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Chapter 5

Chapter 5 Notes – Sensation and Perception.docx

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Psychology 1000
Mark Cole

Chapter 5 Notes – Sensation and Perception Sensation  stimulus is received by certain types of sensory receptors  receptors translate stimulus properties into nerve impulses (transduction)  feature detectors analyze stimulus features  stimulus features are reconstructed into neural representation  neural representation is compared with previously stored information in the brain  matching process results in recognition and interpretation of stimulus  Perception Sensation – stimulus-detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses Perception – making sense of what our senses tell us through the active process of organizing this stimulus input and giving it meaning Sensory Processes - The only reason we can contact with the outer world in our brain is because certain neurons have developed into specialized sensory receptors that transform the energy forms into “code” for the nerve impulses - Sensory equipment of any species is an adaptation to the environment where it lives - ENERGY must be converted into NERVE IMPULSES Transduction – the process where the characteristics of a stimulus are converted into nerve impulses Psychophysics – studies relations between the physical characteristics of stimuli and sensory capabilities - It concerns: 1. Absolute limits of sensitivity 2. Differences between stimuli Absolute Threshold – the lowest intensity at which a stimulus can be detected correctly half the time (50%)  Lower absolute threshold = greater sensitivity  No such thing as a fixed absolute threshold Decision Criteria - a standard of how certain they must be that a stimulus is present before they will say they detect it Signal Detection Theory – factors that influence sensory judgments (changes)  Have four possible outcomes (hit, false alarm, miss, correct rejection)  At low stimulus intensities, both the participant and the instructor’s characteristics influence the decision criteria  Perception is a part of a decision Subliminal Stimulus – a stimulus that is so weak tat it cannot be perceived consciously and the stimulus is well below the absolute threshold  Can be seen when examining patients with specific types of brain damage Difference Threshold – the smallest difference between two stimuli that people can perceive half the time (50%)  A.K.A just noticeable difference (JND) Weber’s Law – states that different threshold is directly proportional to the magnitude of the stimulus with which the comparison is being made  Is expressed as a Weber’s fraction  Example) Weber’s fraction is 1/50 (meaning if you lift a 50 gram weight, a comparison weight must be at least 51 grams in order for you to feel a difference in the weight)  1/50 = 10/500 **  Shows humans are more sensitive to their visual sense than to their sense of smell  Shows humans are highly sensitive to differences in pitch but less sensitive to differences in volume Sensory Adaptation – the brains ability to filter out redundant and insignificant information - Sensory neurons respond to constant stimulus by decreasing their activity, and the diminishing sensitivity to the unchanging stimulus is sensory adaptation - Adaptation occurs in all sensory modalities - The brain splits the information up to various areas (multi-tasking) - Example: when you no longer notice the ticking of a clock - When the information does not get reintegrated precisely, we demonstrate optical illusions  It reduces our overall sensitivity BUT it is adaptive because it frees our senses from the constant to pick up information changes in the environment (can be important for survival) Vision EEG (electromagnetic energy) – are light waves, which are measured in nanometers  700nm (red) to 400nm (blue-violet)  ROY G BIV Eye – focuses incoming light energy on the photoreceptors of the retina - Three layers: external, intermediate, and internal External Layer – white, tough, fibrous protective layer called Sclera - Light enters the eye through the Cornea (transparent part of the sclera at the front of the eye)  Sclera – sides and back protect and support the eye  Cornea – bends light rays into eye - Where light enters the eye Intermediate Layer – called Choroid, which absorbs stray light rays that are not detected by photoreceptors - It also contains blood vessels which nourish the eye - Front of the choroid forms the Iris, which allows light to enter inner eye through the Pupil and adjusts its size based on light income; also known as adaptation  Choroid – sides and back absorb scattered light and contain blood vessels  Iris – regulates the amount of light that enters the eye  Pupil – opening for light to enter inner eye - Located behind the cornea - Dilate = low levels illumination/ letting more light into they eye - Constriction = bright light  Ciliary Muscles – changes shape of the lens in order to focus Internal Layer - Behind the iris, the choroid thickens and forms ciliary muscle which attaches to the lens and focuses images on the retina  Retina is the internal layer of the eye which is a thin layer of tissue containing rods and cones (photoreceptors) - The rods are sensitive to light intensity (brightness) and the cones are packed densely at the back of the eye in the fovea centralis - Rods and cones send sensory impulses to the brain by the optic nerve  Rods – photoreceptors that are sensitive to dim light - Black and white brightness receptors - Located on the peripheral portion of the retina - Important for night vision - Insensitive to red wavelengths  Cones – photoreceptors that are sensitive to different wavelengths of light (colour) - Best in bright illuminations  Fovea Centralis – contains high density of cones and provides acute vision - DOESN’T include rods Other Parts of Eye:  Lens – focuses light rays onto the fovea centralis/retina - Can change shape due to its flexibility - Located behind the pupil - Becomes thinner to focus on distant objects and thicker to focus on nearby objects - Reverses the image from right to left and top to bottom when it is projected onto the retina  Optic Nerve – transmits sensory information to the brain  Aqueous Humour – maintains the shape of the cornea and provides oxygen and nutrients for the surrounding cells, including the lens and cornea - Clear watery liquid in the anterior chamber  Vitreous Humour – helps to maintain the shape of the eyeball and supports the surrounding cells - Jelly-like fluid in the posterior chamber, surrounded by the retina  Myopia – nearsighted, eyeball elongated so the focused light falls in front of the retina (concave lenses) * - Have trouble seeing faraway objects but can see close up objects - The lens focuses the image in front of the retina  Hyperopia – farsighted, eyeball is short so the focused light falls behind the retina (convex lenses) * - Have trouble seeing close up objects but can see faraway objects - The lens doesn’t thicken enough and the image is focused behind the retina Photoreceptors (path of light): A. In the retina, the rods are extremely sensitive to light, but do not enable us to distinguish colour (black and white) B. The rods also detect motion and are responsible for peripheral vision C. In the retina, the cones are the colour-detecting sensors D. The cones are packed most densely at the fovea centralis at the back of the retina E. When light stimulates the rods and cones, they stop releasing an inhibitory neurotransmitter into the synapse F. The bipolar cells then transfer neural impulse to the ganglion cells G. Ganglion cells are in the layer closest to the vitreous humour and form the Optic nerve H. The optic nerve fibers that emerge from the back of the eye transmit visual images to the occipital lobe of the brain I. Colour Blindness is caused by a lack or deficiency of cones  Ganglion cells – merge to form the optic nerve  Bipolar Cells - where rods and cones send their messages to the brain  Optic Nerve – nerve that carries messages from the photoreceptors in the retina to the brain  Blind Spot – doesn’t contain photoreceptors and is incapable of detecting light 1. The retina must send information to the optic nerve 2. Then, the information travels to the thalamus and then to the occipital lobe of the cerebral cortex for interpretation 3. The image is then split because the left optic nerve carries information about the right portion of the visual field and vise versa (carry opposite information) 4. In the cerebrum, the various pieces of visual information are processed and integrated and the image is perceived right-side up - Rods and cones point light away from the entering light so we receive only a fraction of the light energy that enters the eye - Many rods and some cones are connected to the same bipolar cells so they can combine their individual electrical messages to the bipolar cells - Visual acuity occurs when the visual image projects directly onto the fovea Visual Transduction: Light to Nerve Impulses - Photopigments – protein molecules that light waves to nerve impulses through rods and cones - Light absorption produces chemical reactions that change the rate of neurotransmitter release at the receptors synapse with the bipolar cells - If nerve impulses are triggered at rods, cones, bipolar cells, and ganglion cells, then the message is on its way to the visual relay station in the thalamus, and then to the visual cortex of the brain Brightness Vision and Dark Adaptation - Brightness sensitivity depends on the length of the wavelength of the light - Rods have better brightness sensitivity than cones, EXCEPT at the red end - Red end = no rods - Greenish/yellow = where cones are most sensitive Dark Adaptation – progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination  Photopigment molecules are regenerated and the receptors sensitivity increases  Rods photopigments regenerate slowly  Rods are important in night vision Colour Vision - We can distinguish up to 7.5 million hue variations Trichromatic Theory: - Additive colour mixture is when we produce a combination of red, blue and green in order to see all colours - Advanced by Thomas Young - He believes there are three types of colour receptors in the retina and that individual cones are most sensitive to wavelengths that correspond to either red, blue or green - His theory states that the visual system combines the signals to recreate the original hue - If all three cones are equally activated, that is when we will perceive white - Theory Problems: o Yellow is produced by activity of red and green receptors, yet some people with red-green colour blindness can sill see yellow o Colour afterimage where an image in a different colour appears after a colour stimulus has been viewed steadily and then with-drawn Opponent-process Theory: - Formulated by Ewald Hering - Assumed there are three types of cones and they each respond to two different wavelengths - One type responds to red or green, another type responds to blue or yellow, and a third responds to black or white - Believes after a while, the neural processes that register colours becomes fatigued and you see the opposite colour - Theory Problems: o Doesn’t occur at the level of the cones Dual- process Theory: - Combines the trichromaic and opponent-process theory to account for the colour transduction process - The cones do contain one of three different protein photopigments that are most sensitive to wavelengths corresponding to red, blue and green - Different activity in these colours produce a pattern of neural activity that corresponds to any hue in the spectrum - Discovered that certain ganglion cells in the retina, as well as some neurons in the visual relay stations and visual cortex, respond in opponent-process by altering their rate of firing - Activity of blue-sensitive cones directly stimulates the blue process farther along in the visual field - Activity of yellow is triggered by simultaneous input from the red and green- sensitive cones Colour-deficient Vision - Trichromats are people with normal colour vision - About 7% males and 1% females have a deficiency in the red-green or yellow-blue system - Dichromat is someone who is colour blind in only one of the systems (red- green or yellow-blue) due to an absence of hue-sensitive photopigment in a certain cone type - Monochromat is someone who is sensitive only to the black-white system and is totally colour blind Visual Scene Pathway 1. Optic nerve sends nerve impulses to a visual relay station in the thalamus 2. The input is routed to various parts of the cortex, particularly the primary visual cortex 3. There is a point-to-point correspondence between the regions of the retina and groups of neurons in the visual cortex 4. The fovea is represented by a large area of the visual cortex due to all the high visual activity from synapses of cones with bipolar cells 5. Neurons within the primary visual cortex are organized to receive and integrate sensory nerve impulses originating in specific regions of the retina (feature detectors) 6. Feature detectors fire selectively in response to stimuli that have specific characteristics 7. Certain neurons fire most frequently when lines of certain orientations were presented (bars, slits, and edges in certain positions – Hubel and Wiesel) 8. In the cortex, this information is analyzed by complex feature detector systems to produce our perception of objects (for bars, slits and edges) 9. Other types of feature detectors respond to colour, depth or movement by subdividing the v
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