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 Fechner The founder of psychophysics The term ‘Psychophysik’ was coined by Fechner 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.

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 average the 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 observer underestimated 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

The method of adjustment (5.2.4)

Method of limits To illustrate this technique, suppose we want to measure when a figure looks perfectly square. This question might be motivated by a well-known visual 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

Method of Limits 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.

The method of limits (5.2.1 )

Method 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 preliminary observations and this is an important part of such an experiment. We might, for example, use another method such as the method of adjustment to find such levels.] Once we have found the appropriate light levels we can now do our experiment.

As 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 presented or 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 experimenter knows whether or not a flash was presented, the observer's performance can be measured. The amount of light necessary for a certain level of performance (the threshold) can therefore be found.

A hypothetical threshold function with perfect discrimination

Results of constant stimuli experiment Psychometric Function

After obtaining such results, Fechner defined the threshold as that point at which the observer detects the stimulus 50% of the time. Note that this ‘definition’ maintains the concept of a threshold, but assumes that the threshold varies from to trial. Later we will question the assumption of a threshold.

Method of Constant Stimuli Advantages and Disadvantages: 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.

Method of constant stimuli (5.2.5)

Adaptive Methods To cope with some of the problems in the classical psychophysical methods, so-called adaptive methods have been developed. Many of these adaptive methods are variants of the classical method of limits. One of the main problems with the method of limits is that it is inefficient, because many trials that are well below or well above 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 depend on how the observer has already responded such that the next stimulus in a series depends upon the response the observer just made.

One adaptive method is a ‘staircase’ method Basically the procedure involves starting at some stimulus value above threshold, for example, and decreasing this value until the observer says 'no’ or ‘not seen’. On the next presentation the value is increased and if the observer says 'no', it is increased until she says yes. When The observer says 'yes' or ‘seen’ the value of the stimulus is decreased until he/she says 'no' and so on.

Staircase methods (5.2.3)

Double staircase-method It is possible, however, that an observer may become aware of the simple 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 two staircases simultaneously, randomly switching from one to the other.

Forced-Choice Procedures While the Method of Constant Stimuli eliminates some of the problems of the previous methods, it still has some problems of response criteria: the point at which "No" responses become"Yes" responses is determined not only by the stimulus threshold, but also by other ‘subjective’ factors such as the response criterion. A method developed to circumvent the problem of response criteria is the Method of Forced-Choice.

Method of Forced Choice Here subjects are presented with two or more alternatives, and must select one on each trial even if the stimulus was not clearly seen.

Square 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.

Threshold is 75% correct Chance 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 plays a major role in contemporary psychophysical research Of course, researchers are aware of the problems with the different techniques and design their experiments using the best techniques