Sometimes we are able to attend to more than one input at a time. This notion of divided attention led Kahneman (1973) to suggest that a limited amount of attention is allocated to tasks by a central processor. Many factors determine how much attentional capacity can be allocated and how much is needed for each task.
Kahneman provided a more flexible explanation of attention than the focused attention theorists did - we can attend to more than one thing at a time, particularly if we are skilled at a task. However, the capacity model fails to explain exactly how the allocation decisions are made.
Allport (1980) proposed that a number of limited-capacity processing modules exist. This notion can explain how we can easily divide our attention between dissimilar tasks (using different modules) but not between similar tasks (competing for resources from the same module).
This theory does not state how many modules there are, so any number could be made up to explain good performance in tasks requiring divided attention.
The more we practise, the less attention we have to allocate to a task. It may become automatic if we can carry out the task without having to think about it.
Schneider & Shiffrin (1977) make a distinction between controlled and automatic processing:
|Controlled processing||Automatic processing|
|limited capacity||no limited capacity|
|serial processing - requires focused attention||parallel processing - requires no attention|
|enables flexibile processing of novel situations||learned through practice and difficult to alter|
This distinction is supported by evidence that simple visual features, such as shape or colour, can be detected automatically but combined features, such as shape and colour, need directed attention. With practise, these controlled processes can become automatic and difficult to change.
It seems that automatic processes can interfere with tasks requiring controlled processing. An example is the Stroop effect. Try naming the colour these words are written in as fast as you can:
You probably found it hard to ignore the word's meaning - if you see blue ink but the word says "red", it is very difficult to choose between the colour and the meaning of the word. This is because the meaning is normally more important than the colour so we process that information automatically.
One criticism is that there is no explanation of how a controlled process becomes automatic. Does practice speed up the processing or change the way it is processed?
People often make mistakes when carrying out automatic processes. These action slips have been investigated by Reason (1992), who asked participants to keep a diary of the mistakes they made over two weeks.
The errors could be placed into five categories:
|Mixing up the objects used for different tasks||Mistaking a pencil for a pen||Discrimination failure|
|Forgetting what you have done and doing it again||Writing the same thing twice||Storage failure|
|Leaving out or muddling up the stages in a task||Sealing the envelope before putting the letter in||Subroutine failure|
|Forgetting what you intended to do and doing something else||Going to the post box and ending up at the shop||Test failure|
|Mixing up the stages of different tasks||Posting your pen and putting the letter in your pocket||Programme assembly failure|
Reason concluded that actions are controlled by:
- closed loop: slow processing involving conscious and deliberate attention for learning and switching tasks
- open loop: fast processing involving no attentional control for well practised tasks
Automatic tasks are not subject to conscious monitoring, therefore action slips occur in these tasks under open loop control.
However, the results collected from diary studies are unreliable because people may not be aware of all the slips they make and not write them down. Trying to create situations to result in action slips in the lab leads to problems of ecological validity.
Other researchers have explained action slips in terms of existing schemata (memory sets for processes, things and events) being used automatically when conscious control should have taken over (Norman, 1981).