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Chapter 5

PSYA01H3 Chapter Notes - Chapter 5: Postcentral Gyrus, Ganglion Cell, Optic Disc


Department
Psychology
Course Code
PSYA01H3
Professor
Steve Joordens
Chapter
5

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CHAPTER 5: SENSATION
SENSORY PROCESSING
Sensation: detection of simple properties of stimuli such as brightness, colour, warmth, sweetness
Perception: detection of objects, their locations, their movements and backgrounds
Perceptions dependant on learning, sensations involve innate, no clear boundary between the two
Sensory Mechanisms: visual, auditory, gustatory, olfactory, somatosensory
Transduction
Only sense receptors brain processes detect things like temperature, salt concentration of blood, and these receptors
can’t inform it about what is going on outside (outside info gathered by sense organs outside the brain)
Detected stimuli by sense organs transmitted to brain through neural impulse, task of sense organs to transmit
signals to brain that are coded to represent faithfully the events that have occurred in environment, brain analyzes
and reconstructs
Transduction: process by which sense organs convert energy from environmental events into neural activity
In most senses specialized neurons, receptor cells, release chemical transmitter substances that stimulate other
neurons altering rate of firing
In somatosenses dendrites of neurons respond directly to physical stimuli without receptor cells
Location, Environmental Stimuli, Energy Transduced:
Eye, Light, Radiant||Ear, Sound, Mechanical||Vestibular System, Head Tilt/Rotation, Mechanical||
Tongue, Taste, Recognition of Molecular Shape||Nose, Odor, Recognition of Molecular Shape
Skin, Internal Organs, Touch, Temperature, Vibration, Mechanical, Thermal, Mechanical
Muscle, Pain, Stretch, Chemical, Mechanical
Sensory Coding
Anatomical Coding: means by which nervous system represents info; different features coded by activity of
different neurons
Used to interpret the location, and type of sensory stimulus according to which incoming nerve fibres are active
Sensory coding for body surface is anatomical, primary somatosensory cortex contains a neural map of the skin
(receptors in skin send info to different parts of primary somatosensory cortex) same with visual cortex and visual
field
Temporal Coding: means by which nervous system represents info; different features coded by activity of
different neurons
Coding of information in terms of time, simplest is rate; firing faster or slower because of intensity of stimulus can
communicate quantitative info to brain
Psychophysics
Psychophysics: systematic study of relation between physical characteristics of stimuli and sensations they
produce
The Principle of Just Noticeable Difference (JND)-Weber-smallest change in magnitude of a stimulus that a
person can detect
JND directly related to magnitude of stimulus below are Weber Fractions
Two weights feel the same unless they differed by a factor of 1 in 40(can barely tell a 40g rate from 41)
Differ between two brightness is 1 in 60
Fechner’s largest contribution show how a logarithmic equation could be derived from Weber’s principle

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Gustav Fechner, German physiologist, used Weber’s JND to measure people’s sensations, assumed it was basic unit
of sensory experience and measured magnitude of sensation in JNDs {two frosted glass, increase one light till
noticeable, then match the other one (that is one JND) then do again which is two, and so on, graph is steep curve
turning to a standstill each point more apart}
Pain takes less energy at higher intensities to produce JND
Stevens made power function to relate physical intensity to magnitude of sensation S=kIb (S is psychological
magnitude, I is intensity of physical stimulus, k is mathematical constant)
Value of b for saccharin is 0.8, salt 1.3
Signal Detection Theory mathematical theory of detection of stimuli which involves discriminating a signal
from the noise in which it is embedded and takes into account participant’s willingness to report detecting the
signal
Psychophysical methods highly rely on threshold (the line between perceiving and not perceiving)
JND = difference threshold
Absolute Threshold: minimum value of a stimulus that can be detected, discriminated from no stimulus at all
Threshold is not absolutely fixed value, thus it is the point where the person detects it 50% of the time
Every stimulus event need distinction between signal(stimulus) and noise(background stimuli and random nervous
system activity)
Experiment-sitting in room when light flashes may hear tone, at first easy, then the tone gets so faint can barely hear
Can’t tell if can really hear it or if we are imagining
Response bias: tendency to say yes or no when not sure whether we detect stimulus
Hits: yes when it is present, Miss: no when it is present, False Alarm: Yes when not, Correct Negative: No when not
Affect the response bias through payments, the curve goes from $1 fine for false alarm, 50C for hit $1 fine for false
alarm, $1 for hit, fine for false alarm, $1 for hit, 50C fine for false alarm, $1 for hit-THIS is a receiver operating
characteristic curve (ROC) named in Bell Laboratories to measure intelligibility of speech through phone
Detectability measured by relative distances of the curves from 45 degree angle
VISION
Light
Wavelength: the distance between the waves of radiant energy
Gamma, X-rays, Ultraviolet, Visible Color, Infrared, Radar, TV/Radio, AC Circuits 380nm Violet, 760 Red
Bees can see ultraviolet, snakes have special organs that detect infrared radiation
The Eye and Its Functions
Cornea: transparent tissue covering front of eye forms bulge at front of eye, lets light through
Sclera: tough outer layer of the eye (the white)
Iris: pigmented muscle of the eye that controls the size of the pupil
Space immediately behind Cornea is aqueous humour which nourishes cornea and other portions of the front of the
eye fluid circulated and renewed
If produced too quickly or if passage that returns to blood blocked, damage vision (Glaucoma)
Lens: transparent organ behind the iris of the eye, focuses an image on the retina, curvature causes images to be
focused on inner surface of back of the eye
Cornea is fixed, Lens flexible and alters shape to obtain images of near or distant objects
Accommodation: changes in thickness of lens of the eye that focus images of near or distant objects on retina

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Length of eye matches bending of light rays produced by cornea and lens
If not matched retina is out of focus, (prescription glasses correct discrepancy)
Eye too long, nearsighted, concave|| Eye too short, farsighted, convex glasses
Retina: tissue at back inside surface of eye that contains photoreceptors and associated neurons
Performs sensory functions of the eye
Embedded are +130million photoreceptors: receptive cell for vision either rod or cone
Info from photoreceptors transmitted to neurons that send axons to the optic disc at the back of the eye
Optic disc: circular structure located at the exit point from the retina of the axons and the ganglion cells that
form the optic nerve
There are no photoreceptors directly in front of the optic disc which is our blind spot
Johannes Kepler credited with suggestion retina not the lens that contains receptive tissue of eye
Christoph Scheiner demonstrated lens is simply focusing device(ox eye, peel sclera away and see upside-down
image of world)
Retina has three layers light passes through: Ganglion cell layer (front), bipolar cell layer (middle), photoreceptor
layer (back)
Bipolar cell: neuron in retina that receives info from photoreceptor and passes it on to ganglion cells from
which axons proceed through optic nerves to brain
Ganglion Cell: a neuron in the retina that receives info from photoreceptors by means of bipolar cells and
from which axons proceed through the optic nerves to the brain
Visual info passes through 3cell chain to brain: photoreceptor, bipolar, ganglion, brain
Single photoreceptor only responds to light in immediate vicinity; ganglion can receive info from many different
photoreceptors
Retina contains neurons that interconnects adjacent photoreceptors and adjacent ganglion cells
Human retina contains two types of photoreceptors: 125million rods, 6million cones
Rods: photoreceptor very sensitive to light but can`t detect color
Cone: photoreceptor responsible or acute daytime vision and color perception
Fovea: small pit near centre of retina containing densely packed cones responsible for most acute and
detailed vision
Fovea contains only cones and we move our eyes so that the place we are focusing on falls directly on fovea
Farther away from fovea, cones decrease, rods increase, up to 100 rods can converge on one ganglion, because many
from different areas connect to the same ganglion, visual info lack sharpness
Transduction of Light by Photoreceptors
Molecule from Vitamin A is the transduction of energy of light into neural activity
In absence of light, attaches to a protein molecule to for photopigment: complex molecule found in
photoreceptors, when struck by light splits and stimulates the membrane of the photoreceptor its in
There are 4 kinds of photopigments 1 for rods, 3 for cones
Split of photopigments start transduction, causes a series of chemical reactions that stimulate the photoreceptors and
cause it to send a message to the bipolar cell with which it forms a synapse
Rhodopsin: photopigment contained by rods (is pink)
Once photopigments are split, they lose their color (discovered by Franz Boll)
Boll`s discovery led investigators to suspect chemical reaction responsible for transduction of light into neural
activity
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