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Chapter 5

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Steve Joordens

Psychology Chapter 5 SENSATION Transduction Sensory CodingPsychophysics Function of the sense organs is to provide information that can help guide behaviour The translation of information about environmental events into neural activity is called transduction In the neural system information must be translated into two types of neural codes ANATOMICAL OR TEMPORAL Psychophysics is the study of the relationship between the physical characteristics of stimuli and the perceptions they produce Purpose our senses are the means by which we experience the world everything we learn is detected by sense organs and transmitted into our brain by sensory nerves otherwise we would have no purpose The visual scene received by our eyes changes rapidly as we move our body our head and our eyes Sound isnt as variable as vision Its intensity changes as we move toward or away from the as we move toward or away from its origin on the whole these changes are more gradual than those faced by the visual system Furthermore sounds carry around obstacles in ways that light does not Our auditory sense then has more time to process signals As we will see it uses time discriminate complex waveforms of the underlying sounds Speech is important for human culture as well as audition is important for social behaviourWith vision it provides information about distant events as does the sense of smell which can tell us about sources of aromatic molecules for upwind SENSORY PROCESSINGSensation the detection of elementary properties of stimulusbright colourswarmth colour and sweetness Perception the detection of more complex properties of a stimulusIts the detection ofobjects animate and inanimatetheir locations their movements and their backgroundsThis means that seeing the colour red is sensation While seeing a RED apple object is perception Seeing a movement is sensation but seeing a soccer ball coming toward us and realizing that we will have to move left to block it is perception It was believed that perception was dependent on learning but sensation meant innate psychological mechanisms proven to be falseEXPERIENCE is essential to the development of some of the most elementary features of the nervous systemSensory mechanisms visual auditory gustatory olfactory and somasensory systemsSomasensory systems have components that are able to detect touch warmthcoolness vibration physical damage painhead tilt head movement limb movement muscular contraction and various events occurring with our bodiesTRANSDUCTION The only sense receptors that the brain possesses detects such things as temperature and salt concentration of the blood and these receptors cannot inform it about what is going on outside Sense organs detect stimuli provided by light sound odour taste or mechanical contact with the environment1 Information about these stimuli is transmitted to the brain through neural impulses action potentials carried by the axons in the sensory nerves2 The task of sensory organs is to transmit signals to the brain that are coded in such a way to represent the events that have occurred in the environment 3 The task of the brain is to analyze this information and reconstruct what has occurred Transduction is the process by which sense organs convert energy from environmental events into neural activity Each sense organ responds to a particular form of energy given off by an environmental stimulus and translates that energy into a neural firing into which the brain can respond In most senses specialized neurons called receptor cells release neurotransmitters that stimulate other neurons thus altering the rate of firing of their axons In the samotosesnses body senses dendrites of neurons respond directly to physical stimuli without the intervention of specialized receptor cellsTYPES OF TRANSMISSION accomplished by the sense organsLocation of the sense organ Environmental Stimuli Energy transduced Eyelight radiant energy EarSoundMechanical energyVestibular systemTilt and rotationMechanical energy Tongue taste Recognition of molecular shape Nose Odour Recognition of molecular shape Skin Internal organsTouch Mechanical energy Temperature Thermal energy VibrationMechanical energy MusclePain Chemical reactionStretchMechanical energy Sensory CodingSensory information must accurately represent the environment Nerves are bundles of axons each of which can do no more than transmit action potentials These action potentials are fixed in size and durationthey cannot be altered Different stimuli cannot be translated into different types of action potentials Yet we can detect an enormous number of different stimuli with each of our sense organsThe information from the sense organs must somehow be coded in the activity of axons carrying information from the sense organs to the brainAnatomical Coding a means by which the nervous system represents information different features are coded by the activity of different neurons Sensory organs in different parts of the body send their information to the brain through different nerves Because they brain has no direct information about the physical energy impinging on a given sense organ it uses anatomical coding to interpret the location and type of sensory stimulus according to which nerve fibers are activeo If you rub your eye you will mechanically stimulate the lightsensitive receptors located there This stimulation produces actionpotentials in the axons of the nerves that connect the eyes with the brain optic nervesThe visual system of the brain has no way of knowing that the light sensitive receptors have been activated by a non visual stimulus o So the brain acts as if the neural activity in the optic nerves was produced by light so you see stars and flashes Experiments performed during surgery have shown that artificial stimulation of the nerves that convey taste produces a sensation of taste
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