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Chapter 5

Chapter 5 Textbook - Sensation

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Steve Joordens

CHAPTER 5 SENSATIONOur senses are the means by which we experience the worldEverything we learn is detected by sense organs and transmitted to our brain by sensory nervesDifferences between sources of information have important consequences in terms of which sensory systems process that information because different sense organs are attuned to different aspects of our world SENSORY PROCESSINGSENSATION detection of the elementary properties of a stimulus ie brightness colour warmth sweetnessInvolves innate prewired physiological mechanismsPERCEPTION detection of the more complex properties of a stimulus ie objects its location its movement its backgroundsInvolves learningCannot establish a clear boundary between sensation simple and perception complexExperience is essential to the development of some of the most elementary features of sensory systems TransductionSense organs located outside the brain detect environmental stimuli provided by light sound odour taste or mechanical contacttransmit signals to the brain that are coded in a specific way to represent theTask of the sense organsevents that have occurred in the environment informs the brain with the information gathered regarding the external worldInformation is transmitted to the brain through neural impulsesTask of the brain to analyze this info and reconstruct what has occurred respond by producing useful actionsTRANSDUCTION conversion of physical stimuli into changes in the activity of receptor cells of sensory organsProcess by which the sense organs convert energy from environmental events into neural activityEach sense organ responds to a particular form of energy and translates that energy into neural firingMeans of transduction are VERY diverseRECEPTOR CELL neuron that directly responds to a physical stimulus ie light vibrations aromatic moleculesRelease neurotransmitters that stimulate other neuronsalters the rate of firing of the axonsSomatosenses respond directly to physical stimuli without the use of receptor cellsSome of these neurons have specialized endings that enable them to respond to particular kinds of sensory information TABLE 51 Types of Transduction Accomplished by the Sense Organs Sensory CodingSensory information must accurately represent the environmentHowever different stimuli cannot be translated into different types of action potentialsAs a result the information from the sense organs are coded in the activity of axons carrying information from the sense organs to the brainAllows the brain to detect an enormous number of different stimuliCODING means by which the nervous system represents information 1 ANATOMICAL CODING different features are coded by the activity of different neuronsinterpret the location and type of sensory stimulus according to which incoming never fibres are activeReceptors that has been activated by an artificial stimulus create typical responses ie if you rub your eye 2 TEMPORAL CODING different features are coded by the pattern of activity of neuronsThe coding of information in terms of time ie rateAn axon can communicate quantitative information to the brain by firing at a faster or slower rate according to the intensity of the stimulusThe firing of a particular set of neurons tells where the body is being stimulated anatomical code whereas the rate at which these neurons fire tells the intensity of the stimulation temporal code PsychophysicsPSYCHOPHYSICS measures the quantitative relation between physical stimuli and perceptual experience the sensation they produce Two reliable methods to measure sensations 1 The Principle of the JustNoticeable Difference Ernest WeberJnd also called difference threshold is directly related to the magnitude of that stimulusWEBER FRACTION the ratio between a jnd and the magnitude of a stimulus is reasonably constant over the middle range of most stimulus intensities ie weightsGustav Fechner measured the absolute magnitude of a sensation in jnds ie strength of a persons sensation of a light of a particular intensityGraphed the relation between the strength of a sensory experience perception to the physical intensity of the stimulusProblem Some sensations take less energy to produce a jnd at higher intensity while some take less energy to produce a jnd at lower intensitySS Stevens suggested a power function to relate physical intensity to magnitude of sensation bSKI b SKI whereas S is the psychological magnitude of the sensation I is the intensity of the physical stimulus K is a constant that adjusts for the way physical intensity is measured and b is a value that is specific to each stimulusProvided a principle that can account for both types of results and compare different sensory systems 2 Signal Detection Theory Best methodTHRESHOLD point at which a stimulus or a change in the value of a stimulus can just be detectedDIFFERENCE THRESHOLD minimum detectable difference between two stimuliABSOLUTE THRESHOLD minimum value of a stimulus that can be detected from no stimulus at allThreshold is not a fixed value because there is inherent variability of activity in the nervous systemBy definition threshold is the point at which a participant detects the stimulus 50 of the timeSIGNAL DETECTION THEORY involves discriminating between a signal stimulus from the noise in which it is embedded consists of background stimuli and random activity of the nervous system ie warning light experimentTakes into account random changes in the nervous systemsFour possibilities hit false alarm miss correct negative correct responsesResponse bias can seriously affect an investigators estimate of the threshold of detectionMethods have been developed to assess peoples sensitivity regardless of their initial response bias by manipulating the response biases and observing the results of these manipulations on the peoples judgement ie payoff conditionsPerson decides whether a stimulus occurred and the consequences of making hits or false alarms can bias this decision
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