organization, interpretation of sensory input. Involves giving sensory
input meaning. Involved building uniform model from integrated input of all senses.
Transduction: conversion of one form of energy to another (e.g. sense organs convert enviro.
effects into neural activity)
Psychophysics: relation between distal and proximal stimulus, how the characteristics of the
environmental stimulus and the sensations they produce are related.
Threshold: dividing point between energy levels that are or aren't detectable
Absolute Threshold: minimum amount of stimulation needed to detect stimulus, no single
intensity that is detectable, involves perception for individuals, gradual process leading to
Just Noticeable Difference (JND): smallest difference of stimulation that is detectable.
Absolute threshold = first JND, JND from nothing. OJ example
• x-axis: measure of physical intensity
• y-axis: extent of perception
Weber's Law: size of JND = constant proportion to initial stimulus, Weber's fraction
(percentages and fractions increasing).
Fechner's Law: magnitude of sensory experience proportional to # of JNDs past absolute
threshold, perception cannot be measured on absolute scales, progressively smaller differences
when perceiving sensation.
Anatomical Coding: diff. types of energy coded by diff. types of neurons, brain interprets
through which fibres are active
Temporal Coding: some features coded by patterns of neural activity
Signal-detection Theory: detecting stimuli involves both decision and sensory processes (both
of which are influenced by more than just stimulus intensity). Responses depend on criterion
individual sets of how sure they must be before reacting.
• Hits: detect signals when they are present
• Misses: fail to detect when present
• False Alarm: detect when NOT present
• Correct Rejections: NOT detecting when absent 2
Noise: all irrelevant stimuli in environment. More noise, harder to detect weak signal.
Subliminal Perception: registering sensory inputs without conscious acknowledgement, below
e.g. hidden messages
• subliminal inputs produce effects though they are quite weak
• only measurable in very controlled settings
Sensory Adaptation: gradual decline in perceived intensity (sensitivity to stimuli) due to
prolonged stimulation, focus on changes rather than constant.
Response Bias: tendency to respond yes or no when unsure of answer
ROC Curve: sensitivity of subject vs. response bias, detectability measured through 45 degree
angle line (closeness to curve)
Amplitude: height of wave, affects perception of brightness
Wavelength: distance between peaks, affects perception of colour
Purity: how varied mix is, affects perception of colour
• In order to see, visual input needs to be converted in neural impulses
The Eye: channels light to retina (neural tissue that receives light, back surface of eye), houses
• light enters front of eye through cornea
• cornea and crystalline lens (right behind cornea) form upside-down image on retina
• Lens: transparent, focuses light rays that fall on retina
• Accommodation: altering visual focus through manipulation of shape and thickness of
• Nearsightedness: focus too soon (cross too early, before reaching retina)
• Farsightedness: focus too late (don't cross before reaching retina)
• Fovea = focal point
• pupil helps regulate amount of light passing into rear chamber of eye
• Saccades: brief focus on various parts of stimulus, scanning visual environment 3
• retina processes images and plays crucial role in central nervous system
• axons from retina to brain converge at optic disk (hole in retina where axons/ganglion
cells leave eye, creates blind spot at optic nerve)
• optic nerve: collection of axons which connect eye to brain
Cones: receptors that play crucial role in daylight/colour vision, do not respond well to dim light,
better visual acuity (sharpness and precise detail) than rods, concentrated in centre of retina
and fall off in density toward periphery.
Rods: receptors that play crucial role in night/peripheral vision, density is greatest outside of
fovea, fall off in density toward periphery, rods outnumber cones (100-125 million rods, 5-6.4
Rhodopsin: photopigment of rods, colour at rest, breaks into component molecules when
stimulated with light (without colour)
Dark Adaptation: eyes become more sensitive to light in low illumination, cones adapt more
quickly than rods.
Light Adaptation: eyes less sensitive to light in high illumination.
Information Processing in the Retina:
• light strikes retina's receptors and triggers neural signals
• neural signals send impulses along optic nerve
• axons carry visual info (in form of neural impulses) to the brain
Receptive Field: area of retina that, when stimulated, affects firing of said cell. Light falling in
centre of field has opposite effect of when falling in surrounding area. (e.g. rate of firing
increased when stimulating centre, decreased when surrounding area) When stimulated,
signals sent to brain and laterally (to surrounding cells).
Lateral Antagonism: neural activity within cell opposes activity in surrounding cells
Visual Pathway to the Brain: axons leave back of eye and form optic nerves which travel to
Optic Chiasm: crossover of optic nerves from each eye, project to opposite half of
brain, ensures signals from both eyes reach both hemispheres of brain.
• after optic chiasm, optic nerve fibres take two pathways
• main pathway projects to thalamus (brain's relay station)
• @ Thalamus, 90% of axons from retina synapse in LGN (lateral geniculate nucleus) 4
• visual signals processed in LGN and distributed to areas of occipital lobe that make up
the primary visual cortex.
• 2nd pathway goes through superior colliculus (area of midbrain) and then travels through
thalamus to occipital lobe
• 2nd pathway's function: coordinate visual input with other sensory inputs
• Main pathway has specialized pathways: magnocellular and parvocellular
• Channels engage in Parallel processing: simultaneous abstraction of different kinds of
info from same input
• e.g. parvocellular handles perception of colour, magnocellular handles brightness
Info Processing in Visual Cortex: microelectrodes in primary visual cortex of animals,
individual cells don't respond to spots, respond to lines.
Specialized Cells in Primary Visual Cortex:
• Simple Cells: line with correct width, length, angle, and position in correct receptive field
• Complex Cells: correct width/orientation, any location in receptive field
Ventral Stream: processes details of what objects are out there
Dorsal Stream: processes where the objects are
Trichomatic Theory: human eye has 3 types of receptors for different wavelengths (red, green,
• colour mixing naturally by eye to detect other colours, modified of red, green, and blue.
Colour-blindness: inability to accurately distinguish between colours.
Dichromats: "colour blind" making due with two colour channels instead of three.