Introduction

David Navon’s paper about the speed with which people process global and local information is extremely popular (Navon, 1977). The paper has been cited nearly 1,800 times (checked in the beginning of 2016).

The basic idea of Navon’s study is that when objects are arranged in groups, there are global features and local features. For example, a group of trees has local features (the individual trees) and the feature of a forest (the trees together).

The basic finding of Navon’s work is that people are faster in identifying features at the global than at the local level. This effect is also known as global precedence.

An example of a Navon figure is shown below. The figure has a global feature, it looks like an H. Its local feature are the many small letters X the figure is made of. People are typically quicker detecting an H than an X.

navon Hx

The global precedence effect is not just observed in this specific setup. For example, generally words are recognized quicker than its individual letters. Can you think of other examples?

About this implementation

This specific implementation is designed by PsyToolkit developer Gijsbert Stoet. There are 50 trials. On each trial, you get up to 4 seconds to decide whether you see a target letter (H or O) at the local or global level, or not. On each trial, you need to respond with a key press. There are clear instructions at the start on how to exactly respond, and there is feedback at the end. According to the theory, you should respond fastest (and made fewest mistakes) in the global condition.

The distance between the screen and your eyes might influence how easy the global features can be detected. For example, if you sit very close to the screen, the local features will be more obvious that the global features.

Run the demo

In this example, you will see Navon types of letters. You will need to decide if a letter is or contains the letters H or O. If you an H or O, you press the b button. If you do not see an H or O, you press the n button (think n stands for no).

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

The letter. For example, Hs stands for a big H made of small letters S

2

The level where the target is, or none if there is no target letter at all

3

Same as column 2, but in numbers. 0: no target. 1: target at local level. 2: target at global level

4

Status. 1: correct, 2: wrong response, 3: too slow

5

Reaction time in milliseconds

PsyToolkit code

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

bitmaps
  Lu
  Uo
  Uu
  Ul
  Ll
  Lo
  Ou
  Ol
  Tt
  Ts
  Th
  St
  Ss
  Sh
  Ht
  Hs
  correct
  wrong
  tooslow
  navon_title
  instruction1
  instruction2
  instruction3  

fonts
  arial 18

table navon
  "Lu none 0"    Lu 2
  "Uo local 1"   Uo 1
  "Uu none 0"    Uu 2
  "Ul none 0"    Ul 2
  "Ll none 0"    Ll 2
  "Lo local 1"   Lo 1
  "Ou global 2"  Ou 1
  "Ol global 2"  Ol 1
  "Tt none 0"    Tt 2
  "Ts none 0"    Ts 2
  "Th local 1"   Th 1
  "St none 0"    St 2
  "Ss none 0"    Ss 2
  "Sh local 1"   Sh 1
  "Ht global 2"  Ht 1
  "Hs global 2"  Hs 1

task navon
  table navon
  keys b n
  show bitmap @2
  readkey @3 4000
  if STATUS == CORRECT
    show bitmap correct
  fi
  if STATUS == WRONG
    show bitmap wrong
  fi
  if STATUS == TIMEOUT
    show bitmap tooslow
  fi
  delay 1000
  clear 1 2
  delay 500
  save @1 STATUS RT

block myblock
  message navon_title
  message instruction1
  message instruction2
  message instruction3  
  delay 500
  tasklist
    navon 50 all_before_repeat
  end
  feedback
    text color yellow
    text align left
    set &GlobalRT mean c5 ; select c4 == 1 && c3 == 2
    set &LocalRT  mean c5 ; select c4 == 1 && c3 == 1
    set &NoneRT   mean c5 ; select c4 == 1 && c3 == 0
    set &GlobalPE count ; select c4 != 1 && c3 == 2
    set &LocalPE  count ; select c4 != 1 && c3 == 1
    set &NonePE   count ; select c4 != 1 && c3 == 0
    text -200 -200 "Feedback Navon task"
    text -200 -150 "Reaction Times per experimental condition:"
    text -200 -100 &GlobalRT ; prefix "Global level (H or O): " ; postfix " ms"
    text -200 -50  &LocalRT ; prefix "Local level (h or o):" ; postfix " ms"
    text -200 0    &NoneRT ; prefix "No target at all:" ; postfix " ms"
    text -200 50   "Error count per experimental condition:"
    text -200 100  &GlobalPE ; prefix "Global level (H or O): " ; postfix " errors"
    text -200 150  &LocalPE ; prefix "Local level (h or o):" ; postfix " errors"
    text -200 200  &NonePE ; prefix "No target at all:" ; postfix " errors"
    text -200 250  "press space bar to continue"
  end

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.

Further reading

  • Navon, D. (1977). Forest before trees - Precedence of global features in visual-perception. Cognitive Psychology, 9, 353-383.

  • Navon, D. (2003). What does a compound letter tell the psychologist’s mind? Acta Psychologica, 114, 273-309.