Lecture 02. Measuring Perception.pdf

Up to a two line subtitle, generally used to describe the takeaway for the slide Fundamentals of Sensation and Perception 1 Introduction Whom and how do we study perception?...

Up to a two line subtitle, generally used to describe the takeaway for the slide Fundamentals of Sensation and Perception 1 Introduction Whom and how do we study perception? something inenvironmentthrough protecting Psychologists typically study sensation and perception sensel experience chowmeasuy.it Also studied by biologists, computer scientists, linguists, neuroscientists, and many other fields The study of sensation and perception is a scientifichtwpoopla pursuit and smalleltamruntofsometh requires scientific methods Thresholds: Finding the limits of what can be perceived. Scaling: Measuring experience.scale 1 10howpainful Youlanseeorhear Sensory neuroscience: The biology of sensation and perception.Measursensatio usingelectrode Neuroimaging: An image of the mind (e.g., PET, fMRI). Fingat brainactivitymeasunearo toxaman.mn Patient Studies: individuals with brain damage. neuron brainfunctionofthosebrainareas Brain Stimulation: Transcranial Magnetic Stimulation (TMS), Transcranial direct current stimulation (tDCS). 2 What is sensed by Humans? damagetoourtissues respondstotemp capasesinspicyfood activatether moreleptors 3 Studying the behaviour Dawn of Psychophysics Gustav Fechner (1801–1887) invented “psychophysics” and is often considered to be the true founder of experimental psychology Psychophysics: The science of defining quantitative relationships between physical and psychological (subjective) events 4 Studying the behaviour Dawn of Psychophysics Psychophysics adopted several new concepts for understanding sensation and perception Kiki till nolongerhear lowtohigh tillable Absolute threshold: Minimum amount of stimulation tohear necessary for a person to detect a stimulus 50% of the time. smallest amount ofstimulusabletodetect Method oflimits canstartat one extreme and makeway to otherend Method of Adjustment: “quick and dirty”. Adjust using a dial / buttons until stimulus is perceived. 5 Studying the behaviour Dawn of Psychophysics Method of Constant Stimuli: select number of stimuli that cover a range likely to include absolute threshold. a) simple yes/know response on one block of trials. b) Psychometric function from many blocks. tedious time needtobepresented andated times toobigspacing between moreaccurate intensities description loudness 6 Studying the behaviour Dawn of Psychophysics Staircase Method: intensity of stimulus one up or down from previous trial. 144MEur It turningpoints everytime I person change theiranswer change direction Montinueincreasing average turning points threshold 7 Studying the behaviour Dawn of Psychophysics tellingthe differencebetween 2stimuli Difference threshold: same or different? (also called just noticeable difference, or JND). 8 Studying the behaviour Dawn of Psychophysics Difference threshold: can be measured using method of adjustment and constant stimuli. JustNoticeable C Differences Change difference between comparison of stimuli 9 Studying the behaviour Dawn of Psychophysics Ernst Weber (1795–1878) discovered that the smallest change in a stimulus (e.g., the weight of an object) that can be detected is a constant proportion of the stimulus level. and aslongas2 C E.g., for 100g, JND is 2g (or 2%). For 1000g, JND is 20g (or 2%). proportion staysthesame ALWAYS 2010 Weber’s law JND=kl : The principle describing the relationship between stimulus and resulting sensation intensity (l) that says the JND is a constant fraction (k) of the comparison stimulus. 10 Studying the behaviour Psychophysical Scaling Measuring how changes in stimulus intensity relate to changes in perceived intensity Fechner’s law, S = k ln I/Io: Variant of Weber’s law, A principle describing the relationship between the perceived intensity (S) of a stimulus as it changes in comparison to the natural log ratio between the physical intensity (I) and the intensity at absolute threshold (Io). Logarithmic relationship Fechner's law perceived intensity 11 Studying the behaviour Psychophysical Scaling Problem: Weber’s law good for brightness and loudness, but not for other sensations, like electric shock. Stevens law, S = cln: relationship between the peceived intensity of a stimulus (S), its physical instensity (I), the exponent n is different for each perceptual dimension, and c is the constant that depends on which units are being used for S and I. 12 Noise in Responses At neuronlevel o work deterministically 13 Noise and the Psychometric Function too muchoverlap hard to distinguish between difference groups 14 Signal Detection Theory considering decision making by people 15 Signal Detection Theory considering decision making by people Signal detection theory: A psychophysical theory that quantifies the response of an observer to the presentation of a signal in the presence of noisy perceptual evidence. There are four possible stimulus/response situations in signal detection theory: expected the tobe same 16 Signal Detection Theory considering decision making by people Signal detection theory: A psychophysical theory that quantifies the response of an observer to the presentation of a signal in the presence of noisy perceptual evidence. There are four possible stimulus/response situations in signal detection theory: eagerto avoidfalsealarms causes misses type I error type II error 17 Signal Detection Theory considering decision making by people Perfectscore Receiver operating characteristic (ROC): In studies of signal detection, the graphical plot of the hit rate as a function of the false alarm rate Chance performance will fall along the diagonal Good performance (high sensitivity) “bows out” towards the upper left corner Plotting the ROC curve allows one to predict the proportion of hits for a given proportion of false alarms, and vice-versa 18 Signal Detection Theory considering decision making by people Signal detection theory makes a distinction between an observers’ ability to perceive a signal, and their willingness to report it. These are three separate concepts Sensitivity: A value that defines the ease with which an observer can tell the difference between the presence and absence of a stimulus or the difference between stimulus 1 and stimulus 2 Criterion: An internal threshold that is set by the observer. If the internal response is above criterion, the observer gives one response (e.g., “yes, I hear that”). Below criterion, the observer gives another response (e.g., “no, I hear nothing”). Bias: observer tendency to be liberal or conservative in response, indicated by value of criterion. 19 Signal Detection Theory considering decision making by people s performance Yhff falsealarm Imiss performance 20