Re: Donders' Subtractive Method

From: Champion, Jim (jdc295@soton.ac.uk)
Date: Sun May 26 1996 - 17:23:25 BST


Describe Donders Subtractive Method for timing cognition

Cognition is a complex activity, and the best way to try and understand
it is to try and break it down into its constituent parts. Donders
subtractive method makes use of the fact that all mental operations
take time. Although the individual stages in cognition may not be
directly observable, they may be uncovered by the effects they have on
the brain (time of activity, and the location). This tool for finding
out more about cognition is best described using an example.

When considering how we mentally process visible English words, the
task can be divided up into four steps:

1) Reacting to the visual stimulus (registering the sticks and curves
   that are the shapes of the letters).

2) Recognising the letters of the English alphabet.

3) Recognising that the letters are arranged in a way that can be
   pronounced according to the rules of English.

4) Recalling a meaning for the word.

If subjects are presented with visual stimuli of four varieties (each
variety containing the features required for the four different stages)
then the brain activity can be measured using electrodes on the scalp.

Assuming the process to be a serial one (the stages are carried out one
after the other) then the time for the whole operation is the sum of
the times taken to perform each step. The time taken, for example, to
recognise the meaning of the word can be found by subtracting the time
to do task 3 from the time of task 4 (as shown in the diagram below)

1. * |------->| | | |
2. q g m |--------|------------->| | |
3. g a n |--------|--------------|--------------->| |
4. g u n |--------|--------------|----------------|------------->|
         | | | | |
        t=0 Recognition Pronounceable Meaning
                    of letters word

This is the essence of Donders subtractive method. It is not restricted
to the timing of cognition (finding out the "when?") - it can be used
in finding out more about "where" cognition takes place. The parts of
the brain involved in assigning meaning to a word (to use the above
example) can be seen by subtracting a brain acivity image of someone
performing task 3 from a task 4 scan. You can infer from this that what
remained after subtraction was the part of the brain involved in
assigning meaning to a word.



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