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Franciscus Donders Contribution to cognitive psychology
Franciscus Donders (27th May, 1818 - 24th March, 1889) was one of the first people to attempt to measure and analyse the component processes in the brain, which is the cognitive process of a simple task, such as decision making. Donders was largely influenced by the work of Helmholtz who was interested in the nerve transmission time which he later used the subtraction method .Donders created the method of subtraction which is a given task which has a set of paradigms that take place sequentially to find the time taken for cognitive decision making processes put across three paradigms; Simple response time task,  go/no-go discrimination response task and choice response time task.

Reaction Time
Franciscus Donders  was interested in the different processes in the mind and he did this by analysing cognitive activity by separating it in to discrete stages , much of his experiments were performed in the early 1860s and were using reaction time tasks. These tasks involved participants making decisions with stimuli being presented and the response given he then would look in to the time taken for the participant to react to the stimuli presented. Donders was interested in 'timing the mind' he wanted to see how long it took to make a decision and created a subtraction technique to time the different mental processes.His concept of subtraction can be seen to be the basis of much of the functional brain imaging work seen today.

Paradigms for method of subtraction
The subtraction method can be seen to have three different process or paradigms, these processes  are the basis for the subtraction method and the speed of psychological process of which we make a cognitive decision. When making a cognitive decision we move through a sequence of three processes. A simple response, a discrimination response and a choice response task and Donders measured each of these with their own separate tasks see figure 1 table displaying the order in which the different paradigms would produce a reaction.

Paradigm A; Simple response time is to see the time taken to respond to a predetermined response, this would show the time taken to respond. An example of the task he used consisted of a single signal stimulus which the participant is expected to respond as quickly as they can this would be done by making a predetermined response, this could be anything such as clicking when a light is shown.

paradigm B; A go/no-go discrimination response Is to show the time taken to make a decision and react to the stimuli. Donders believed that presenting two or more stimuli and only make one stimuli have a response all other stimuli should not be responded to, this should eliminate motor choice time and only leave the time to discriminate between the different stimuli. An example of the task is there are two stimuli but only one of the stimuli has a response, for example there is a green light and red light and only clicking the buzzer at red would be correct.

Paradigm C; Choice response time this can show the dual choice and intermittency in human performance, donders stated that when there are two or three on more stimuli presented the participants must choose what response is needed for the stimulus so the choice reaction time will include both the time to discriminate to the stimulus from the different ones and the time to choose the reaction.He later stated that the difference between a Simple reaction and the a choice reaction is displaying any time to discriminate and Motor choice.An example of a task is consisted of two signal stimuli the participant would then make two different responses one for each stimuli, such as one click is for a square and two clicks for a circle.

The method of subtraction has been criticised by some researchers that if the method id accepted then the models which processing overlap other processing stages may be considers only valid because of the response selection and not because of the response actualisation. When the go /no-go method was later used it was argued by Wundt (1984) that there is a motor choice which has to be made, making or not making a response .Most of the experiments carried out in the laboratories by  Wundt(1984) was expanding on or repeating Donders previous work on reaction times. Figure 1. Displaying the Method of subtraction in table format for each paradigm.

Use in modern psychology
The model of subtraction has been extended from the temporal to the spatial domain. The logic of donders subtraction method has been seen to be fruitfully applied to isolate the localisation of functionally defined activations in the brain and this resulted in cognitive neuroimaging as we know. Sternberg's (1969) reinterpretation of the subtraction theory does differ from the subtraction theory as it does not involve deleting or inserting whole stages, it concentrated on manipulating the variables to affect the amount of time taken for each stage he called this method the addictive factors method .Gottsdanker and Shragg, (1985) carried out research for the verification of the subtraction method and found that the results supported Donders findings that the Choice and simple reactions only differentiate because of the selection and discrimination operations in the choice task .Donders experiment showed  that mental responses such as choosing which response goes with a certain stimuli cannot be measured directly and must be inferred from the behaviour. Although there have been many methods to measure reaction time most methods have used Donders theory of measuring the behaviour to determine a property of the mind. It has been stated that donders experiment and theory could illustrate the basic principles cognitive psychologists should use too see the way the mind works by the overt behaviour. The subtraction theory is still widely used neuroimaging as it has being found a successful way of interpreting functional neuroimages.