Psychological Refractory Period Paradigm (PRP)

Introduction

The Psychological Refractory Period (PRP) is a fascinating and intensively studied phenomenon which goes at least back to the work by Welford (1952) and is strongly associated by the extensive body of work on this topic by famous psychologist Harold Pashler (1994).

The wikipedia link does a good job explaining the phenomenon.

In short, you do two tasks shortly after each other (task 1 and task 2). It turns out that the shorter the time interval between stimulus one and two (the SOA, see figure below, inspired by Pashler’s 1994 paper, p. 222, figure 1), the longer response time in task 2. One of the explanations for this is that the central processing of stimulus 2 has to wait (i.e., the refractory period) until the participant is done processing stimulus 1. The idea is that people can only handle the central processing of one task at the time. This is the central bottleneck.

The PRP paradigm has been used to distinguish between a single bottleneck model (e.g., McCann & Johnston, 1992) of information processing and a resource model. A good critical approach against the bottleneck model is the paper by Navon & Miller (2002).

All sort of smart methods have been used to prove that there is a single central bottleneck, although the debate continues until this day. One of the interesting manipulations is the difficulty level of task 1 (as is provided in the implementation here).

This topic is related to multitasking. As explained in the experiment library entry under multitasking, there are different ways to measure multitasking, and this is one of them. In this case, the multitasking really focuses on the simultaneous processing of information.

Some researchers have studied the effects of practice on PRP and whether or not the effect can be eliminated through practise Van Selst, 1999.

About this implementation

PRP paradigms come in all shapes and forms. Pashler’s 1994 paper gives a good overview of it. The implementation here is inspired by Pashler’s paper, but some of the parameters have been chosen by me.

The difficulty of task 1 is manipulated by having sounds that are easy to classify as low or high or difficult to classify as low and high.

In this example, you need speakers or headphone, because sound stimuli are used. Sounds have been created with the popular completely free-of-cost sound editor audacity.

Here is how you can generate your own tones using audacity. From the menu, select "generate". Then select "tone". You can select any waveform. The sounds selected for this experiment are probably not as nice as sine waves. Then you can play the sound, and if happy, from the "file" menu option, select "export audio". I selected the mp3 file format, because this creates very small audio files, which means that the experiment will start up faster.

Each square wave sound stimulus is 200 ms long, and the following frequencies have been used: 220, 440, 660, 880 Hz.

Make sure that sounds are audible on all computers. Some lower sine waves might not be audible on some hardware, because they are simply too low for the hardware. For example, on my laptop, I can hear a 220 Hz square wave, but not a 220 Hz sine wave.

Run the demo

There are 20 training trials and 100 real trials — hopefully you do not find this too long! The sound discrimination can be considered difficult in the beginning, because you need to be able to call 440 Hz as low and 660 Hz as high. Are you up to it? Anyway, that is the difficulty manipulation of this experiment!

Click here to run a demo

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	blocktype (1=training, 2=realdata)
2	s1: tone (lo or hi)
3	s2: color (yellow or blue)
4	task 1 difficulty (easy or difficult)
5	low/high tone as number (1=low, 2=high)
6	yellow/blue as number (1=yellow, 2=blue)
7	soa (75,150,300,600)
8	response time 1
9	response time 2
10	status of response 1 (1=correct, 2=wrong, 3=timeout)
11	status of response 2 (1=correct, 2=wrong, 3=timeout)

PsyToolkit code

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

options
  bitmapdir stimuli
  sounddir stimuli

sounds
  lo1 square220hz_200ms.mp3
  lo2 square440hz_200ms.mp3
  hi1 square660hz_200ms.mp3
  hi2 square880hz_200ms.mp3

bitmaps
  instruction1
  instruction2
  instruction3
  instruction4  
  readyreal
  readypractise
  fixpoint1
  fixpoint2  
  errorfeedback
  listen
  thankyou

fonts
  courier 18
  
table prptable
  lo1  1 3 1 "lo yellow easy"
  lo2  1 3 2 "lo yellow diff"
  hi1  2 3 2 "hi yellow diff"
  hi2  2 3 1 "hi yellow easy"
  lo1  1 4 1 "lo blue   easy"
  lo2  1 4 2 "lo blue   diff"
  hi1  2 4 2 "hi blue   diff"
  hi2  2 4 1 "hi blue   easy"

task prptask
  table prptable
  keys z x comma period
  ## first set some more variables to values
  set $prpsoa random from 75 150 300 600
  ## create yellow rectangle colors  
  if @3 == 3
    set $redchannel   255
    set $greenchannel 255
    set $bluechannel  0
  fi
  ## create blue rectangle colors
  if @3 == 4
    set $redchannel   100
    set $greenchannel 100
    set $bluechannel  255
  fi
  ############################################
  ## start events, start with nice fixpoint
  delay 500
  show bitmap fixpoint1
  delay 100
  show bitmap fixpoint2
  delay 100
  clear 1 2
  show bitmap fixpoint1
  delay 100
  clear -1
  ############################################
  ## now the task relevant events
  sound @1
  readkey @2 $prpsoa
  set $rt1 RT
  set $status1 STATUS
  show rectangle 0 0 200 100 $redchannel $greenchannel $bluechannel
  ## the following if is true if people responsed quicker than the PRP SOA
  if $status1 != TIMEOUT
    set $realrt1 RT
    readkey @3 3000
    set $realrt2 RT
    set $status2 STATUS
  fi
  if $status1 == TIMEOUT
    readkey @2 3000
    set $rt1 RT
    set $status1 STATUS
    set $realrt1 expression $prpsoa + $rt1    
    readkey @3 3000
    set $realrt2 RT
    set $status2 STATUS
  fi
  ############################################
  ## check if values were correct
  if $status1 != CORRECT || $status2 != CORRECT
    show bitmap errorfeedback
    delay 3000
    clear -1
  fi
  save &blocktype @5 @2 @3 $prpsoa $realrt1 $realrt2 $status1 $status2

block prp_training
  set &blocktype 1 # training
  pager instruction1 instruction2 instruction3 instruction4
  message listen
  sound lo1
  delay 300
  sound lo2
  delay 600  
  sound hi1
  delay 300
  sound hi2
  delay 600
  sound lo1
  delay 300  
  sound lo2
  delay 600  
  sound hi1
  delay 300    
  sound hi2
  delay 600
  message readypractise
  tasklist
    prptask 20
  end

block prp_real
  set &blocktype 2 # real
  message readyreal
  tasklist
    prptask 100
  end
  feedback
    text align left
    set &soa1_rt1 mean c8  ; select c7 == 75  && c1 == 2 && c9 == 1 && c10 == 1
    set &soa2_rt1 mean c8  ; select c7 == 150 && c1 == 2 && c9 == 1 && c10 == 1
    set &soa3_rt1 mean c8  ; select c7 == 300 && c1 == 2 && c9 == 1 && c10 == 1
    set &soa4_rt1 mean c8  ; select c7 == 600 && c1 == 2 && c9 == 1 && c10 == 1
    set &soa1_rt2 mean c12 ; select c7 == 75  && c1 == 2 && c9 == 1 && c10 == 1
    set &soa2_rt2 mean c12 ; select c7 == 150 && c1 == 2 && c9 == 1 && c10 == 1
    set &soa3_rt2 mean c12 ; select c7 == 300 && c1 == 2 && c9 == 1 && c10 == 1
    set &soa4_rt2 mean c12 ; select c7 == 600 && c1 == 2 && c9 == 1 && c10 == 1
    text -350 -200 &soa1_rt1 ; prefix "RT1 SOA=50  " ; postfix " ms"
    text -350 -150 &soa2_rt1 ; prefix "RT1 SOA=200 " ; postfix " ms"    
    text -350 -100 &soa3_rt1 ; prefix "RT1 SOA=400 " ; postfix " ms"
    text -350 -50 &soa4_rt1  ; prefix "RT1 SOA=800 " ; postfix " ms"
    text -350  0 &soa1_rt2   ; prefix "RT2 SOA=50  " ; postfix " ms"
    text -350  50 &soa2_rt2  ; prefix "RT2 SOA=200 " ; postfix " ms"    
    text -350  100 &soa3_rt2 ; prefix "RT2 SOA=400 " ; postfix " ms"
    text -350  150 &soa4_rt2 ; prefix "RT2 SOA=800 " ; postfix " ms"
    text -350  200 "Write down numbers. Press space bar to continue."
  end
  message thankyou

Download

If you have a PsyToolkit account, you can upload the zipfile directly to your PsyToolkit account. Watch a video on how to do that. If you want to upload the zipfile into your PsyToolkit account, make sure the file is not automatically uncompressed (some browsers, especially Mac Safari, by default uncompress zip files). Read here how to easily deal with this.

Psychological Refractory Period Paradigm (PRP)

Introduction

About this implementation

Run the demo

Data output file

PsyToolkit code

Download

Further reading