Presentation on theme: "Approaches to the Study of Perception"— Presentation transcript:
1Approaches to the Study of Perception Introduction to Psychophysics
2Approaches to the study of perception Phenomenology DescriptionBehaviour and Psychophysics MeasurementAnatomy and Physiology InferenceOne goal of sensory neuroscience is to link perceptual behaviour (and experience) to physiological mechanisms
3The Phenomenological Approach Often the starting point - sparks curiosityProvides a description of phenomenaProvides the raw material for research
5The Psychophysical Approach Examines the relationship between physical stimuli and sensory experienceMay provide the basis for inferring activity at the neural level
6The Anatomical/Physiological Approach A direct examination of the underlying structures and mechanismsCan be done independently or in parallel with psychophysicsMay provide the basis for explaining psychophysical data
7Introduction to Visual Psychophysics Psychophysics is the study of the relationship between physical stimuli and the perceptual behaviourHistorically, psychophysics was directed at answering philosophical questions about the relationship between mind and body.Modern psychophysics is mainly a set of tools for investigating the nervous system - information processing capacities
8Gustav Fechner The founder of psychophysics The term ‘Psychophysik’ was coined byFechner as a result of a mystical visionthat he experienced on October 22, 1850.‘Fechner Day’ (October 22) is nowcelebrated by psychophysicists in memoryof Fechner and his contributions.
9The contributions of Fechner: Was interested in solving the mind-body problemWanted to establish the relationship between changes in the physical domain and changes in subjective sensationHe developed the “classical” psychophysical methods as tools to investigate this relationshipHe formulated the first psychophysical “law”
10Psychophysics is concerned with functional relationships between stimulus characteristics and perceptual behaviourModern psychophysics uses these data to try to draw inferences about underlying physiological mechanismsThe data are gathered using different psychophysical methods
11The Domains of Psychophysics Detectionabsolute thresholdDiscriminationjust noticeable difference (jnd)point of subjective equalityScalingrating subjective magnitude
12Thresholds Much of psychophysics is concerned with thresholds Threshold - means the 'beginning point'. In early psychologyit referred to the boundary point which separated that which wasnot conscious from that which was conscious.
13Method of adjustmentTo illustrate the method of adjustment, let’s imagine that we conductan experiment on the Müller-Lyer illusion. In this illusion the line inthe top figure with the arrows pointing out looks shorter than the linein the bottom figure with the arrows pointing in even though the twolines are identical in length. Suppose we want to measure the extentof this illusion.
14Suppose we developed a computer program in which we could vary the length of lines and the computer stored the values of these linelengths. We could then have observers adjust the length of one of thelines until both lines looked identical in length. For example, supposeObservers adjusted the left line and the right line was the ‘standard’(180 units (e.g. pixels) in length). We would use both 'ascending' and'descending' trials in our experiment.
15A D The standard (ST) = 180 pixels Point of Subjective Equality (PSE) = pixels(Mean setting for both ascending and descending trials)The PSE indicates the setting in which the left line looks on averagethe same as the right line.Constant Error (CE) = PSE-ST = = pixelsThe CE indicates the average amount by which the observerunderestimated the standard line length and in this case is 36.5 pixels.Variable Error (VE) = 4 pixels (mean standard deviation (SD))The difference between the PSE and the observer’s setting on any trialis called the variable error (VE) as it varies in magnitude from the PSEover trials. VE is measured by the standard deviation.
16Method of Adjustment Advantages and Disadvantages: Advantages - relatively high test-retest reliability- more interesting for observer- quite efficientDisadvantages - method is not suitable for use with variables thatdo not vary continuously- errors of anticipation- subject to observer bias
18Method of limitsTo illustrate this technique, suppose we want to measure when a figurelooks perfectly square. This question might be motivated by a well-knownvisual illusion called the horizontal-vertical illusion. In this illusion, avertical line bisects a horizontal line. Even though both lines are thesame length, the vertical line looks longer. This experiment could tellus whether such an illusion operates in simple closed figures like a square.
19In our experiment we present rectangles in which we vary one dimension (e.g. the Y dimension) by a fixed amount- this is ourstep size. Here are some samples below. The figure in the middleis a perfect square while the rectangle on the far left has a Y axis10% longer than the X axis, and that at the far right has a Y axis 10%shorter than the X axis.
20We could keep track of our data like this Threshold value = -1.8
21Method of Limits Advantages and Disadvantages: Advantages - it is simple- reduces biasDisadvantages - tends not to be precise- it is inefficient since many trials farabove and below threshold are often performed.
23Method of Constant Stimuli How much light is required for an observer to detect it?Suppose we choose 9 levels of light intensity from ‘subthreshold’to ‘suprathreshold’ levels.We will present the stimuli briefly in a light flash.[To find these levels we would first have to make some preliminaryobservations and this is an important part of such an experiment.We might, for example, use another method such as the method ofadjustment to find such levels.]Once we have found the appropriate light levels we can nowdo our experiment.
24As in all psychophysical research, the task involves presenting a stimulus to an observer and measuring a response a 'simple' response.In this case, the task on each trial is to tell whether a flash was presentedor not. A trial is a designated interval in which a stimulus is presented(or not in some cases as we will see later). Since the experimenterknows whether or not a flash was presented, the observer's performancecan be measured. The amount of light necessary for acertain level of performance (the threshold) can therefore be found.
25A hypothetical threshold function with perfect discrimination
26Results of constant stimuli experiment Psychometric Function
27After obtaining such results, Fechner defined the threshold as that pointat which the observer detects thestimulus 50% of the time.Note that this ‘definition’ maintains the concept of athreshold, but assumes that the threshold varies fromto trial.Later we will question the assumption of a threshold.
28Method of Constant Stimuli Advantages and Disadvantages:Advantages - very precise and reliable- reduces biasDisadvantages - inefficient-requires some prior knowledge of thethreshold value of interestin order to be able to select the range ofvalues to be tested.
30Adaptive MethodsTo cope with some of the problems in the classical psychophysicalmethods, so-called adaptive methods have been developed.Many of these adaptive methods are variants of the classical methodof limits. One of the main problems with the method of limits is thatit is inefficient, because many trials that are well below or wellabove threshold are presented.Also,it is open to several sources of bias because observers canguess what is going on.In the adaptive methods, the stimuli that are presented dependon how the observer has already responded such that the nextstimulus in a series depends upon the response the observer just made.
31One adaptive method is a ‘staircase’ method Basically the procedure involves starting at some stimulus value abovethreshold, for example, and decreasing this value until the observer says'no’ or ‘not seen’. On the next presentation the value is increasedand if the observer says 'no', it is increased until she says yes. WhenThe observer says 'yes' or ‘seen’ the value of the stimulus is decreaseduntil he/she says 'no' and so on.
33Double staircase-method It is possible, however, that an observer may become aware of thesimple stimulus presentation scheme used in a simple staircase -this knowledge could influence the observer’s responses.To deal with this and other problems researchers often run twostaircases simultaneously, randomly switching from one to the other.
34Forced-Choice Procedures While the Method of Constant Stimuli eliminates some of the problemsof the previous methods, it still has some problems of response criteria:the point at which "No" responses become"Yes" responses is determinednot only by the stimulus threshold, but also by other ‘subjective’ factorssuch as the response criterion.A method developed to circumvent the problem of response criteria isthe Method of Forced-Choice.
35Method of Forced Choice Here subjects are presented with two or more alternatives, andmust select one on each trial even if the stimulus was not clearly seen.
36Square on Left or Right? (2 AFC) The observer must identify some characteristic of the stimulus (i.e. square) other than its intensity, for example. In this case, the observer must indicate its location. This proves that the observer can detect the stimulus.
38Forced Choice Methods- factor out criterion differences among observers- produces lower ‘thresholds’- shows that nervous systems register more information than one is usually aware of
39The determination of ‘thresholds’ still plays a major role in contemporary psychophysicalresearchOf course, researchers are aware of the problemswith the different techniques and design theirexperiments using the best techniques