LUIDA Docs
Cookbook

Different stimuli per condition

Show one stimulus in condition A, another in condition B. The if-toggle on a state-listening action does it — no custom code required.

Goal

In one experiment, half the trials (or half the participants) see stimulus A, the other half see stimulus B. The choice has to be tied to a LUIDA condition variable, not a coin flip in your script.

Smallest setup

  • A LUIDA scene with at least one trial state.
  • Two stimulus GameObjects in the scene (Stimulus_A and Stimulus_B), both with the LUIDA state-listener component attached.
  • The Configure experiment automation editor open at LUIDA › Configure experiment automation.

The recipe

Declare the variable

Open the Variables tab. Decide whether you want this to vary within each participant or between participants:

  • Within-subjects — every participant sees both stimuli (in random order, across trials). Add a within-subjects variable named e.g. stim with values A,B.
  • Between-subjects — each participant only sees one stimulus across all their trials. Add a between-subjects variable named e.g. stim with values A,B. LUIDA's balancer alternates assignments across sessions.

Save the variables config. Trial count auto-recalculates.

Hide both stimuli by default

In the State-listening Items tab, find the row for Stimulus_A and the column for the trial state. Under On State Start, add a HideItem() action. Do the same on Stimulus_B.

Now both stimuli are hidden when the trial begins.

Conditionally show each stimulus

In the same Stimulus_A cell, under On State Start, add a second action: ShowItem(). Then click the if toggle on that action row. Configure:

  • Variable: stim
  • Equals: A

The generated code becomes:

HideItem();
if (CONDITION["stim"] === "A") {
  ShowItem();
}

Repeat for Stimulus_B, gated on CONDITION["stim"] === "B".

Clean up on state exit (optional but good hygiene)

In the same row for each stimulus, under On State Exit of the trial state, add an unconditional HideItem(). This guarantees a clean slate going into the next trial regardless of which branch fired this trial.

Press Play. On each trial, LUIDA picks the next CONDITION["stim"] (alternating A/B with random shuffling, by default), and only the matching stimulus's ShowItem() fires.

Why this works

CONDITION is the per-trial variable dictionary that LUIDA fills in from your variable declaration before each trial begins. The if toggle on the state-listening editor wraps the action in a CONDITION[var] === value check, executed inside the same lifecycle slot — so the gate happens at the moment of action firing, with the current trial's variables.

HideItem / ShowItem are global state writes on the item itself (isVisible), networked to all clients in the Cluster instance. Both branches of your "if" race-free, because the editor generates them as straight-line code inside a single lifecycle handler — there's no await or two-frame gap where both stimuli could briefly be visible.

For background on within vs between assignment, see Concepts → Within vs between subjects. For the balancer that assigns between-subjects variables across sessions, see Concepts → Conditions, sessions, participants.

Variants

More than two stimuli. Use values A,B,C,D. Add four hidden stimuli, each with a ShowItem() gated on its matching CONDITION value. Scales linearly.

Stimulus is a property of one shared GameObject, not a separate one. Use a single Stimulus GameObject and toggle its text or texture instead of its visibility:

// On State Start, in a Customized Action
if (CONDITION["text"] === "Red") {
  SetText("Red");
} else if (CONDITION["text"] === "Blue") {
  SetText("Blue");
}

The Stroop tutorial uses this pattern for the colored words. See Reference → SetText and Reference → SetChildPosition for related single-item-multi-config patterns.

Combine with a second variable. A 2×2 design — font: R, B × text: Red, Blue — gives four cells. Show your Stimulus once, but configure it with a Customized Action that reads both CONDITION["font"] and CONDITION["text"]. This is exactly Tutorial 2's setup.

Per-condition avatars instead of per-condition stimuli. Conceptually identical pattern, but use AssignAvatarToParticipant instead of ShowItem. See the unique-avatar-per-participant cookbook.

Where to go next

On this page

GoalSmallest setupThe recipeDeclare the variableHide both stimuli by defaultConditionally show each stimulusClean up on state exit (optional but good hygiene)Why this worksVariantsWhere to go next