Task

Two types of tasks are used in psychophysical experiments: detection tasks and discrimination tasks. For all examples in this app, two stimulus intervals were presented. In detection tasks, one of the intervals was empty, the null stimulus. The other stimulus, the target, varied in an attribute of interest, (e.g., contrast) and observers were asked to decide in which interval the target stimulus appeared. In discrimination tasks, both intervals contain a stimulus, and the stimuli vary in one or more aspects. Observers were asked to compare the stimuli on one attribute (e.g. 'Which stimulus has the highest contrast?'). Task type can only be varied in this app by selecting different example data.

Response

Two different response options are illustrated in this app: binary and ternary response options. When responses are binary (2AFC), the observer is forced to make a choice, even when they are not sure. When responses are ternary, there is a third response option that represents uncertainty (e.g., 'I don't know'). Response options can only be varied in this app by selecting different example data.

Type

This parameter has two levels – 'same' and 'different' – and indicates whether the same psychophysical function is assumed to hold for both standard and test stimuli, or not. For detection tasks, this is set to 'same' by default. For discrimination tasks, 'same' indicates that the standard and test stimuli only differ in the target attribute. 'Different' indicates that the standard and test stimuli differ in more than one attribute, and thus require two different psychophysical functions.

Alpha

Alpha represents the stimulus level below which detection tasks are meaningful. Above this value, stimuli are assumed to be easy to detect, and discrimination becomes more meaningful. To fit the data, the lower and upper bounds of this alpha value have to be set. A good convention is to set the lower bound to the minimum stimulis level minus two timesthe range of stimulus levels, and to set the lower bound to the maximum stimulus level. Alpha is only meaninful in detection tasks, and therefore cannot be set in discrimination tasks.

Beta

Beta is a shape parameter of the psychophysical function that determines the slope of the function in the discrimination range (above the detection threshold). A larger beta indicates a smaller slope, and vice versa. To fit the data, the lower and upper bounds of this beta value have to be set. The beta value has to be positive.

Plots

Three plots can be selected: the psychophysical function, the psychometric function, and a visualisation of the decision boundaries in decision space. The plots include a short description of the meaning of the plot.