Participants
The participants for the experiment were family and friends of the researcher. None of them had any specific knowledge of psychology. They were recruited by asking if they would be happy to take part in a short psychological experiment. The age range of the participants was 24 – 64 years and there were five men and five women. This data was added to the provided data, so that the total participants came to twenty four.
Materials
A clock with a second hand was used to time how long it took each participant to complete the task. The lists of words for each condition consisted of thirty six words, presented in two columns of eighteen on a sheet of A4 paper. In each column, nine words were relevant to the condition (e.g. they were a colour or a number) and these words appeared in the same position in the column for both conditions. The non-relevant words were the names of animals and were the same in both conditions. Each word had a box next to it for the participant to place a mark in if the word was a colour (condition one) or a number (condition two). The researcher used a sheet of paper with a list of sums on it to read out as the participant was completing the word selection task – the same sheet was used in both conditions. Copies of these materials can be found in the appendix.
Procedure
Each participant was asked if they would be willing to be tested as part of an experiment. A coin toss determined which condition the participant was allocated to (until one of the conditions had enough participants, then the remaining condition was used). The age and sex of the participant was recorded. The participant was then asked to turn the piece of paper in front of them over when signalled, and to place a tick next to all the words that were the names of colours/numbers (depending on the condition) as quickly as possible. The participant was also told that they would be asked a series of mathematical sums at the same time as completing the word task. The exact instructions given to the participants in each condition can be found in the appendix. The test commenced when the second hand of the clock was at a suitable position. As the participant worked through the list ticking the relevant words, the mathematical sums were read out. The mathematical task continued until the word selection task was completed, then the time was noted again. This allowed the time taken for the task to be calculated and recorded. The participant was then told how long it had taken them to do the task, and asked whether they had any questions or comments.
Results
The experimental hypothesis being tested was:
Participants will take longer to complete a dual-task that requires responding to similar stimuli than a dual-task that requires responding to dissimilar stimuli. This was a one tailed hypothesis.
The data collected was the time (seconds) it took each participant to complete the task. The mean and standard deviation for each condition are shown below.
The mean time taken to complete the task when dissimilar stimuli are used (condition 1) is shorter than the time taken to complete the task when similar stimuli are used (condition 2).
The data was analysed using an independent-samples t-test – this showed that the difference between the mean time taken to complete the task when dissimilar stimuli were used and the mean time taken to complete the task when similar stimuli were used was statistically significant (t = -1.728, df = 22, p < 0.05). Note that the SPSS generated value for p was halved to relate to the one tailed hypothesis given above.
On the basis of these inferential statistics the null hypothesis was rejected.
Discussion
The results of the experiment show that it took longer for participants to complete the dual-task when similar stimuli were used to prompt a response than when dissimilar stimuli were used. This supports the prediction of the experimental hypothesis.
From the evidence gained from dual-task experiments carried out by Posner and Boies (1971) and McLeod (1977), it seems response times to stimuli were affected by the type of response required, (e.g. two manual responses at the same time, or one manual and one verbal response.) However, this experiment shows that the type of stimulus used to elicit a response is also important. When the stimuli were similar – selecting numbers while answering numerical questions – the response time was slower than when the stimuli were dissimilar. A possible explanation for this result is that there are separate pools of resources for processing different types of stimuli – as is believed to be the case for processing different types of response (Posner and Boies, 1971, McLeod, 1977). When a participant was asked to respond to similar stimuli, this can be thought of as drawing on one pool of resources for the two tasks. This has the effect of slowing down response time compared to that of a participant responding to two dissimilar stimuli using two separate resource pools. The fact that two similar stimuli are being processed at the same time could also be said to have an ‘off putting’ effect, since the two stimuli could become confused and cause the participant to become mixed up or ‘lose their place’ in the part of the dual-task they are currently attending to. This is less likely to occur when the stimuli are dissimilar.
Although the experiment provides evidence for the theories mentioned, it is important to look at confounding variables which could affect the results. In particular, the mental arithmetic task is a problem. The ability of the different participants to perform mental arithmetic will have varied widely. Participants who are poor at mental arithmetic will have had longer response times for this reason alone. It is therefore likely that some of the difference in response times between the two conditions is simply due to differences in mental arithmetic ability rather than similar stimuli competing for resources. This could be made less of a problem by using a within-participants design instead of between-participants, as it would minimize individual differences in mental arithmetic abilities. Alternatively, participants could be matched for their mathematical ability to try and reduce the effect of this variable.
Further experiments could use different types of stimuli for the similar/dissimilar conditions and see whether these results were replicated. It may prove easier to minimize confounding variables if the stimuli used do not rely on an ability that varies widely within the participants, as mental arithmetic ability does. The experimental design would also be improved if a much larger sample were used. However, despite its limitations this experiment has none the less produced data which is supportive of the theory that we have separate pools of limited resources for processing different types of stimuli.
References
Posner and Boies (1971, cited in Edgar, 2002)
McLeod (1977, cited in Edgar, 2002)
Edgar, G. (2002) ‘Perception and Attention’ in , in Miell, D., Phoenix, A. and Thomas, K. (eds) Mapping Psychology 2, Milton Keynes, The Open University, pp. 12-13
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Appendix
- Instructions given to participants
- Response sheet
- Extra data provided
- Condition 1 sheet used with participants
- Condition 2 sheet used with participants
- Auditory task sheet used in both conditions
- SPSS output for independent samples t-test
