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Chapter 5

Chapter 5

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Queen's University
PSYC 100
Meredith Chivers

Week 8: Chapter 5: Sensation p. 127-165 - everything we learn is detected by sense organs and transmitted to our brains by sensory nerves - sense organs and sensory nerves provide us with useful information about the outside world - but how specific that information is depends on things like: the specific modality of the information, the characteristics of the information, the state of the brain at the time it receives it information from different sources in the environment are processed differently by sensory systems - ex. there is a clear difference between vision and audition - vision: we see different things every second, but we have a sense that our visual world is stable the visual system must provide that stability - sound: it is not so variable-> the intensity of sound changes depending on how far we are from the source, but these changes are more gradual than those faced by the visual system also, sounds can go around obstacles, unlike light our auditory sense has more time to process signals - so our senses are attuned to different aspects of our world, and they combine together to give us a rich experience of the world - audition is important for social behavior, vision for information about distant events, sense of smell can tell us about sources of aromatic molecules far upwind - taste and touch deal with events occurring immediately nearby Sensory Processing - experience is traditionally studied by distinguishing between sensation and perception - sensation: the detection of the elementary properties of a stimulus ex. seeing the colour red, seeing a movement - perception: the detection of the more complex properties of a stimulus, including its location and nature; involves learning ex. seeing a red apple, seeing a soccer ball coming and realizing you need to move to block it - detecting a sound is not the same as identifying what the source of the sound means - long ago, they thought that perceptions depended on learning, and sensations involved innate, prewired physiological mechanisms - but until now, there has been no clear boundary between “simple” sensations and “complex” perceptions - research: experience is essential to the development of some of the most elementary features of sensory systems - we will look at our sensory mechanisms: the visual, auditory, gustatory, olfactory, somatosensory systems - tradition-> 5 senses; but we actually have several more - ex. the somatosensory system can be separated into various components for: touch, warmth, coolness, vibration,physical damage, head tilt, etc. it just depends if you want to use the term “sense” for them Transduction - the brain is separated from the outside world and the only receptors it has looks at temperature and salt concentration of the blood-> no receptors for the outside world - useful actions require information from the external world, so this information is gathered by the sense organs located outside of the brain - sense organs detect stimuli (from light, sound, odor, taste, mechanical contact) from the environment-> transported brain through neural impulses it is the task of the sense organs to transmit signals to the brain that are coded to represent events in the environment the brain analyzes this information and reconstructs what just happened - transduction: the conversion of physical stimuli into changes in the activity of receptor cells of sensory organs energy from environmental events-> neural activity each sense organ responds to a particular form of energy and translates it into neural firing - the means of transduction for most cases is specialized neurons called receptor cells - receptor cell: a neuron that directly responds to a physical stimulus, such as light, vibrations, or aromatic molecules they release chemical transmitter substances that stimulate other neurons, thus altering the rate of firing of their axons - for the somatosensory systems, dendrites of neurons respond directly to physical stimuli without the intervention of specialized receptor cells but there may still be a bit of specialization: some of these neurons have specialized endings that let them respond to particular kinds of sensory information The Types of Transduction Accomplished by the Sense Organs Location of Sense Environmental Energy Transduced Organ Stimuli Eye Light Radiant Energy Ear Sound Mechanical Energy Vestibular System Tilt & Rotation of Mechanical Energy Head Tongue Taste Recognition of Molecular Shape Nose Odour Recognition of Molecular Shape Skin, internal organs Touch Mechanical Energy Temperature Thermal Energy Vibration Mechanica Energy Muscle Pain Chemical Reaction Stretch Mechanical Energy Sensory Coding - sensory information is translated into firing of action potentials, but there aren’t different types of action potentials - but we can still detect a lot of different stimuli with each of our sense organs ex. we can discriminate among 7.5 million different colours, we can recognize up to 10 000 odors discriminate the degree of pressure involved, sharpness or bluntness, softness or hardness, and temperature of the object we’re touching - you may ask, if action potentials can’t be altered, how do the sensory organs tell the brain a red apple or a yellow lemon that you see, for ex? the information must be coded somehow in the activity of the axons, and it is in two general forms: anatomical coding and temporal coding Anatomical Coding - recall Muller’s doctrine of specific nerve energies-> the brain learns what is happening through the activity of specific sets of neurons sensory organs are in different places and send their information to the brain through different sets of nerves - the brain uses anatomical coding to interpret the location and type of sensory stimulus according to which incoming nerve fibres are active anatomical coding: a means by which the nervous system represents information; different features are coded by the activity of different neurons - ex. rub your eyes-> light-sensitive receptors are there, and they are mechanically stimulated-> action potentials are produced in the optic nerves-> brain acts as if the neural activity in the optic nerves was produced by light-> you see stars and flashes - this can be seen in the other senses too artificially stimulate the nerves that convey taste-> sensation of taste electrical stimulation of the auditory nerve-> sensation of a buzzing noise Temporal Coding - temporal coding: a means by which the nervous system represents information; different features are coded by the pattern of activity of neurons it is in terms of time - the simplest form of temporal code is rate: by firing at a faster/slower rate according to the intensity of a stimulus, an axon can communicate quantitative information to the brain - ex. light touch-> encoded by a low rate of firing; more forceful touch-> by a high rate of firing - anatomical coding: The firing of particular set of neurons tells where the body is being touched - temporal coding: the rate at which these neurons fire tells how intense that touch is Psychophysics - psychophysics: a branch of psychology that measures the quantitative relation between physical stimuli and perceptual experience physics of the mind - scientists have to find ways to measure people’s sensations, and there are two methods: the just-noticeable difference the procedures of signal detection theory The Principle of the Just-Noticeable Difference - Weber looked at the ability of humans to discriminate between various stimuli, he measured the just-noticeable difference (jnd) - just-noticeable difference (jnd): the smallest difference between two similar stimuli that can be distinguished; also called difference threshold - the jnd is directly related to the magnitude of that stimulus - give people two metal objects and ask if they were different in weight- participants said they were the same unless they differed by a factor of 1 in 40 (they could barely distinguish 40 gand 41 g; 80 g and 82 g, etc.- the difference in weight between 40 and 41g, and 80 and 82 g is the jnd - different senses had different ratiosex. ratio for differences in brightness of white light is 1 in 60 - Weber fraction: the ratio between a jnd and the magnitude of a stimulus; reasonably constant over the middle range of most stimulus intensities - Fechner: used Weber’s concept of the jnd to measure people’s sensations assumption: the jnd was the basic unit of a sensory experience he measured the absolute magnitude of a sensation in jnds - experiment: put participant in dark room and have two bulbs (one is sample, one is comparison) sample is turned off, comparison is brightened until they notice a difference-> the value is one jnd set sample to one jnd, comparison is brightened until they notice a difference- the value is two jnds continue this until the lights become uncomfortably bright - construct a graph that shows the strength of a sensation of brightness (in jnds) in relation to the intensity of a stimulus(the shape of the graph is looks like the top left part of a circle) y axis: perception of brightness (each unit is a jnd–subjective) x axis: intensity of stimulus (measures of physical intensity– objective) so the graph shows a mapping between the physical and the psychological worlds - from graph: the amount of physical energy necessary to produce a jnd increases with the magnitude of the stimulus - note: shape of the curve: rises steeply at first, then levels off-> characteristic of logarithm - exception: there are some sensations that takes LESS energy to produce a jnd at higher intensities the shape of the graph would be like the bottom right part of a circle ex. the pain from an electric shock - Stevens: he suggested a power function to relate physical intensity to the magnitude of sensationS= kIb S: psychological magnitude of the sensation I: intensity of the physical stimulus k: a mathematical constant that adjusts for the way physical intensity is measured - when 01, curve looks like bottom right corner of circle - ex. sense of taste: value of b for saccharin is 0.8 if we were to taste two solutions, the second one containing twice as high a [ ] as the first one, the second one would only taste 1.7 times sweeter - ex. value of b for salt is 1.3 the second one would taste 2.5 times saltier - therefore, the power law provides a systematic way to compare different sensory systems Signal Detection Theory - threshold: the point at which a stimulus, or a change in the value of a stimulus, can just be detected the line between not perceiving and perceiving - difference threshold: an alternative name for the jnd minimum detectable difference between two stimuli - absolute threshold: the minimum value of a stimulus that can be detected discriminated from no stimulus at all - so when we compared a dark bulb with one with light, we are measuring an absolute threshold - when we compared the bulb at one jnd with another bulb, we are measuring a difference threshold - even early psychophysicists realized that a threshold was not an absolutely fixed value ex. flash a very dim light, sometimes the participant will see it, sometimes they won’t - by convention: the threshold is the point at which a participant detects the stimulus 50% of the time the nervous system is always firing, and sometimes when a very weak stimulus occurs, neurons may or may not detect it if neurons happen to be quiet, the brain will detect it if neurons happen to be firing, effects of the stimulus may be thought of as noise - and so we have another way of measuring a person’s sensitivity to changes in physical stimuli that takes into account the random changes in the nervous system - signal detection theory: a mathematical theory of the detection of stimuli, which involves discriminating a signal from the noise in which it is embedded and which takes into account participants’ willingness to report detecting the signal every stimulus event requires discrimination between signal (stimulus) and noise (background stimuli/random activity in the nervous system) - you’re in a quiet room and when the light flashes, there may be a faint tone and you must say whether there was a flash or not (yes/no) depending on whether you heard the tone - first, it’s easy and the tone is loud, but it gets fainter and fainter and you’re not sure if you hear it or not, even though you do see the light flash - at this point, your response bias, your tendency to say yes or no when you’re not sure you saw a stimulus, can have an effect - there are 4 possibilities in judging the presence or absence of a stimulus: - say you want to be very sure you are correct when you say yes, then your response bias is to avoid false alarms, even at the risk of making misses - someone else’s response bias might be to be in favour of detecting all stimuli, even at the risk of making false alarms - a person’s response bias can seriously affect an investigator’s estimate of the threshold of detection someone with a response bias to avoid false alarms will appear to have a higher threshold than will someone who does not want to let the tone go by without saying yes - to avoid the problem: there is a method of assessing people’s sensitivity, regardless of their initial response bias purposely manipulate the response biases and observe the results of these manipulations on the people’s judgments - you get a dollar for each hit, no penalty for false alarms-> you will say yes more - you are fined a dollar for each false alarm, no reward for hits-> you will say no more - you get a dollar for each hit, fined 50 cents for every miss-> you’ll say yes whenever reasonably sure of the tone - you get 50 cents for each hit, fined a dollar for each false alarm-> you will be more conservative - receiver operating characteristic curve (ROC curve): a graph of hits and false alarms of participants under different motivational conditions; indicates people’s ability to detect a particular stimulus it was used by Bell to measure the intelligibility of speech transmitted through the phone system - the curve shows people’s performance when the sound is difficult to detect sound is louder-> rarely need to doubt, so many hits and very few false alarms the few misses would be due to when you wanted to be sure that you heard something the few false alarms would be when guessing didn’t matter because the fine for being wrong was low/non-existent - louder tones are easier to detect than softer tones - the signal detection method is the best way to determine a person’s sensitivity to the occurrence of a particular stimulus - the concept of threshold is not used, we just say that a stimulus is more or less detectable - the person decides whether a stimulus occurred and the consequences of making hits/false alarms can bias this decision - so the signal detection theory shows us that sensory experience has to do with more than just sensory systems, but also motivation and prior experience Vision - a quick glance and we can recognize people, objects and landscapes in depth and full colour - the stimulus is light Light - the eye is sensitive to light-> radiant energy similar to radio waves that oscillates as it is transmitted from its source - wavelength: the distance between adjacent waves of radiant energy; in vision, most closely associated with the perceptual dimension of hue ex. EM energy travels at 298 000 km/s, if the frequency is 88.5 MHz, the wavelength is 3.4 metres - the wavelength of light is much shorter: 380–760 nm (violet–red) all other radiant energy is invisible to our eyes - increasing in nm: gamma rays, X-rays, UV rays, visible light, infrared rays, radar, television/radio broadcast bands, AC circuits - the entire range of wavelengths is called the electromagnetic spectrum, the part that we see as light is called the visible spectrum - the definition of the visible spectrum depends on the visual system (humans or other species of animals) ex. bees can see UV rays that we can’t some plants have flowers that contain dyes that reflect ultraviolet radiation and present patterns to attract bees - ex. some snakes (pit vipers) have organs that detect infrared radiation, so they can find their prey in the dark by detecting the heat emitted from the animals in the form of infrared radiation The Eye and Its Functions - the eye is a delicate and important sense organs, it is well protected: eye is house in a boney socket- eyelid: to keep dust and dirt out eyelashes: keep foreign matter from falling into open eye eyebrows: prevent sweat from forehead from dripping into eyes reflex mechanisms: sudden approach of an object toward face/touch on surface of the eye causes automatic eyelid closure and withdrawal of the head - cornea: the transparent tissue covering the front of the eye - sclera: the tough outer layer of the eye; the “white” of the eye - iris: the pigmented muscle of the eye that controls the size of the pupil consists of two bands of muscles that can control how much light is admitted into the eye controlled by the brain size of pupil is constricted in bright light, dilated in dim light - the space immediately behind the cornea is filled with aqueous humour that means watery fluid this fluid is produced by tissue behind the cornea that filters the fluid from the blood the aqueous humour is like blood vessels, it nourishes the cornea and other portions of the front of the eye, and this fluid must circulate and be renewed - if aqueous humour is produced too quickly or if the passage that returns tit to the blood is blocked, the pressure within the eye can increase and cause damage to vision (glaucoma) - the cornea is nourished with aqueous humour instead of with blood because of its transparency - lens: the transparent organ situated behind the iris of the eye; helps focus an image on the retina the image is upside down and reversed from left to right, but the brain compensates for this alteration - the lens has a special imitation: it must remain transparent, so it contains no blood vessels, so it is functionally dead tissue - the shape of the cornea is fixed, but the lens is flexible - a special set of muscles can alter the shape of the lens so that the eye can obtain images of either nearby or distant objects - accommodation: changes in the thickness of the lens of the eye that focus images of near or distant objects on the retina - usually, the length of the eye matches the bending of light rays produced by the cornea and the lens, so that there is a sharp image on the retina but for some people, the image on the retina is out of focus, and they require an extra lens in front of their eyes to correct the image - eyes are too long (from front to back)-> nearsighted-> use concave lens - eyes are too short-> farsighted-> use convex lens as people get older, their lenses become less flexible so it’s hard to focus on objects near them - retina: the tissue at the back inside surface of the eye that contains the photoreceptors and associated neurons performs the sensory functions of the eye there are 130 million photoreceptors embedded in the retina - photoreceptor: a receptive cell for vision in the retina; a rod or a cone they are specialized neurons that transduce light into neural activity information from the photoreceptors is transmitted to neurons that send axons toward one point at the back of the eye: the optic disc - optic disc: a circular structure located at the exit point from the retina of the axons of the ganglion cells that form the optic nerve all axons leave the eye at the optic disc and join the optic nerve which travels to the brain there are no photoreceptors directly in front of the optic disc, this portion for the retina is called the blind spot - before 17th century: we thought that the lens sensed the presence of light - Kepler: suggested that the retina, not the lens, contained the receptive tissue of the eye - Scheiner: demonstrated that the lens was just a focusing device took an ox’s eye, peeled away the sclera from the back of the eye and could see an upside-down image of the world through the thin, translucent membrane that remained so he recognized the function of the lens of the eye: to cast images - there are 3 principal layers to the retina: light passes from front to back ganglion cell layer-> bipolar cell layer-> photoreceptor layer the first two layers are transparent - visual information passes through a three-cell chain to the brain: photoreceptor-> bipolar cell-> ganglion cell-> brain - bipolar cell: a neuron in the retina that receives information from photoreceptors and passes it on to the ganglion cells, from which axons proceed through the optic nerves to the brain - ganglion cell: a neuron in the retina that receives information from photoreceptors by means of bipolar cells and from which axons proceed through the optic nerves to the brain - a single photoreceptor responds only to light that reaches its immediate vicinity a ganglion cell though, can receive information from many different photoreceptors - the retina also has neurons that interconnect photoreceptors with adjacent ganglion cells - this neural circuitry shows that some kinds of information processing are performed in the retina - there are 2 general types of photoreceptors: 125 million rods, 6 million cones - rod: a photoreceptor that is very sensitive to light, but cannot detect changes in hue mostly function in dim light, are sensitive to light, insensitive to differences between colours - cone: a photoreceptor that is responsible for acute daytime vision and for colour perception cones function when it’s bright enough to see things clearly, responsible for colour vision - fovea: a small pit near the centre of the retina containing densely packed cones; responsible for the most acute and detailed vision 1 mm in diameter, has only cones most cones are connected to one ganglion cell each when we look at a point in our visual field, we are actually moving our eyes so that the image of that point falls on the cone-packed fovea - farther away from the fovea, the number of cones decrease, the number of rods increase - there could be a maximum of 100 rods connected to one single ganglion cell (the ganglion cell must also be very sensitive to low levels of light) - rods are good because they help us see dim light, but since so many rods share just one ganglion cell, the visual information provided is usually not that sharp Transduction of Light by Photoreceptors - all species require a molecule derived from vitamin A that is the central ingredient in the transduction of the energyof light into neural activity carrots are good for vision because they contain a substance that the body can convert into vitamin A - when there is no light, this molecule is attached to a protein, and together, they form a photopigment photopigment: a complex molecule found in photoreceptors; when struck by light, it splits and stimulates the membrane of the photoreceptor in which is resides - photoreceptors of the human eye contain 4 kinds of photopigments (one for rods, three for cones)- they have thes ame basic mechanism - photon hits photopigment–> photopigment splits into its 2 molecules-> a series of chemical reactions that stimulate photoreceptor–> photoreceptor sends a message to bipolar cell (with which it forms a synapse)–> bipolar cell sends a message to the ganglion cell–> ganglion cell sends a message to the brain - intact photopigments have a characteristic colour ex. the photopigment contained by rods is called rhodopsin, it is pink - but once the photopigment is split, it loses its colour, they are bleached - how did they find this? Boll removed an eye from an animal, pointed it toward a window (with a bright scene) looked at the retina under dim light, and found that the image of the scene was still there retina was pink where little light had fallen, pale where the image had been bright - suggestion: a chemical reaction was responsible for the transduction of light into neural activity - after the light splits the photopigment and bleaches it, energy from the photoreceptor’s metabolism makes the two molecules recombine, and then is ready to be bleached again - each photoreceptor has many thousands molecules of photopigment - the number of intact, unbleached molecules of photopigment in a given cell depends on the rate at which they are being split by light and put back together - the brighter the light, the more bleaching there is of the photopigment Adaptation to Light and Dark - detection of light requires that photons split molecules of rhodopsin or one of the other photopigments - when there is high illumination, the rate of regeneration of rhodopsin falls behind there are little rhodopsin intact, so the rods are not very sensitive to light - go from bright room to dark room, there are too few intact rhodopsin molecules, so eyes can’t respond immediately to dim light the probability that a photon will strike an intact rhodopsin molecule is very low - after awhile, the regeneration of rhodopsin overcomes the bleaching effects of the light, the rods become full of unbleached rhodopsin, a photon passing through a rod is likely to find a target the eye just went through dark adaptation - dark adaptation: the process by which the eye becomes capable of distinguishing dimly illuminated objects after going from a bright area to a dark one Eye Movements - when our gaze is fixed on a place, it’s called the fixation point - our eyes make fast, aimless, jittering movements, as well as occasional slow movements away from the target theyare fixed on which are terminated by quick movements that bring the image of the fixation point back to the fovea - you may think that the jerky movements of our eyes at rest are random, but they have a useful function - there was an experiment that showed that elements of the visual system are not responsive to an unchanging stimulus - project stabilized images onto the retina (the images stay in the same location on the retina) - wear contact lens with a mirror and bounce light off of it and onto a white screen - the image on the screen moved in perfect synchrony with the eye movements - findings: details of visual stimuli began to disappear, first the image was clear, but then there was a fog over the field of view - after awhile, some images couldn’t be seen at all - finding: the photoreceptors and/or the ganglion cells stop responding to a constant stimulus - the small, involuntary movements of our eyes keep the image moving, and so keep the visual system responsive to the details of the scene before us - without these movements, our vision would become blurry soon after we fixate on a single point - the eyes also make 3 purposeful movements: vergence movements, saccadic movements, pursuit movements - vergence movement: the co-operative movement of the eyes, which ensures that the image of an object falls on identical portions of both retinas ex. when you bring your finger to your face, you see only one finger because your eyes will make vergence movements toward your nose you then look at something far in the room and your eyes will rotate outward and you will see two fingers - saccadic movement: the rapid movement
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