chapter 2 notes
chapter 2 notes Psyc4106W
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This 10 page Class Notes was uploaded by Jennifer Gittleman on Wednesday October 28, 2015. The Class Notes belongs to Psyc4106W at George Washington University taught by Dr. Philbeck in Fall 2015. Since its upload, it has received 20 views. For similar materials see Sensation and Perception in Psychlogy at George Washington University.
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Date Created: 10/28/15
Light and Focusing a Light The Stimulus for Vision V Visible light a band of energy within the electromagnetic spectrum Electromagnetic spectrum a continuum of electromagnetic energy that is produced by electrical charges and is radiated as waves Wavelength distance between the peaks of the electromagnetic waves energy in the electromagnetic spectrum 1 Extremely short gamma ray one tenbillionth of a meter 2 Long radio wave 10000 meters Visible light 1 The energy within the electromagnetic spectrum that humans can perceive Has wavelengths from 400 to 700 nanometers Short wavelengths blue color Middle wavelengths green color Long wavelengths red orange yellow colors Photons small packets of energy in light one photon is smallest possible packet of energy U39lhUUN b The Eye VI vii viii Eye contain receptors for vision Pupil re ected light from object in environment enters through here Cornea 1 Transparent covering of front of eye 2 Responsible for 80 of eye s focusing power 3 Fixed in place so can t adjust its focus 1 Responsible for 20 of eye s focuses power 2 Can change its shape to adjust eye s focus for objects that are at different distances Retina where sharp images are formed Rods and cones visua receptors that contain light sensitive chemicals Visual pigments lightsensitive chemicals Optic nerve conducts signals toward brain All of these things shape what we see by creating two transformations 1 The transformation from light re ected from an object into an image of an object 2 The transformation from the image of the object into electric signals c Light is Focused by the Eye i Ciliary muscles 1 Does action of changing shape for eye 2 Increases focusing power of the lens by increasing its curvature bends light ii Adjustable lens controls accommodation and helps prevent blurring iii Accommodation 1 Change in the lens s shape that occurs when the ciliary muscles tighten and increase curvature of lens so that it gets thicker 2 Increased curvature helps to create a sharper image on the retina iv Near point distance where your lens can no longer accommodate to bring close objects into focus d Loss of Accommodation with Increasing Age i Presbyopia distance of the near point increases as a person gets older lens hardens with age and ciliary muscles become weaker ii Near point for ages 1 20 year olds 10 cm 2 30 year olds 14 cm 3 40 year olds 22 cm 4 60 year olds 100 cm e Myopia nearsightedness i Inability to see distant objects clearly ii Focus point for parallel rays of light is located in front of the retina so the image is blurred iii Problem caused by either two factors 1 Refractive myopia cornea andor lens bends the light too much 2 Axial myopia the eyeball is too long iv Far point distance at which light becomes focused on the retina v Solution 1 Corrective eyeglassescontact lenses a Bend incoming light so that it s focused as if it were at the far point 2 LASIK surgery a Sculpting the cornea with a type of laser f Hyperopia farsightedness i Inability to see nearby objects clearly ii Focus point for parallel rays of light is located behind the retina iii Eyeball is too short II Receptors and Perception a Transforming Light Energy into Electrical Energy i Transduction transformation of one form of energy into another form of energy ii Transformation of light energy into electrical energy for vision occurs in rods and cones iii Outer segments where molecules of lightsensitive visual pigments are contained iv Visual pigments have two parts 1 Opsin long protein 2 Retinal small lightsensitive component 3 When opsin and retinal are combined the resulting molecule absorbs visible light v Isomerization 1 When a visual pigment molecule absorbs one photon of light the retinal changes shape from being bent to straight 2 Creates a chemical chain reaction that activates thousands of charged molecules to create electrical signals in receptors activation of the receptor vi Visual pigments 1 Create electrical signals in receptors 2 Shape aspects of our perceptions a Ex determine how well we are able to adjust to darkness b Adapting to the Dark i Dark adaptation process of increasing sensitivity in the dark ii Distribution of the Rods and Cones 1 Fovea only contains cones where the object s image fails when we look at something 2 Peripheral retina contains both rods and cones contains more rods than cones 3 Macular degeneration common in older people destroys the conerich fovea and a small area that surrounds it creates a blind region in central vision when a person directly looks at something they lose sight of it 4 Retinitis pigmentosa degeneration of the retina passed from one generation to next rst attacks peripheral rod receptors and results in poor vision in severe cases foveal cone receptors are attacked too and results in blindness 5 Blind spot one area in retina where there are no receptors where the optic nerve leaves the eye iii Measuring the Dark Adaptation Curve 1 Dark adaptation curve function relating sensitivity to light to time in the dark 2 3 4 As adaptation proceeds observer becomes more sensitive to light Darkadapted sensitivity sensitivity at the end of dark adaptation about 100000 times greater than the lightadapted sensitivity measured before dark adaptation began Rod monochromats people who have no cones iv Measuring Cone and Rod Adaptation 1 2 When the light is turned off the sensitivity of both the cones and rods increases The cones are much more sensitive at the beginning of the dark adaptation so we see with our cones right after the light is turned off After about 7 minutes in the dark the rods become more sensitive than the cones Rodcone break place where the rods begin to determine the dark adaptation curve Visual pigment regeneration reason why the rods take much more time to reach their maximum sensitivity 2030 mins than the cones 34 mins v Visual Pigment Regeneration 1 2 3 Visual pigment bleaching process where the change in shape and separation of retinal from the opsin causes the molecule to become lighter in color when pigments are in lighter bleached state they aren t useful for vision Visual pigment regeneration retinal returning to its bent shape and become reattached to opsin so they can do the job of changing light energy into electrical energy William Rushton a Made a procedure to measure the regeneration of visual pigment in humans by measuring the darkening of the retina that occurs during dark adaptation b Cone pigment takes 6 minutes to regenerate c Rod pigment takes more than 30 minutes to regenerate d Two connections between perception and physiology i Our sensitivity to light depends on the concentrations of a chemical the visual pigment ii The speed at which our sensitivity increases in the dark depends on a chemical reaction the regeneration of the visual pigment 4 Pigment epithelium layer that contains enzymes necessary for pigment regeneration 5 Detached retina bleached pigment s separated opsin and retinal cannot be recombined and person becomes blind c Spectral Sensitivity i The eye s sensitivity to light as a function of the light s wavelength ii Spectral sensitivity curves relationship between wavelength and sensitivity iii Monochromatic light light of a single wavelength iv Cone spectral sensitivity curve having an observer look directly at a test light so it stimulates only the cones in the fovea v Rod spectral sensitivity curve measuring sensitivity after the eye is dark adapted and presenting test ashes in peripheral retina off to the side of the xation point vi Rods more sensitive to shortwavelength light as vision shifts from cones to rods during dark adaptation we become more sensitive to blue and green vii Purkinje shift enhanced perception of short wavelengths during dark adaption viii Rod and cone absorption spectra 1 Amount of light absorbed versus the wavelength of the light 2 Rod pigment absorbs best at bluegreen 3 Three absorption spectra for cones a Shortwavelength pigment 419 nm b Mediumwavelength pigment 531 nm c Longwavelength pigment 558 nm 4 Spectral sensitivity curve is determined mainly by the medium and long lll Electrical Signals a Neurons i Where electrical signals occur ii Key components 1 Cell body contains mechanisms to keep the cell alive 2 Dendrites branch out from the cell body to receive electrical signals from other neurons 3 Axon nerve ber lled with uid that conducts electrical signals iii Sensory receptors neurons specialized to respond to environmental stimuli iv Each eye contains more than 100 million receptors each transmits signals to neurons within the retina and then the signals are transmitted out of back of eye in optic nerve to a group of neurons called the lateral geniculate nucleus and then to the visual receiving area in the cortex Important to record the signals from single neurons because it provides valuable information what is happening in the nervous system b Recording Electrical Signals in Neurons iv V c Basic i V Electrical signals are recorded from the axonsnerve bers using small electrodes to pick up the signals When the axonnerve ber is at rest the difference in potential between the tips of the two electrodes is 70 miivots Resting potential the difference in charge between the inside and outside of the nerve ber when the ber isn t conducting electrical signals value is 70 miivots which stays the same as long as there are no signals in the neuron As the signal passes through the electrode the charge inside the axon rises up to 40 miivots compared to the outside as the signal passes the electrode the charge inside becomes negative again until it returns to the resting potential Action potential rapid increase in positive charge in a nerve ber that travels down the ber Properties of Action Potentials Propagated response once the response is triggered it travels all the way down the axon without decreasing in size important because it enables neurons to transmit signals over long distances The action potential remains the same size no matter how intense the stimulus is Refractory period interval between the time one nerve impulse occurs and the next one can be generated in the axon creates a limit 500800 impulses per second to the number of nerve impulses per second that can be conducted down an axon Spontaneous activity action potentials that occur in the absence of stimuli from the environment establishes a baseline level of ring for the neuron Presence of stimulation usually causes an increase in activity above the spontaneous eve d Chemical Basis of Action Potentials Action potentials create electricity in the wet environment of the body ii Components of the neuron s liquid environment 1 iii How 1 2 Neurons are bathed in a liquid solution rich in ions a Ions molecules that carry an electrical charge created when molecules gain or lose electrons ions result in the action potential Action potential begins traveling down the axon As it approaches Na ions rush into the axon because channels in the membrane have opened to allow Na to ow across the membrane Opening of sodium channels represent increase in the membrane s permeability to sodium a Permeability the ease with which a molecule can pass through the membrane Rising phase of the action potential a In the axon the decrease in negativity from 70 mV to 40 mV that occurs during the action potential this increase is caused by the in ow of Na ions into the axon Once the charge inside the neuron is 40 mV the sodium channels close membrane becomes impermeable to sodium and potassium channels open membrane becomes permeable to potassium Positively charged potassium rushes out of the axon and causes the charge inside to become more nega ve Falling phase of the action potential a In the axon the increase in negativity from 40 mV to 70 mV that occurs during the action potential this increase in negativity is associated with the ow of positively charged potassium ions out of the axon Sodiumpotassium pump a Stops sodium from building up inside the axon and potassium form building up outside the axon by continuously pumping sodium out and potassium in the ber e Transmitting Information Across a Gap i Synapse 1 Small space between neurons ii Neurotransmitters 1 Chemicals that are released when the action potentials reach the end of a neuron 2 Stored in structures called synaptic vessels 3 Like a key that ts a speci c lock iii Receptor sites 1 Small areas where the neurotransmitter molecules ow into that are sensitive to speci c neurotransmitters 2 When neurotransmitter makes contact here matching its shape it is activated and triggers a voltage change in the receiving neuron iv When the electrical signal reaches the synapse it triggers a chemical process that causes a new electrical signal in the receiving neuron 1 Two types of responses can occur at the receptor sites a Excitatory response occurs when the inside of the neuron becomes more positive called depolarization increase the chances that a neuron will generate action potentials increasing rates of nerve ring i Depolarization causes the charge to change in the direction that triggers an action potential b Inhibitory response occurs when the inside of the neuron becomes more negative called hyperpolarization decreases the chances that a neuron will general action potentials decreases rates of nerve ring i Hyperpolarization causes the charge inside the axon to move away from the level of depolarization needed to generate an action potential IV Neural Convergence and Perception a Neural circuits interconnected groups of neurons b 5 types of neurons that make up retina i receptors bipolar cells iii ganglion cells iv horizontal cells v amacrine cells c Signals generated in receptors travel to the bipolar cells and then to ganglion cells i Receptors and bipolar cells don t have long axons but ganglion cells have them which transmit signals out of the retina in the optic nerve d Signals travel between receptors through the horizontal cells and between bipolar cells and between ganglion cells through the amacrine cells e Neural convergence Occurs when a number of neurons synapse onto a single neuron Ex 120 million rods in retina but only 6 million cones 120 rods send their signals to one ganglion cell but 6 cones send signals to one ganglion cell f Convergence Causes the Rods to Be More Sensitive Than the Cones Rods have greater convergence than the cones 1 Difference in rod and cone convergence translates into differences in the maximum sensitivities of the rods and cones Rods high sensitivity compared to cones is caused by rods greater convergence g Lack of Convergence Causes the Cones to Have Better Acuity Than the Rods iv V Because rods have more convergence cones have better visual acuity because they have less convergence 1 Visual acuity ability to see details highest in the fovea ex looking for something that is hidden among other things such as a cell phone When you present two spots of light for rods the ganglion cell res no matter where the light is placed because it is the same ganglion cell When you present two spots of light for cones it matters where you place the light because it has separate ganglion cells High convergence results in high sensitivity but poor acuity rods Low convergence results in low sensitivity but high acuity cones V Early Events Are Powerful a Rocket was blasted off to place the Hubble space telescope into orbit to provide highresolution images images of stars and galaxies were supposed to be sharp but came out blurry b Cause of problem telescope s lens was on the wrong curvature c Solution a corrective lens was tted over the original lens Vl Infant Visual Acuity a Preferential looking technique PL measures infant visual acuity b Visual evoked potential VEP acuity determined by measuring C an electrical signal recorded by disc electrodes placed on the infant s head over the visual cortex generated if stripes can be detected usually indicates better acuity than PL Both techniques indicate that visual acuity is poorly developed at birth 20400 or 20600 d Acuity increases at 69 months but full adult acuity isn t reached until 1 year old e Infant s poor acuity is because their cones are spaced far apart and the visual area of the brain isn t developed yet
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