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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.
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