Donders' Subtractive Method

From: Dale, Becky (
Date: Fri May 24 1996 - 11:16:12 BST

Describe Donders Subtractive Method.

Donder's work attempts to describe the processes going on in the
mind, by analyzing cognitive activity into separate stages. Until
Donder's work, many scientists had assumed that the mental operations
involved in responding to a stimulus occurred instantaneously.
Donder was particularly interested in "timing the mind" and used a
subtraction technique to time the different mental processes that the
brain goes through when faced with different tasks.

Donders performed experiments using reaction time tasks in 1868. His
was the first attempt to analyze and measure the component processes
of a simple task. He used three tasks :

1. a simple reaction time task - e.g. you are seated in front of a
  panel that contains a light bulb and a response button. When
  the light comes on, you must press the button.

2. a discrimination reaction time task - e.g. you are seated in front
  of a panel with 5 light bulbs and one reponse button. When the
  target light goes on you must press the button - but not if the 4
  other lights come on.

3. a choice reaction time task - e.g. you are seated in front of 5
  light bulbs, each with their own button. You must press the
  button corresponding to the appropriate light.

Donder's then predicted the kinds of processes that might be involved
in each task :

1. A simple time task would require perception and motor stages -
  time to receive and then execute the stimulus.

2. A discrimination reaction time task requires the above plus a
  discrimination stage.

3. A choice reaction time task requires all of the above -
  time to receive and execute the stimulus, and discriminate plus
  a choice stage.

As expected, simple tasks take the shortest amount of time, followed
by discrimination tasks, with choice tasks taking the longest amount
of time. Donders calculated the time required for each stage by
using a subtraction technique :

perception and motor time - time required for simple task
discrimination time - time for discrimination task minus simple task
choice time - time for choice task minus discrimination time.

This demonstrated a simple conclusion - more stages should require
more time.

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