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Chapter 5

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Simon Fraser University
PSYC 100
Russell Day

Chapter 5: Sensation and Perception Synesthesia – means ‘mixing of the senses’ – they experience sounds as colours or tastes as touch sensations that have different shapes - Women are more likely to be synaesthetes than men - Maurer suggested – we are all born synesthetic  neural pathways of infants are fairly indifferentiated and lead to cross modal perceptions Transduction – when a stimuli activated specialised sensory receptors translate this information into the language of nerve impulses – understood by nervous system - Then feature detectors break down and analyze the specific features of the stimuli o Then these numerous stimulus ‘pieces’ are reconstructed into a neural representation that is then compared with previously stored information  like knowledge of objects look, smell or, feel like  This matching of a new stimulus with our infernal storehouse of knowledge allows us to recognize the stimulus and give it to meaning Specific parts of the brain are specialized for different sensory functions - In people with synthesia, there is some sort of cross-wiring  so activity in one part of the brain evokes responses in another art of the brain dedicated to another sensory modality Explanations offered for sensory mixing: 1) The pruning of neural connects that occur in infancy have not occurred in people with synthesia a. So brain regions retain connections that are absent in most people 2) With synthesia, there is a deficit in neural inhibitory processes in the brain that ordinarily keep input from one sensory modality from ‘overflowing’ into other sensory areas and stimulating them Puzzle: binding problem – how do we bind all our perceptions into one complete whole while keeping its sensory elements separate Sensation –process where sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brain Perception – making ‘sense’ of what our senses tell us - The active process of organizing this stimulus input and giving it meaning - Your interpretation, or perception, of an image or character is influence by their context Sensory Processes Is a result of adaption to the environment in which a species lives - Whatever the source of stimulation, its energy must be converted into nerve impulses Transduction – process whereby the characteristics of a stimulus are converted into nerve impulses Five Classical senses: Vision, audition (hearing), touch, gustation (taste), and olfaction (smell) - There are more senses like: o Sense of toucan be subdivided into separate senses of pressure, pain, and temperature 1 o Immune system sensory function allows it to detect foreign invaders and receive simulation from the brain - Human sensory systems are designed to extract from the environment the information that we need to function and survive - We have specialized sensors that can detect many different kinds of stimuli with considerable sensitivity Psychophysics – studies relations between the physical characteristics of stimulus and sensory capabilities - Is concerned with two kinds of sensitivity: o 1) Concerns the absolute limits of sensitivity  Ex: What is the softest sound or the weakest salt solution that humans can detect o 2) Concerned with differences between stimuli  Ex: what is the smallest difference in brightness that we can detect Stimulus Detection: The Absolute Threshold How intense must a stimulus be before we can detect its presence? Because we are often unsure of whether we have actually sensed very faint stimuli, researchers designate the absolute threshold – the lowest intensity at which a stimulus can be detected correctly 50% of the time - Lower the absolute threshold the greater the sensitivity Signal Detection Theory People’s apparent sensitivity can fluctuate quite a bit - Concept of a fixed absolute threshold is inaccurate because there is no single point on the intensity scale that separates nondetection from detection of a stimulus - There is a range of uncertainty and people say their own: o decision criterion – a standard of how certain they must be that a stimulus is present before they will say they detect it  changes from time to time depending on like fatigue, expectation, and the potential significance of the stimulus o Signal Detection Theory – concerned with factors that influence sensory judgements  Shows that’s perception is, in part, a decision The Difference Threshold Difference Threshold – the smallest difference between two stimuli that people can perceive 50% of the time - This is sometimes called the just noticeable difference (jnd) Ernest Weber – discovered there is some degree of lawfulness in the range of sensitivities within our sensory systems Weber’s law – the difference threshold, or jnd, is directly proportional to the magnitude of the stimulus with which the comparison is being made - Can also be called Weber Fraction - Breaks down at extremely high and low intensities of stimulation - It holds up reasonably well within the most frequently encountered range – so provides a reasonable barometer of our abilities to discern differences in the carious sensory modalities Sensory Adaption Sensory systems are finely attuned to changes in the stimulation Sensory neurons respond to a constant stimulus by decreasing their activity 2 Sensory Adaption – diminishing sensitivy to an unchanging stimulus Adaption (sometimes called habituation) - After a while, monotonous background sounds are largely unheard Adaption occurs in all sensory modalities, including vision The Sensory Systems Vision The normal stimulus for vision is electromagnetic energy, or light waves, which are measured in nonometres (one billionth of a metre) Electromagnetic spectrum includes X-rays, TV and radio signals, and infrared and ultraviolet rays The Human Eye Light waves enter the eye through the cornea – a transparent protective structure at the front of the eye Pupil – behind the cornea - An adjustable opening that can dilate or constrict to control the amount of light that enters the eye - Its size is controlled by muscles in the coloured iris that surrounds the pupil - Low levels of illumination cause the pupil to dilate, letting more than light into the eye to improve optical clarity - Bright light triggers constriction of the pupil Lens – behind the pupil - An elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects - Lens e focus the visual image on the light-sensitive retina - Retina – a multilayered tissue at the rear of the fluid-filled eyeball - The lens reverses the image from right to left and top to bottom when it is projected on the retina o But the brain reconstructs the visual input into the image that we perceive - Ability to see clearly depends on the lens ability to focus the image directly onto the retina Myopia – (nearsightedness) - When you have good vision for nearby objects but have difficulty seeing faraway objects - Condition generally occurs because the eyeball is longer (front to back) than normal Hyperopia – farsightedness - When you have excellent distance vision but have difficulty seeing close up objects clearly - Happens when the lens does not thicken enough and the image is therefore focused on a point behind the retina too far from the lens) - Ageing process typically causes the eyeball to become shorter overtime, contributing to the development of hyperopia Age related shortening of the eyeball often improves the vision of myopic people because as the retina moves closer to the lens, it approaches the point where the ‘nearsighted’ lens is projecting the image Eyeglasses and contact lenses are designed to correct for the natural lens’s inability to focus the visual image directly onto the retina 3 Photoreceptors: The Rods and Cones Retina is actually an extension of the brain Retina contains 2 types of light sensitive receptors cells 1) Rods – there are about 120 million rods in the human eye - Functions best in dim light - Is primarily black-and-white brightness receptors - Are about 500 times more sensitive to light than the cones are but do not give rise to colour sensation 2) Cones – 6 million in human eye - Are colour receptors - Functions best in bright illumination In humans, rods are found throughout the retina except in the fovea Fovea – a small area in the centre of the retina that contains only cones - Cones decrease in concentration as one moves away from the centre of the retina, and the periphery of the retina contains mainly rods Rods and cones send their messages to the brain via two additional layers of cells: 1) Bipolar Cells – have synaptic connections with the rods and cones - These cells, in turn, synapse with a layer of about one million ganglion cells 2) Ganglion Cells – axons are collected into a bundle to form the optic nerve Rods and cones are connected to the bipolar and ganglion cells - Rods and cones form the rear layer of the retina but their light-sensitive ends actually point away from the direction of the entering light so that they receive only a fraction of the light energy that enters the eye - The manner in which the rods and cones are connected to the bipolar cells accounts for both the greater importance of rods in dim light and our greater ability to see fine detail in bright illumination, when the cones are most active - Typically, many rods are connected to the bipolar cells o So can combine their individual electrical messages to the bipolar cell, where the additive effect of the many signals may be enough to fire it - Cones that lie in the periphery of the retina also share bipolar cells o But in fovea, the densely packed cones each have their own ‘private lines’ to a single bipolar cell  Visual Acuity, as a result, is greatest when the visual image projects directly onto the fovea  Such focusing results in the firing of a large numbers of cones and their private-line bipolar cells Visual Acuity – our ability to see fine detail Blind Spot – Optic nerve formed by the axons of the ganglion cells exits through the back of the eye not far from the fovea - Where there is no photoreceptors Visual Transduction: From Light to Nerve Impulses Transduction – The process where stimulus are converted into nerve impulses Photopigments – Rods and cones translate light waves into nerve impulses through the action of protein molecules 4 Absorption of light by protein molecules produces a chemical reaction that changes the rate of neurotransmitters release at the receptor’s synapse with the bipolar cells - The greater the change in transmitter release, the stronger the signal passed on to the bipolar cell and, in turn, to the ganglion cells whose axons form the optic nerve - If nerve responses are triggered at each of the three levels (rod or cone, bipolar cells, and ganglion cell), the message is instantaneously on its way to the visual relay station in the thalamus and then on the visual cortex of the brain Brightness Vision and Dark Adaption Rods are far more sensitive than cones under conditions of low illumination - the brightness sensitivity of both the rods and the cones depends in part on the wavelength of the light - Rods have much greater brightness sensitivity than cones throughout the colour spectrum except at the red end, where rods are relatively insensitive Cones are most sensitive to low illumination in the greenish-yellow range of the spectrum Although rods are by nature sensitive to low illumination, they are not always ready to fulfill their function Dark Adaption – the progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination - After absorbing light, a photoreceptor is depleted of its pigment molecules for a period of time - If the eye has been exposed to conditions of high illumination, a substantial amount of photo pigment will be depleted - During process, the photopigment molecules are regenerated, and the receptor’s sensitivity increases greatly Vision researchers have plotted the course of dark adaption as people move from conditions of bright light into darkness (Figure 5.8 page 153) - First part of curve – is due to dark adaption of the cones o Cones gradually becomes sensitive to fainter lights as time passes, but after 5-10 minutes in the dark, their sensitivity has reached its maximum o Rods, whose photopigments regenerate more slowly, do not reach their maximum sensitivity for about half an hour Colour Vision There are 2 different theories of colour vision that have been used to explain colour vision The Trichromatic Theory Additive Colour Mixture – 1800 – Any colour in the visible spectrum can be produced by some combination of wavelengths that correspond to the colours blue, green, and red - Was the basis for the theory Theory advanced by Thomas Young and Hermann van Helmholtz Young Helmholtz’s theory  Trichromatic Theory – there are 3 types of colour receptors in the retina - Although all cones can be stimulated by most wavelengths to varying degrees, individual cones are most sensitive to wavelengths that correspond to either blue, green, or red - Each of these receptor classes sends messages to the brain, based on the extent to which they are activated by the light energy’s wavelength 5 - The visual system then combines the signals to recreate the original hue o If all 3 cones are equally activated, a pure white colour is perceived - Is consistent with the laws of colour mixture  but several facts did not fit the theory o Like: o Yellow is produced by red and green receptors but people with red-green colour blindness can still see yellow Second phenomenon that posed problem of this theory  afterimage – an image in a different colour appears after a colour stimulus has been viewed steadily and then withdrawn Opponent-process Theory Formulated by Ewald Hering in 1870 Opponent-Process Theory – each of the 3 cones types respond to 2 different wavelengths: 1) red or green 2) Blue or yellow 3) Black or white What happens: ex: red-green cones responds with one chemical reaction to a green stimulus and with its other chemical reaction (opponent process) to a red stimulus Dual Processes in Colour Transduction Both theories had yielded research with a win-win verdict Dual-Process Theory – today’s theory - Combines trichromatic and opponent-process theories to account for the colour transduction process Trichromatic theories were right about the cones - They do contain 1 of 3 different protein photopigments that are most sensitive to wavelengths roughly corresponding to the colours blue, red, and green - Different ratios of activity in the red-, blue-, and green-sensitive cones cans produce a pattern of neural activity that corresponds to any hue in the spectrum Opponent-Process theory was partly correct but does not occur at the level of the cones Certain ganglion cells in the retina, as well as some neurons in visual relay stations and fashion by altering their rate of firing The red-green opponent processes are triggered directly by input from the red- or green-sensitive cones in the retina The blue-yellow opponent process: - Activity of blue-sensitive cones directly stimulates the ‘blue’ process farther along in the visual system - The yellow opponent process is triggered not by a ‘yellow-sensitive’ cone but by simultaneous input from the red- and green-sensitive cones Colour-Deficient Vision Trichromats – people with normal colour vision - They are sensitive to all 3 systems: o Red-green o Yellow-blue o Black-white 6 - Only about 7% of the male population and 1% of the female population have a deficiency in the red-green system, yellow-blue system, or both o Defieciency is caused by an absence of hue-sensitive photopigment in certain cone types Dichromat – a person who is colour-blind in only one of the systems (red-green or yellow-blue) Monochromat – sensitive only to the black-white system and is totally colour-blind Most colour-deficient people are dichromats and have deficiency in the red-green system A colour-blind person cannot discern certain numbers embedded in the circles Analysis and Reconstruction of Visual Scenes Feature Detectors From retina, optic nerve sends nerve impulses to a visual relay station in the thalamus  the brain’s sensory switchboard - Then the input is routed to various parts of the cortex, particularly the primary visual cortex in the occipital lobe at the rear of the brain There is a point-to-point correspondence between tiny regions of the retina and groups of neurons in the visual cortex - The fovea where the one-to-one synapses of cones with bipolar cells produces high visual acuity, is represented by a disproportionately large area of the visual cortex - There is more than one cortical ‘map’ of the retina  there are at least 10 duplicate mappings Groups of neurons within the primary visual cortex are organized to receive and integrate sensory nerve impulses originating in specific regions of the retina - Some of these cells are known as feature detectors – they fire selectively in response to stimuli that have specific characteristics Other classes of feature detector respond to colour, depth, or movement Parallel Processing Brief, high-frequency ‘bursts’ of firing in sensory neurons may function as feature detectors and can signal the occurrence of important stimuli in the sensory field Final stages in the process of constructing a visual representation occur when the information analyzed and recombined by the primary visual cortex is routed to other cortical regions known as visual association cortex - Here more complex features of the visual scene are combined and interpreted in light of our memories and knowledge Audition Stimulation for hearing are sound waves  form of mechanical energy Sound is actually pressure waves in air, water, or some other conducting medium Sound waves have 2 characteristics: 1) Frequency: the number of sound waves, or cycles per second a. Hertz (Hz) – the technical measure of cycles per second i. 1 Hz = one cycle per second b. Sound wave frequency is related to the pitch that we perceive 7 i. The higher the frequency (Hz), the higher the perceived pitch ii. Humans are capable of detecting sound frequencies from 20 Hz to 20 000 Hz 2) Amplitude: the vertical size of the sound waves – the amount of compression and expansion of the molecules in the conducting medium a. Sound wave’s amplitude is the primary determinant of the sound’s perceived loudness b. Differences in amplitude are expressed as decibels (db) – a measure of the physical pressures that occur in the eardrum i. Each increase in 10 decibels represents a tenfold increase in loudness Auditory Transduction: From Pressure Waves to Nerve Impulses Transduction system of the ear is made up of tiny bones, membranes, and liquid-filled tubes designed to translate pressure waves into nerve impulses Sound waves travel into an auditory canal leading to the eardrum, a moveable membrane that vibrates in response to the sound waves Beyond the eardrum is the middle ear, a cavity housing three tiny bones (the smallest in the body, each the size of a grain of rice) - The vibrating activity of these bones – the hammer(malleus), anvil (incus), and stirrup (stapes) – amplifies the sound waves more than 30 times - Hammer is attached firmly to the eardrum - The stirrup is attached to oval window – which forms the boundary between the middle ear and the inner ear - The inner ear contains the cochlea- a coiled, snail-shaped tube about 3.5 cm long that is filled with fluid and contains the basilar membrane – a sheet of tissue that runs its length - Resting on the basilar membrane is the organ of Corti – which contains about 16 000 tiny hair cells that are the actual sound receptors o The tops of the hair cells are attached to the tectorial membrane that overhands the basilar membrane along the entire length of the cochlea
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