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Chapter 5

PSYA01 FINAL EXAM NOTES Chapter 5.docx

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Steve Joordens

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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