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Page Revision: 2012/03/18 07:53

Staircase element allows an easy implementation of the weighted up/down staircase method with the fixed predefined performance rate (Kaernbach, 1991).

Table of Contents [Hide/Show] Description Snapshots Practical Use     Technique for creating a single staircase Notes Properties     Generic Properties

Description ¶

Staircase element assists in implementing the single staircase method that efficiently estimates participant's detection threshold in regards to stimulus intensity (e.g. contrast or duration). The element continuously calculates adjustments for stimulus intensity on the basis of participant's consecutive responses. In addition, the element estimates the threshold of the given participant, collects various statistics and even generates the real-time staircase plot for the status screen. In general, element's staircase algorithm resembles the classical staircase procedure introduced by von Békésy (1947). More particularly, the current algorithm is a replica of the weighted up/down staircase method with the fixed performance rate (Kaernbach, 1991). The element implements a single staircase. If multiple staircases, interleaved or not, are required by an experimental paradigm, several Staircase elements can be used in parallel.

Snapshots ¶

Snapshot 1

Snapshot 2

Practical Use ¶

Technique for creating a single staircase ¶

Design time

1. Add Staircase element to any of experiment's event. The event choice is not important for elements working in the global scope
2. Set a value of Initial Intensity property of the element. The numerical value should correspond the initial level of stimulus intensity that can be easily detected. For example, the number can be related to the opacity of a picture, or to duration of an audio.
3. Set a value of Staircase Step property. The signed value denotes an adjustment of the stimulus intensity when the correct response is given by a participant. The adjustment on the wrong response is calculated by the element automatically as suggested by the weighted method. Smaller step value may lead to more accurate but slower staircase convergence.
4. Set a percentage value of Performance Level property. The staircase procedure will be automatically adapted to keep participant's performance on the given fixed level. Thus, the staircase results will always show participant's detection threshold on the given performance level, e.g for 80% of detection rate. The chance level corresponds to the value 50.
5. Set a value of Reversal Number property to define a total number of reversals necessary to complete the staircase procedure. Reaching the maximal reversal number does not stop the staircase procedure, the element simply signals this moment by setting true to its Is Completed property, which, in turn, can be used to control the experiment flow logic.
6. Set a value of Ignorable Reversal greater than zero, if it's necessary to exclude several initial reversals from the threshold estimation, e.g. to avoid occasional outliers before convergence of the staircase close to the real threshold level.
7. Create proxy variables for AddResponse and Next Intensity properties of the element. The first proxy will be used to add participant's responses to the staircase whereas the second for the adjusting stimulus intensity.
8. Create proxy variables for the selected properties of Runtime Results category. The proxies will allow to monitor staircase statistics on the Status Screen and save results to Report.
   Runtime
9. In order to add a single participant response to the staircase, the trial outcome has to be added via AddResponse proxy (usually, at the end of a trial):
   

    
   
   if (Response==1)
   
   AddResponse=true;
   
   else
   
   AddResponse=false;
   
   

   
   Integer values can be assigned to AddReponse proxy as well:
   

    
   
   AddResponse=Response; /// the Response here is an integer variable, which is equal 1 or 0 depending on the trial outcome
   
   

10. Adjust the intensity of the stimulus with Next Intensity value. A new value is calculated each time after adding a participant response (via AddResponse). At the start of the staircase Next Intensity is equal to Initial Intensity
    

     
    
    RendererOpacity=NextIntensity; /// adjusts the opacity of the visual stimulus
    
    

    
    or,
    

     
    
    EventDuration=Convert.ToUInt64(NextIntensity); /// adjusts duration of the stimulus presentation 
    
    

11. After completion of the current staircase, it can be reset and used again in a new sequence of trials. Use the following code:
    

     
    
    ResetStaircase=true; /// ResetStaircase is a proxy variable linked to element's property of the same name. 
    
    

12. The overall staircase progress (changes on the stimulus intensity over time) can be written to Report with the following code:
    

     
    
    Report="Stairscase of the stimulus intensity:\r\n";
    
    for (int i=0;i<StaircasePoints.Length;i++)
    
    Report=Report+StaircasePoints[i]+";"; /// where StaircasePoint is a proxy variable linked to element's property of the same name.
    
    

13. The graphical representation of the staircase progress can be monitored on the status screen with Staircase Plot property. The following code for Status Screen shows XAML binding to a proxy linked with the Staircase Plot property:
    

     
    
    <Image Source="{Binding StaircasePlot}"/>	
    
    

    
    

Notes ¶

Adding participant's response to the staircase and adjusting the stimulus intensity with the next value MUST NOT be done in the same code snippet (the element estimates a new intensity after completion of the snippet where a new response is added). Instead, use separate consequent snippets in the experiment flow for these two actions. The example of the incorrect code is below:

 
/// Incorrect use of the staircase !!!
AddResponse=Response;
EventDuration=Convert.ToUInt64(NextIntensity); /// Next Intensity is not updated yet 

Properties ¶

Generic Properties ¶

Name	Description	Attributes	Value Type	Proxy Reflex
Settings
Initial Intensity	Defines the initial value for the stimulus intensity (e.g. noise level, picture opacity, audio stimulus duration).	Normal..	Double	Assigning a value at runtime causes a complete reset of the staircase estimation
Staircase Step	Defines a size of the change in stimulus intensity for the correct response. A sign of the value defines the staircase direction relative to the initial value. The reverse step size for the wrong answers is calculated automatically based on the Staircase Step and the value of Performance Level property. Both step sizes are fixed for entire estimation procedure.	Normal..	Double
Inverse Step	ndicates a size of the change in stimulus intensity for the wrong response. The value is calculated automatically, based on Perfomance Level and Staircase properties.	Normal..	Double
Performance Level	Defines the fixed participant's performance level for the estimated threshold, in percentages. The staircase steps on correct and wrong responses will be different if this value is not equal change level-50%. The valid input range is between 0 and 100.	Normal..	Double
Reversal Number	Defines the total number of reversals, or staircase length, required to complete the threshold estimation	Normal..	Int32
Ignorable Reversals	Defines a number of the initial reversals that are excluded from the threshold estimation (but still be counted in the staircase length)	Normal..	Int32
Runtime Input
Add Response	Assigning a new value to this property in code snippets adds a single trial outcome to the staircase estimator. The value should be either boolean or integer: true/1 for the correct response and false/0 for the wrong response.	WriteOnly..	Int32 or Boolean	Assigned value gets added to the staircase estimator
Reset Staircase	Assigning "true" in snippets resets the current staircase and start a new estimation	Runtime Command..	Boolean	Resets the current staircase
Runtime Results
Next Intensity	Returns a value for the stimulus intensity in the next trial as suggested by the staircase estimator. The value is updated after adding responses to the staircase estimator.	Status..	Double
Correct Rate	Indicates the real correct rate calculated over all participant's responses, in percentages	Status..	Double
Is Completed	Indicates whether the estimation of the threshold is completed - the total number of reversal is reached	Status..	Boolean
Estimated Threshold	The current estimate of participant's threshold	Status..	Double
Reversal Count	Indicates the number of the observed reversals in the current staircase (including the ignorable reversals)	Status..	Int32
Response Count	Indicates the number of the collected responses in the current staircase	Status..	Int32
All Responses	Returns a boolean array of the collected responses in the current staircase	Status..	Boolean[]
Staircase Points	Returns a double array of all intensity values in the current staircase. The array values are time-ordered and the value with the zero index corresponds to the initial intensity.	Status..	Double[]
Staircase Plot	Contains a plot showing a real-time progress of the staircase estimation. For performance sake, the plot is created and gets updated only if the given property is binded in Status screen. If plot is created, it's final state can be accessed via Property Panel after experiment finishes. Then plot image can be copied to the clipboard.	Status..	System.Windows.Interop.InteropBitmap

+=== Properties inherited from clElement === Inherited properties of clElement

Name	Description	Constraints	Value Type	Upon Change
Control
Is Enabled	If set to false the element is completely omitted when the experiment is run.	Design Time..	Boolean
Title	Title of the element.	Design Time..	String