Introduction
Alternative Forced Choice experiments are a psychophysical paradigm that allow measurement of a participant’s sensitivity to specific stimuli. In these tasks, the participant must select one of a set of possible choices in response to a perceived stimulus.
As of Version 1.13, LabMaestro now supports Alternative Forced Choice experiments using the Staircase and Quest Methods. This tutorial explains how to prepare such an experiment.
Prerequisites
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LabMaestro Version 1.13 is installed and activated
Template Project
The template project above shows a contrast sensitivity experiment in which the participant must answer if the grating orientation is vertical (Left key) or horizontal (Right key). For every correct answer, the grating contrast diminishes, and vice versa.
Enabling X-Alternative Forced Choice
To enable Alternative Forced Choice in an experiment, start by adding a method to your project (Staircase or Quest). Then, in your method’s properties, check the Enabled box under Alternating Forced Choice. From this same box, you can also specify the Mode (either Single or Sequential) and the number of choices (2, 3, or 4).
If we follow the template project linked above, which measures contrast sensitivity, we can select 2 choices. The bottom boxes (StaircaseUpInput & StaircaseDownInput) will be renamed to ChoiceOneInput and ChoiceTwoInput, but will still function the same way. For the template project, the input for Choice One is bound to Left, while the input for Choice 2 is bound to Right.
Once the inputs have been defined, you can then go to Alternating Forced Choice Timelines from the left panel. This tab lets you link specific timelines to different choices, changing how stimuli are presented. For example, in the template project, we link grating orientation to the different choices: the timeline linked to the first choice shows vertical gratings, while the one linked to the second choice shows horizontal gratings.
To make a timeline eligible for selection in this tab, it must be under the method in the Project panel, under Experiment Design.
Contrary to what would ordinarily happen, both timelines will not be shown on every trial; LabMaestro will automatically show only one timeline per trial.
Unlike staircase procedures, where one input increased the value of the dependent variable, and the other decreased it, Alternative Forced Choice paradigms have correct answers reduce the value of the dependent variable, while incorrect answers increase it. The correct answer for a given timeline will be the input key you’ve linked with its corresponding choice.
All other aspects of the method object, whether you use the Staircase or Quest method, will work the same way regardless of whether Alternating Forced Choice is enabled.
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