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5.2 Allocating Stimuli and Attributes

5.2.1 Events, Stimuli, and Attributes

In programming ODEVs, we make a distinction between a an event, and its stimulus, and within the stimulus itself we distinguish between its stimulus values its attributes.

An event is a period of time with a well-defined onset and duration within a trial, during which a stimulus may be displayed. A stimulus of an event is every part of the event which relates to what is displayed during the event and how it is displayed, independant of its onset and duration.

With in a stimulus, the term stimulus value, or stimulus values, generally refers to what is displayed, while the term attributes generally refers to all those things which influence how it is displayed.

Take, for example, the following event definition from a factor-format script:

		Stimulus: "Hello World"
		EventType: Text
		Duration: Mouse[click]
		StartRef: 500 after START
		Font: Helvetica
		Size: 24
		Port: Center 300 33% 100 1

This entry defines an event of type "Text" that is to start 500 msec after the start of the trial and last until the mouse is clicked. The stimulus of this event is the text "Hello World" printed in the Helvetica font, in size 24, in a port 300 pixels wide by 100 pixels high, horizontally centered an vertically positioned 33% of the way down the screen, with a 1 pixel wide frame surrounding it.

In the stimulus described above, the text "Hello World" would be considered to be its stimulus value, while the font, size, and port are its attributes.

How an event's stimulus values and attributes are represented in the script depends upon which script format is being used. Most scripts these days are written in Factor format, in which an event is represented by an entry in the script with a special attribute, called "stimulus", or "stimuli" to denote the stimulus value or values (see "5.2.2 Multiple Stimulus Values"), while the rest of the entry's attributes contain various modifiers of the stimulus values. There are other, less used, scripting formats, however, which read the script differently. The stimlist format, for example, uses individual tokens on the content line of the experiment entry (or a block entry) as stimuli, and uses the attributes that each has inherited through evaluation of scripting expressions as attrbutes. Furthermore, it's possible that sometime in the future, yet another scripting format will be devised.

In any case, the interface between the ODEVs and the script is the same. When an ODEV is asked to allocate a new stimulus, it will be passed a reference to the stimulus and its attributes, which can then be used with a number of PsyScope Toolbox routines to retrieve the actual content of the event. The details of the conversion from script to ODEV are handled elsewhere (in the IMSB, actually).

PSYX Programmer's Manual - 24 AUG 95
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