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 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
b
Stevens made power function to relate physical intensity to magnitude of sensation S=kI (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 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 After light causes photopigment to split and become bleached, energy from photoreceptor`s metabolism causes them
to recombine. Each photoreceptor contains thousands of photopigments. Brighter the light, the more bleached
photopigments there are
Adaption to Light and Dark
Detection of light requires the split of photopigments. When a lot of light hits eye, the regeneration of
photopigments is lower than the bleaching causing to be not very sensitive to light
Dark adaption: process by which eye becomes capable of distinguishing dimly illuminated objects after foing
from a bright area to a dark one
Eye Movements
Eyes never at rest even when gaze is fixed on particular place, always make fast aimless jitters similar to when we
try to keep our hands still. Also make slow movements away from target fixated on and then quick movements that
bring target back in the fovea
Riggs, Ratliff, Cornsweet: stabilized images onto retina (mirror contacts, multiple mirrors and screen) visual system
not responsive to unchanging stimulus; photoreceptors, ganglion perhaps both cease to respond to constant stimulus
Eyes also make 3 purposeful movements: vergence, saccadic, pursuit
Vergence Movements: cooperative movement of the eyes that ensures the image of an object falls on identical
portions of both retinas
Saccadic Movements: rapid movement of the eyes used in scanning visual scene
Scialfa and Jolfe – found these movements important to way we search for an obect; Ross and Ma-Wyatt found
enhance the McCollough effect
Pursuit Movement: movement eyes make to maintain image of a moving object upon the fovea
Colour Vision
Light as humans define consists of radiant energy with wavelengths between 380 and 760 nm
Three types of cones in human eye, each containing different type of photopigments, light of particular wavelength
most readily causes a particular photopigment to split
Entire spectrum does not include all colors we see (brown, metallic etc.)
The Dimensions of Color
Most colors described in terms of three physical dimensions: wavelength (hue) {length of oscillation of light
radiation}, intensity (brightness) {amount of energy of light radiation}, purity (Saturation){intensity of dominant
wavelength relative to total radiant energy
Lot of brightness, lot of light, minimum is black
Fully saturated only has one wavelength of color
Colour Mixing: perception of two or more lights of different wavelengths seen together as light of
intermediate wavelength
Vision is synthetic sensory modality, it puts together rather than take apart like in auditory we hear separate sounds,
but see one wavelength of color
Mixing two beams of different wavelengths always yield brighter color; red+green show yellow, we cannot tell pure
yellow from a synthesised one
The color we see is the wavelength not absorbed by an object , white reflects all colors of light, to make white
though we only need red blue and green
Color Coding in the Retina
Thomas Young 1982 – proposed trichromatic theory: color vision accomplished by three types of
photoreceptors each maximally sensitive to different wavelength of light Young’s suggestion better incorporated by Helmholtz
Cones in human eye contain three types of photopigments, (blue-violet, green, yellow-green) but is referred to as
blue red and green
Red and green equal proportions, blue much fewer
Ewald Hering – noted four primary hues appeared to be paired in opposing colors red/green, blue/yellow, hypothesis
about nature of photoreceptors wrong but principle described info retinal ganglion cells send to brain
Both ganglion fire at steady rate when not stimulated, red yellow speed, green blue slow, this is opponent process:
representation of colors by rate of firing of two types of neurons
Negative Afterimages: image seen after portion of retina is exposed to intense visual stimulus; negative
afterimage consists of colors complementary to the physical stimulus
Complementary Colors go together to make white
The after image is after certain ganglion cells firing at rates suddenly go back to normal try to compensate but
overshoot so red looks green, blue looks yellow, etc.
McCollough experiment look at red/black stripes vertical for 3 sec, green/black horizontal for 3 sec back and forth
then black/white bars, half horizontal half vertical, the white spaces between horizontal will look tinted green,
vertical be pinkish, this is longer lasting sometimes even for days
Defects in Color Vision
1 in 20 have defective color vision, more common in males for the gene to produce photopigments are in the X
chromosome
Red-green colorblind – confuse red and green, blue and yellow are main colors, red and green both look yellow-ish
Protanopia: form of hereditary anomalous colour vision, caused by defective red cones
Lack of red cones, sharpness they have of images suggest they have red cones but filled with green photopigment
If lacked red cones, vision would be a lot less acute, red to them looks darker than green
Deuteranopia: form of hereditary anomalous colour vision caused by defective green cones
Same as above but opposite
Tritanopia: form of hereditary anomalous colour vision caused by lack of blue cones
Much rarer, 1 in 10,000, see world in red/green, bright blue sky would be green, yellow looks pink
Not carried in a gene, equally affects men and women,
Since much less blue cones, have not discovered if they are missing or if they are filled with another color
photopigment
AUDITION
Sound
Consists of rhythmical pressure changes in air, as object vibrates, air around it moves
When object moves to you molecules of air compresses, as it moves away, pulls molecules farther apart
Pressure wave arrives at your ear, bends eardrum in. Following wave of negative pressure causes eardrum to bulge
out
Sound waves measured in frequency units of cycles per second called hertz; human ear can hear between 30-20000
Hz.
Sound waves can vary in intensity and
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