Jittered RT task

Measure choice reaction times with an unpredictable interval before each stimulus.

When to use it

Use this whenever reaction time is the measurement and anticipation is the enemy. With a fixed interval before every stimulus, participants learn the rhythm and start responding to the schedule rather than the stimulus, which compresses reaction times and produces false starts. Jittering the pre-stimulus interval makes each onset unpredictable, so responses reflect processing of the stimulus itself. It fits any speeded choice task (lexical decision, flanker, go or no-go); if your design instead needs every trial to last exactly the same time, use a fixed ITI and rely on a fixed responseWindow.

Trial file

type	stimFormat	stim1	ITI	presTime	responseWindow	keyboard	key
test	.png	arrow_left	800_1600	200	1500	left right	left
test	.png	arrow_right	800_1600	200	1500	left right	right
test	.png	arrow_left	800_1600	200	1500	left right	left
test	.png	arrow_right	800_1600	200	1500	left right	right
test	.png	arrow_left	800_1600	200	1500	left right	left
test	.png	arrow_right	800_1600	200	1500	left right	right

How it works

The point of the jitter is to stop participants timing their response to the trial rhythm instead of the stimulus. ITI set to 800_1600 inserts a blank delay of 800 to 1600 ms before each trial’s stimulus, and the min_max value is re-drawn every time a trial is presented. That per-presentation draw is specific to ITI; random values in presTime or ISI are drawn once per row when the experiment loads and then reused, so the jitter belongs in ITI.

presTime keeps each arrow up for a fixed 200 ms, brief enough that the response is driven by the flash rather than by inspecting a persistent stimulus. The numeric responseWindow of 1500 opens when the stimulus appears and closes 1500 ms later; reaction time is counted from the moment the window opens, and a trial with no response records timeout and moves on, so anticipations and lapses are both visible in the results.

keyboard uses the arrow-key tokens left right; these exact names matter, since an unrecognised multi-character token such as arrowleft decomposes into single letters. key scores the congruent arrow key as correct.

Variations

• Use a discrete jitter instead: ITI of 800|1200|1600 picks one of the listed delays per presentation.
• Fill the jittered interval with a cross by adding fixation set to the exact token ITI, so the delay reads as a fixation period rather than a blank.
• Block anticipatory presses with responseWindow set to min:100;max:1500; responses in the first 100 ms are buffered until the minimum window ends.
• Tighten or relax the deadline by changing the responseWindow timeout; the recorded timeout responses tell you when it is too strict.