Numerical Stroop task / Size Congruency Effect

Introduction

The numerical Stroop task is a great and popular task. Officially, it is called the Size Congruency Effect. The effect was first described by Besner and Coltheart (1979). Another nice early study on the effect was by Henik and Tzelgov (1982). There is a good review by Reike and Schwarz (2017).

Example 1. The Henik and Tzelgov (1982) paper summarizes the effect perfectly:

In this study, subjects were asked to judge which of two digits (e.g., 3 5) was larger either in physical or in numerical size. Reaction times were facilitated when the irrelevant dimension was congruent with the relevant dimension and were inhibited when the two were incongruent (size congruity effect). Although judgments based on physical size were faster, their speed was affected by the numerical distance between the members of the digit pair, indicating that numerical distance is automatically computed even when it is irrelevant to the comparative judgment being required by the task. This finding argues for parallel processing of physical and semantic information in this task.

In short, you are repeatedly shown a pair of numbers. One is physically larger than the other. You need to figure out which one is the physically larger of the pair. If the left one is larger, you press the A key (which is left on the keyboard) and if the right one is larger, you press the L key (which is right under the keyboard).

This can be confusing, because a number can be numerically smaller, yet physically larger. Look at the image below to see some examples.

The difference between how fast you respond in congruent and incongruent trials is the Size Congruency Effect.

Similar to the "real" Stroop task, you respond to stimuli that are sometimes containing "confusing information"

Here is a comparison of the two

	Numerical Stroop	Original Stroop
Stimuli	Numbers	Colour Words
Task	Look which number is physicall largers	Look what the ink color of a word is
Congruent	The smaller number is lower in value than higher value	The ink color and word meaning are the same
Incongruent	The smaller number is higher in value than smaller number	The ink color and word meaning do not match

Numerical Stroop

Original Stroop

Stimuli

Numbers

Colour Words

Task

Look which number is physicall largers

Look what the ink color of a word is

Congruent

The smaller number is lower in value than higher value

The ink color and word meaning are the same

Incongruent

The smaller number is higher in value than smaller number

The ink color and word meaning do not match

For an indepth analysis of response time distributions in this task, see the excellent Master thesis of Kristen Bowman (2020).

About this implementation

This version can be used with the mouse only. Participants need to click or tap the larger number on the screen. Note that in this PsyToolkit implementation, we draw two rectangles behind the numbers so that people do not need to click/tap exactly on the number, but can also tap close to it.

We do not use any images in this task. Everything is simply text. You can make the task more exciting by adding some images.

Also, in this task we do not use a "table" as we do in many other tasks.

Run the demo

Click here to run a demo of the Numerical Stroop task

Data output file

In PsyToolkit, the data output file is simply a textfile. The save line of the PsyToolkit experiment script determines what is being saved in the data output file. Typically, for each experimental trial, you would have exactly one line in your text file, and each number/word on that line gives you the information you need for your data analysis, such as the condition, response speed, and whether an error was made.

Meaning of the columns in the output datafile. You need this information for your data analysis.

Colum	Meaning
1	block number (1=training, 2=real data)
2	Condition name (congruent or incongruent)
3	Condition number (1=congruent or 2=incongruent)
4	Response time (milliseconds)
5	Status (1=correct, 2=wrong, 3=timeout)
6	The left number on the screen
7	The right number on the screen
8	The left font (2 = smaller font, 3 = larger font)
9	The right font (2 = smaller font, 3 = larger font)

PsyToolkit code

Click to expand the PsyToolkit script code (part of zip file below)

options
  background color EEEEEE

fonts
  instructions arial 30   # font 1
  small        arial 100  # font 2
  big          arial 120  # font 3

task sce
  text color black
  #------------------------ set up condition
  set $leftNumber random 1 9
  set &&a range 1 9
  set &&a remove position $leftNumber
  set $rightNumber &&a use random
  set $leftright random 1 2
  if $leftright = 1 ## left = small
    set $leftfont 2
    set $rightfont 3
    set $correctResponse 5 # this is the stimulus number on right side
  else ## left = big
    set $leftfont 3
    set $rightfont 2
    set $correctResponse 3 # this is the stimulus number on left side
  fi
  #----------------------- determine con/inc
  if $leftNumber < $rightNumber and $leftfont = 2
    set %conditionName "congruent"
    set $conditionNumber 1
  fi
  if $leftNumber < $rightNumber and $leftfont = 3
    set %conditionName "incongruent"
    set $conditionNumber 2
  fi
  if $leftNumber > $rightNumber and $leftfont = 2
    set %conditionName "incongruent"
    set $conditionNumber 2
  fi
  if $leftNumber > $rightNumber and $leftfont = 3
    set %conditionName "congruent"
    set $conditionNumber 1
  fi
  #--------------------------------- show stimuli
  show rectangle 0 0 100 10    black  # 1
  show rectangle 0 0  10 100   black  # 2
  delay 500
  clear -1 -2
  font $leftfont
  show rectangle -200 0 400 400 238 238 238 # equals background color # 3
  show text $leftNumber -200 0        # 4
  font $rightfont
  show rectangle  200 0 400 400 238 238 238 # equals background color # 5
  show text $rightNumber  200 0       # 6
  readmouse l $correctResponse 3000 range 3 6 
  clear range 3 6
  #--------------------------------- response feedback
  font small
  if STATUS = CORRECT
    show text "Well done"
    delay 500
  fi
  if STATUS = WRONG
    show text "Wrong"
    font instructions
    show text "Click or tap the physically larger number" 0 100
    delay 3000
  fi
  if STATUS = TIMEOUT
    show text "Too slow"
    delay 1000
  fi
  clear screen
  delay 500
  save BLOCKNUMBER %conditionName $conditionNumber RT STATUS $leftNumber $rightNumber $leftfont 

block training
  text "You see two numbers, one left and one right" 0 -200 black
  text "Look which number if physically largest" 0 -100  black
  text "Click or tap the physically larger number" 0 100 black
  text "Click or tap here to start" 0 200  black
  wait_for_key mouse
  task sce 10
  feedback
    set &congruentTrials mean c4   ; select c1 == 1 && c3 == 1 && c5 == 1
    set &incongruentTrials mean c4 ; select c1 == 1 && c3 == 2 && c5 == 1
    text color black
    text align left
    text -390 -200 &congruentTrials ; prefix "Congruent" ; postfix "ms"
    text -390 -100 &incongruentTrials ; prefix "Incongruent" ; postfix "ms"
    text -390  0   "Click or tap to continue for real block"
    wait_for_key mouse
  end

block real
  text align center
  text "The same task, but now for real" 0 -200 black
  text "Remember:" 0 -100  black
  text "Click or tap the physically larger number" 0 100 black
  text "Click or tap here to start" 0 200  black
  wait_for_key mouse
  task sce 40
  feedback
    set &congruentTrials mean c4   ; select c1 == 2 && c3 == 1 && c5 == 1
    set &incongruentTrials mean c4 ; select c1 == 2 && c3 == 2 && c5 == 1
    text color black
    text align left
    text -390 -200 &congruentTrials ; prefix "Congruent" ; postfix "ms"
    text -390 -100 &incongruentTrials ; prefix "Incongruent" ; postfix "ms"
    text -390  0   "Click/tap to continue"
    wait_for_key mouse
  end

Download

The PsyToolkit code zip file

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Numerical Stroop task / Size Congruency Effect

Introduction

About this implementation

Run the demo

Data output file

PsyToolkit code

Download

Further reading