The Dots Component allows you to present a Random Dot Kinematogram (RDK) to the participant of your study. These are fields of dots that drift in different directions and subjects are typically required to identify the ‘global motion’ of the field.
There are many ways to define the motion of the signal and noise dots. In PsychoPy the way the dots are configured follows Scase, Braddick & Raymond (1996). Although Scase et al (1996) show that the choice of algorithm for your dots actually makes relatively little difference there are some potential gotchas. Think carefully about whether each of these will affect your particular case:
- limited dot lifetimes: as your dots drift in one direction they go off the edge of the stimulus and are replaced randomly in the stimulus field. This could lead to a higher density of dots in the direction of motion providing subjects with an alternative cue to direction. Keeping dot lives relatively short prevents this.
- noiseDots=’direction’: some groups have used noise dots that appear in a random location on each frame (noiseDots=’location’). This has the disadvantage that the noise dots not only have a random direction but also a random speed (whereas signal dots have a constant speed and constant direction)
- signalDots=’same’: on each frame the dots constituting the signal could be the same as on the previous frame or different. If ‘different’, participants could follow a single dot for a long time and calculate its average direction of motion to get the ‘global’ direction, because the dots would sometimes take a random direction and sometimes take the signal direction.
As a result of these, the defaults for PsychoPy are to have signalDots that are from a ‘different’ population, noise dots that have random ‘direction’ and a dot life of 3 frames.
Window
will be used.
See Units for the window and stimuli for explanation of other options.See also
API reference for DotStim