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Approaches to the Study of Perception
Introduction to Psychophysics
Approaches to the study of perception
• Phenomenology Description
• Behaviour and Psychophysics Measurement
• Anatomy and Physiology Inference
• One goal of sensory neuroscience is to link perceptual behaviour (and experience) to physiological mechanisms
The Phenomenological Approach
• Often the starting point - sparks curiosity
• Provides a description of phenomena
• Provides the raw material for research
Hermann Grid
What produces those grey spots?
The Psychophysical Approach
• Examines the relationship between physical stimuli and sensory experience
• May provide the basis for inferring activity at the neural level
The Anatomical/Physiological Approach
• A direct examination of the underlying structures and mechanisms
• Can be done independently or in parallel with psychophysics
• May provide the basis for explaining psychophysical data
Introduction to Visual Psychophysics
• Psychophysics is the study of the relationship between physical stimuli and the perceptual behaviour
• Historically, 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
Gustav FechnerThe founder of psychophysics
The term ‘Psychophysik’ was coined byFechner as a result of a mystical vision that he experienced on October 22, 1850. ‘Fechner Day’ (October 22) is now celebrated by psychophysicists in memory of Fechner and his contributions.
The contributions of Fechner:
• Was interested in solving the mind-body problem
• Wanted to establish the relationship between changes in the physical domain and changes in subjective sensation
• He developed the “classical” psychophysical methods as tools to investigate this relationship
• He formulated the first psychophysical “law”
• Psychophysics is concerned with functional relationships between stimulus characteristics and perceptual behaviour
• Modern psychophysics uses these data to try to draw inferences about underlying physiological mechanisms
• The data are gathered using different psychophysical methods
The Domains of Psychophysics
• Detection – absolute threshold
• Discrimination – just noticeable difference (jnd)– point of subjective equality
• Scaling– rating subjective magnitude
Thresholds
Much of psychophysics is concerned with thresholds
Threshold - means the 'beginning point'. In early psychology it referred to the boundary point which separated that which was not conscious from that which was conscious.
Müller-Lyer Illusion
Method of adjustment
To illustrate the method of adjustment, let’s imagine that we conduct an experiment on the Müller-Lyer illusion. In this illusion the line in the top figure with the arrows pointing out looks shorter than the line in the bottom figure with the arrows pointing in even though the two lines are identical in length. Suppose we want to measure the extent of this illusion.
Suppose we developed a computer program in which we could vary the length of lines and the computer stored the values of these line lengths. We could then have observers adjust the length of one of the lines until both lines looked identical in length. For example, suppose Observers 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.
A D
The standard (ST) = 180 pixels
Point of Subjective Equality (PSE) = 143.5 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 = 143.5-180 = -36.5 pixels
The 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 trial is called the variable error (VE) as it varies in magnitude from the PSE over trials. VE is measured by the standard deviation.
Method of Adjustment
Advantages and Disadvantages:
Advantages - relatively high test-retest reliability - more interesting for observer- quite efficient
Disadvantages - method is not suitable for use with variables that do not vary continuously- errors of anticipation- subject to observer bias
Method of limits
Horizontal-Vertical Illusion
To 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, a vertical line bisects a horizontal line. Even though both lines are the same length, the vertical line looks longer. This experiment could tell us whether such an illusion operates in simple closed figures like a square.
In our experiment we present rectangles in which we vary one dimension (e.g. the Y dimension) by a fixed amount- this is our step size. Here are some samples below. The figure in the middle is a perfect square while the rectangle on the far left has a Y axis 10% longer than the X axis, and that at the far right has a Y axis 10% shorter than the X axis.
We could keep track of our data like this
Threshold value = -1.8
Advantages and Disadvantages:
Advantages - it is simple- reduces bias
Disadvantages - tends not to be precise- it is inefficient since many trials far above and below threshold are often performed.
Method of Limits
Method of Constant StimuliHow 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.
As in all psychophysical research, the task involves presenting astimulus 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.
A hypothetical threshold function with perfect discrimination
Psychometric Function
Results of constant stimuli experiment
After obtaining such results, Fechnerdefined the threshold as that pointat which the observer detects thestimulus 50% of the time.
Note that this ‘definition’ maintains the concept of a threshold, but assumes that the threshold varies fromto trial.
Later we will question the assumption of a threshold.
Advantages - very precise and reliable- reduces bias
Disadvantages - inefficient-requires some prior knowledge of the threshold value of interest in order to be able to select the range of values to be tested.
Advantages and Disadvantages:
Method of Constant Stimuli
Method of constant stimuli (5.2.5)
Adaptive 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 that it 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 can guess 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.
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 decreased until he/she says 'no' and so on.
One adaptive method is a ‘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.
Double staircase-method
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.
Forced-Choice Procedures
Here subjects are presented with two or more alternatives, andmust select one on each trial even if the stimulus was not clearly seen.
Method of Forced Choice
Square on Left or Right?
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.
(2 AFC)
Threshold is 75% correctChance is 50% correct
Forced Choice Methods
- factor out criterion differences among observers- produces lower ‘thresholds’- shows that nervous systems register more information than one is usually aware of
The determination of ‘thresholds’ still playsa major role in contemporary psychophysicalresearch
Of course, researchers are aware of the problemswith the different techniques and design theirexperiments using the best techniques
