The effect of 'interference' was explored during a modified Stroop experiment. This paper reports a study of the response times taken to complete a variation of the Stroop test; to determine whether automatic cognitive processes associated with reading interfere with controlled processes associated with colour naming tasks. It was found that naming ink colour was slower for a list of colour-related words than when neutral words were used, showing that interference does occur during an interaction of automatic and controlled cognitive processes. These findings are concurrent with previous empirical research which has shown that performance decreases when completing more than one task at a time.
The senses receive a phenomenal amount of information from stimuli but it is understood that the brain has a limited capacity of resources available to process this information (Kahneman, cited by Edgar 2007). Attention is the fundamental cognitive process that mediates between what is sensed and perceived, this incorporates the selection and processing of some information and the disregard of. An important concept to consider is that by understanding cognitive processes such as attention and perception, inferences can be made about how behaviour may be affected (Edgar 2007).
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Within the attentional information processing framework, two distinct modes of cognitive functioning exist; controlled processing and automatic processing (Gross 2005, Edgar 2007). Schneider and Shiffrin (cited by Edgar 2007) distinguished several characteristics between controlled and automatic processing. Controlled processing demands heavily on attentional resources, is capacity-limited, slower and involves conscious directing of attention towards a task. In contrast, automatic processing makes little or no demands on attentional resources, is relatively fast, unaffected by capacity limitation and is not subject to conscious awareness (Gross 2005, Edgar 2007).
The ability to perform tasks automatically is advantageous as it means there are still resources available for the processing of other tasks. Everyday experiences and laboratory experiments indicate that it is possible to perform two or more tasks simultaneously and efficiently (Edgar 2007), e.g. holding a conversation while cooking or driving. However Kahneman highlighted disadvantages, claiming that interference can occur when attempting more than one task at a time; each task is thought to compete for resources from a central processer (cited by Edgar 2007). For example it is not possible to read and hold a conversation at the same time. Posner and Boies also observed that despite simplification, performance may decrease when people perform more than one task at a time (cited by Edgar 2007).
To explain, when a task such as reading is well learned and practised, it becomes automatic. The automaticity of reading is so powerful that it is difficult to avoid or change and this can interfere with ability to respond to other tasks such as naming colours (Edgar 2007). This is clearly demonstrated by the Stroop effect, a classic and robust phenomenon associated with cognition and reaction time (cited by Edgar 2007). In 1935 Stroop revealed that if a colour word was written in a conflicting colour (such as 'blue' being written in red) participants would find it difficult to name the colour the words were written in (Gross 2005, Edgar 2007).
The above research into automatic and controlled processing provides the motivation for conducting this experiment. The aim is to test whether interference will intrude on the cognitive processing during a variation of the Stroop Test. The words used in this experiment are manipulated to include a colour-related list (instead of actual colour words used by Stroop) and a neutral list. The research hypothesis is that it will take longer for participants' to name the ink colour of the list of colour-related words than to name the ink colour of neutral words. This is a one-tailed hypothesis. The null hypothesis is that there will be no difference in the time that it takes for participants' to name the ink colour for the lists of colour-related and neutral words.
The experiment was conducted using a within-participants design. The independent variable comprised 2 conditions, a list of colour-related words (the experimental condition) and a list of neutral words (the control condition). The words were printed in different coloured ink, and each participant was required to name the ink colour of each word in both conditions. The dependant variable was the time taken to name the ink colour of the words in each condition. Response times for each condition were measured by the researcher using a stopwatch, and recorded to the nearest second. All participants were read the same standard instructions, and the conditions used were identical except for the manipulated variable; the actual words printed (colour-related/neutral words). To counterbalance for order effects the participants were given a number from 1 to 20, the odd numbered participants completed condition 1 followed by condition 2, and the even numbered participants completed condition 2 and then 1.
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The sample included 20 participants; 16 were recruited by staff at The Open University and were colleagues, friends or family members. The remaining 4 were opportunistically recruited by the researcher; they were family members who volunteered. There were 10 males and 10 females ranging from 30 to 60 years old. All participants were naive to the hypothesis of the experiment but were briefed beforehand, signed a consent form, and were debriefed afterwards. Each participant had normal or corrected to normal vision, ability to distinguish colours, and ability to read and speak English.
The stimuli presented in each condition consisted of a list of 30 words, printed in two columns on a sheet of A4 paper. The experimental condition consisted of the following colour-related words; BLOOD, LEMON, GRASS, CARROT, PLUM and SKY, printed in a colour incongruent with the word (e.g. BLOOD was not printed in red ink, SKY was not printed in blue). The control condition consisted of the following neutral words; BLAME, LEDGE, GRADE, CAREER, PLAN and STY. Each word was printed five times in a random order; both conditions matched for corresponding words, and were printed in same ink colours, size, font and order (see Appendix 1 for both stimuli). Standard written instructions were used (see Appendix 2), and a consent form was completed by each participant (see Appendix 3). A digital stopwatch was used to measure response times, and all data collected were recorded on a response/data sheet (Appendix 4).
Each participant was approached and asked to take part in an experiment, on the understanding that it was associated with naming ink colour of lists of words, as part of an investigation into cognitive processing, and to provide data for the researcher's assignment. The participants were asked if they had any knowledge of the Stroop effect to determine naivety of the hypothesis. Once it was ascertained that those who agreed to participate were suitable for the experiment (with normal or corrected to normal vision, and ability to read and speak English, and distinguish colours), their written consent was obtained. Participants were tested individually; demographic data was recorded including age and sex, before the standard instructions in Appendix 2 were read out verbatim. These told the participant that they would be presented with a list of words, in two columns, that they should work through saying aloud the colour of ink each word is printed with. They were told to complete the task as quickly as possible. An example was given and once each individual confirmed that they understood what they had to do, the first stimuli was placed face down on a desk. When the participant was ready it was turned over and the experimenter started the stopwatch. On completion of the last word the stopwatch was stopped and the time it took to complete the task was recorded to the nearest second on the response sheet. The second stimuli was then placed face down immediately and the procedure was repeated. The odd numbered participants completed Condition 1 then 2 and the even numbered participants completed Condition 2 then 1. After the response time was recorded for the second task, the participant was debriefed. They were informed about the nature of the Stroop effect and the differences between the two conditions. They were also given the opportunity to ask any questions, their anonymity was reiterated and they were thanked for their contribution.
The research hypothesis was that it will take longer for participants' to name the ink colour of the words on the colour-related list than to name the ink colour of the list of neutral words. The time it took for each participant to complete the task was measured in seconds and recorded on a data sheet (see Appendix 4). Table 1 below shows the differences between mean response times and standard deviation for both conditions. The colour-related words condition took 2.9 seconds longer, on average, to complete than the neutral words condition. The standard deviation indicates that there is a greater dispersion of response times (in relation to the mean) in condition 1 than in condition 2. Further SPSS Descriptive Statistics output can be seen in Appendix 5.
Table 1. Mean Response Time (in seconds) For Each Condition
Mean Response Time
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Inferential statistical analysis was conducted on the data, including a paired samples t-test (t (19) = 3.276; p = .002; d = 0.553), which showed that the differences between conditions were statistically significant and not due to sampling error (see Appendix 6 for SPSS inferential analysis output). As the effect size shows a difference of 0.5 standard deviation points, it was considered to be a medium sized effect (Cohen cited by DSE212 Course Team 2007, p172). Based on this analysis, the hypothesis was accepted and the null hypothesis was rejected.
The results of this experiment showed that there was a significant increase in the time taken to name the ink colour of the list of colour-related words compared with the list of neutral words. This was consistent with the Stroop effect and supports the experimental hypothesis of the study. Intrusion that resulted from interference is consistent with Kahneman's model, suggesting that if tasks compete for a single pool of resources interference occurs. It is also consistent with Schneider and Shiffrins descriptions of automatic processes and controlled processes (cited by Edgar 2007).
As reading the colour-related words is automatic and unavoidable, controlled processing is required to remove the automatic colour-related word response and replace it with the correct verbal ink colour response. This takes extra processing time and explains the greater response times measured for the colour-related task. Automatic processing is relatively quick, and requires less mental effort than controlled processes that are comparatively slower and require greater conscious effort. This explains why reading is processed before the colour naming response is processed and verbalised.
It was observed that the even-numbered participants who completed condition 2 first, had a very small mean difference in response times between both conditions compared with those who completed condition 1 first (this is graphically presented in Appendix 7). This shows that counterbalancing for order effects was an effective design. It would be interesting to explore this effect in a repeated but larger study of the same design; it is possible the participants completing the neutral word list first had developed a strategy for naming ink colours during the second condition, such as focusing on the first letter of a word, rather than reading the colour-related word automatically. This could be explored further in future.
There were several limitations to the methodology of the experiment. Although the number of participants used for this study was effective in showing the predicted Stroop effect (for the population tested), the sample size was small and so there cannot be widespread comparison or generalisations. Also, the experiment itself was an unnatural situation and people are unlikely to find themselves in real life situations where they have to read out lists of words or colours. The experiment therefore also lacks ecological validity.
Two of the researcher's participants made mistakes by naming the wrong ink colour, of which they were unaware. Data was not collected during the experiment in relation to numbers of errors that occurred, but this could be considered in future, noting whether errors are corrected or not. A failure of conscious or controlled attention to notice errors has potential consequences in real life situations.
The automaticity of reading has been a focal point of this experiment, but an alternative study could incorporate the use of printed numbers, instead of words to see if similar effects of interference occur with automaticity of recognising numbers as opposed to reading words. Also, while this experiment clearly demonstrates the effects of interference that Kahneman described, it offers no explanation for how tasks performed simultaneously can be done so effectively - this is another area that could be explored in future research through dual-task experiments.