NCMlab / StaircaseJS

Adaptive staircase procedure in JavaScript

Background

In psychophysics, perception is often assessed using adaptive procedures (task difficulty is modified according to participants answers). With staircase procedures (https://en.wikipedia.org/wiki/Psychophysics#Staircase_procedures) the stimulus intensity begins high and is reduced when right answers are provided and increased when wrong answers are provided. Several parameters are generally used:

• The equation converting the old value to the new one (according to the answer provided for the old one)
• The number of right answers n needed to increase the difficulty (when only one wrong answer will decrease it; corresponds to 1-up, n-down).

The StaircaseJS module can be used to manage the value of a stimulus implementing an adaptive procedure, typically for psychophysical experiments made in JavaScript.

You can try an example of StaircaseJS here.

Usage

Create a new staircase will allow you to store and track the values of the stimulus. The parameters of the staircase (equation used to change the value, x-up y-down etc.) has to be set when the contructor is instanciated.

Instanciation

Staircase objects are created using the Staircase constructor with an object as argument:

var stair = new Staircase({
  deltaF: {
    firstVal: 100,
    down: 3,
    factor: 1.25,
    limits: [0, 600],
  }
});

This object has to contain the name of the staircase as a key and its parameters in another object as a property. Thus, the form of the argument is:

{
  staircaseName: {
    parameter1: val1,
    parameter2: val2,
    ...
  }
}

Example

Procedure

In this first example, we will create a stair object to manage the frequency of a sound. At each trial, participants hear two sounds and have to tell which one is higher in frequency. One sound has a fixed frequency of 1000Hz and the other has a changing frequency around 1000Hz. The frequency of this second sound is adaptively chosen according to participant's answers.

Set parameters

var stair = new Staircase({
  deltaF: {
    firstVal: 100,
    down: 2,
    limits: [0, 600],
    factor: 1.25,
  }
});

We will set the first value to 100 cents (one semi-tone) and choose a 1-up 2-down procedure (the difficulty will be increased by an amount x after 2 right answers and decreased of this same amount after 1 wrong answer) with the down parameter. We also set the limit of the frequency difference between 0 and 600 cents with the limits parameter. The factor parameter is used to set the amount of change.

Activate the staircase:

stair.init();

Provide the answer (right: true, wrong: false) in the next method:

stair.next(goodAnswer);

At this time, a new value is stored in the stair object depending of the answer. This new value can be used to change the frequency of the sound. We only have to get the value of our deltaF staircase:

sound.frequency.value = stair.getLast('deltaF');

The historic of the values can be obtained with:

stair.get('deltaF');
// return an array containing all the values

Implementation of the x-up y-down procedure

The x-up y-down procedure can be implemented in two modes. With the wait mode, we will wait y good answers to increase difficulty and x answers to decrease difficulty.

var stair = new Staircase({
  deltaF: {
    ...
    wait: true,
    ...
  }
});

wait mode

// example with a 1-up 2-down procedure and factor = 1.5
// first value = 100
// then good answer
newValue = 100;
// then good answer again
newValue = 100/1.5;
// = 66.66
// then wrong answer
newValue = 66.66*1.5;
// = 100 (first value)

noWait mode

These equations are used to implement the x-up y-down procedure:

• Decreasing (wrong answer): newValue = oldValue / (Math.pow(factor, 1/down)); and increasing (right answer): newValue = oldValue * factor;.

Instead of keeping the same value y times and then change the value by factor, we change the value each time:

// first value = 100
// then good answer
newValue = 100/(Math.pow(1.25, 1/2));
// result = 89.44
// then good answer again
newValue = 89.44/(Math.pow(1.25, 1/2));
// = 80
// then wrong answer
newValue = 80*1.25;
// = 100 (first value)

Multiple staircases

It is possible to use the staircase module to maintain more than one stairecase. To do that, specify multiple objects in the object used to instanciate Staircase().

{
  staircaseName: {
    firstVal: 1,
    down: 3,
    ...
  },
  staircaseName2: {
    firstVal: 2,
    down: 3,
    ...
  },
}

Example

Here is an example of two staircases used to modulate two difficulty parameters in a psychoacoustical task: the frequency difference between two sounds as in the first example and the volume of the sounds.

// create a variable containing 2 staircases that will be used in parallel
var stairs = new Staircase({
  deltaF: {
    firstVal: 100,
    down: 2,
    factor: 1.25,
    limits: [0, 600],
    sameStairMax: 4,
  },
  volume: {
    firstVal: 0.5,
    down: 2,
    factor: 1.25,
    limits: [0, 1],
    sameStairMax: 2,
  }
});

When the stairs variable is initialized, one staircase will be randomly chosen as the active one. A right answer will only increase the difficulty concerning the active staircase.

The sameStairMax property set the maximum number of consecutive trials with the same active staircase.

The method changeActive() is used to change the active staircase: the staircase will be deactivated and another one will be activated (again randomly chosen).

For this example, we will implement the two staircases following these rules:

Change the active staircase if:

• the answer is wrong
• there is two much consecutive right answers (according to the sameStairMax property)
• limit of allowed values is reached

Let's begin by initializing our new stairs object:

// Choose randomly the first active staircase
stairs.init();

To implement the above conditions we will use different properties of our stairs object:

if (stairs.sameStairCount>=stairs.sameStairMax ||
  !goodAnswer ||
  stairs.getActive().limitReached) {
  stairs.changeActive();
}

sameStairCount tracks the number of consecutive trials with the same staircase. It has to be higher than sameStairMax.

The method getActive() return the active staircase. We call the property limitReached of this staircase. It is true when the actual value reach one bound of the limits set in the parameters.

Then, changeActive() will change the active staircase if our conditions are true.

Now the procedure is the same as with one staircase: we call the next value of the updated active staircase:

// goodAns can be true or false
// if it is true, difficulty will be increased
stairs.next(goodAnswer);

Finally, we can use the values of our staircases:

sound.vol = stairs.getLast('volume');
sound.frequency.value = stair.getLast('deltaF');

Stairecase parameters:

These parameters can be used to instanciate Staircase():

• firstVal: the first value of the variable.
• down: how many good answers are required to match one bad answer. For instance, if down=2 the value will decrease (or increase according to the direction of the difficulty, see parameter direction) of an amount x after 2 good answers and will increase of the same amount x for one bad answer. This corresponds to a 1-up 2-down procedure.
• factor: this is the value of the multiplicator used to change the variable.
• direction: tells if a good answer is associated with a decrease (direction=-1) or an increase (direction=1) of the value. The default behaviour is a decrease of the value when a good answer is provided (like for frequency thresholds for example).
• limits: define the lower and upper values that can be used. When reached, the value will stay to the bound until there is an opposite answer.
• operation: can be multiply or add. Determine what operation to do with the factor. If operation: 'multiply', the increased value will be * by factor and the decrease value / by factor. If operation: 'add', the increased value will be + by factor and the decrease value - by factor.
• The sameStairMax option will tell the maximum number of consecutive trials with this staircase.
• lock can be true or false. When true, the staircase will not be active. This can be used to avoid a staircase for a moment.

Properties

name

val

active

limitReached

countSuccGood

sameStairCount

Methods

init()

This method will randomly choose the first staircase to use.

next(goodAns)

The next(goodAns) method takes a boolean as input and will create a new value in the list of values taken by the variable. This new value is computed according to the boolean: if true the answer was right and the difficulty has to be increased; if false the answer was wrong and the difficulty has to be decreased.

In addition, the variable sameStairCount is incremented each time value is changing on the same staircase.

changeActive()

Reset the sameStairCount to 0. Randomly choose another staircase to use (the last active staircase can't be choosen again).

getActive()

Return the active staircase object.

get(stair)

Return the whole array of values of the selected staircase (stair).

getLast(stair)

Return only the last value of the selected staircase.

activate(stair)

Set the active property of stair to true.

deactivate(stair)

Set the active property of stair to false.

isActive(stair)

Return the active property of stair.

setsameStairMax()

Change the number of maximum consecutive values in the same staircase (similar to the option sameStairMax used to instanciate the Staircase module).

lock(stair)

Set the lock property of stair to true.

unlock(stair)

Set the lock property of stair to false.

isLocked(stair)

Return the lock property of stair.

To do

• update doc with example for the lock methods
• fix x-up y-down for add operation....