Psychophysical methods

Recall from Section 2.3 that psychophysics relates perceptual phenomena to the original stimuli, which makes it crucial for understanding VR. Stevens' power law (2.1) related the perceived stimulus magnitude to the actual magnitude. The JND involved determining a differential threshold, which is the smallest amount of stimulus change that is detectable. A special case of this is an absolute threshold, which is the smallest magnitude stimulus (in comparison to zero) that is detectable.

Psychophysical laws or relationships are gained through specific experiments on human subjects. The term psychophysics and research area were introduced by Gustav Fechner [77], who formulated three basic experimental approaches, which will described next. Suppose that $x$ represents the stimulus magnitude. The task is to determine how small $\Delta x$ can become so that subjects perceive a difference. The classical approaches are:

• Method of constant stimuli: In this case, stimuli at various magnitudes are presented in succession, along with the reference stimulus. The subject is asked for each stimulus pair where he can perceive a difference between them. The magnitudes are usually presented in random order to suppress adaptation. Based on the responses over many trials, a best-fitting psychometric function is calculated, as was shown in Figure 2.21.
• Method of limits: The experimenter varies the stimulus magnitude in small increments, starting with an upper or lower limit. The subject is asked in each case whether the new stimulus has less, equal, or more magnitude than the reference stimulus.
• Method of adjustment: The subject is allowed to adjust the stimulus magnitude up and down within a short amount of time, while also being able to compare to the reference stimulus. The subject stops when she reports that the adjusted and reference stimuli appear to have equal magnitude.

Although these methods are effective and widely used, several problems exist. All of them may be prone to some kinds of bias. For the last two, adaptation may interfere with the outcome. For the last one, there is no way to control how the subject makes decisions. Another problem is efficiency, in that many iterations may be wasted in the methods by considering stimuli that are far away from the reference stimulus.