16 Interference tasks and cognitive control
Next, we are going to have a look at slightly more complicated experimental paradigms that have been used to study interference and cognitive control, the Stroop task and the flanker task. In interference tasks, there are at least two sources of information or dimensions presented to the participant; one of these is task-relevant and the other task-irrelevant.
16.1 Stroop task
The Stroop task (Stroop, 1935) is likely the most well-known interference task. In the interference condition, participants are presented with colour words printed in a colour that does not correspond to the meaning of the colour word and are instructed to indicate the colour of the word (and to ignore its meaning). Thus, the colour of the word is the relevant dimension, and the meaning of the word the irrelevant dimension (see Figure 16.1).
In a trial1, the two sources of information can be conflicting, in agreement, or they are unrelated. When they conflict, the trial is incongruent or incompatible. When they are in agreement, the trial is congruent or compatible. And when they are unrelated, the trial is neutral. Figure 16.2 illustrates these three trial types for the Stroop task.
When a Stroop trial is incongruent, the presence of both dimensions creates cognitive conflict that has to be resolved to give a correct response. Cognitive psychologists assume that we need to exert cognitive control (also referred to as executive control, top-down control or attentional control) to resolve this conflict. Resolving this conflict takes time and effort. Therefore, incongruent mean RTs should be slower than congruent mean RTs and neutral mean RTs. Sometimes, but not as consistently, facilitation is observed (i.e., congruent mean RTs can be faster than neutral mean RTs).
Theoretical explanations of the Stroop effect typically stress that we are highly practised readers. According to one influential model (Cohen et al., 1990), this implies that our “word reading pathways” are therefore stronger than our “colour naming pathways”.2 The function of cognitive control mechanisms is to boost the colour naming pathway (and/or, depending on the researcher’s predilections, to inhibit the word reading pathway).
Try the Stroop task
In this task, there will be 36 practice trials during which you will receive accuracy feedback, followed by another 36 experimental trials. Note that the neutral trials in this version involve words that are not usually associated with a particular colour. After completing the task, you will get feedback on your mean response times for congruent, neutral and incongruent trials. You will also have the option to download your own data.
16.2 Interference effects
Interference effects are a measure of the time and effort it takes to resolve the interference. They can be expressed as a slowing down in response time. For example, we might say that participants were on average 200 ms slower in incongruent Stroop trials as compared to congruent Stroop trials. On some trials, the interference resolution mechanisms might fail and the participant gives an incorrect response. Thus, an interference effect can also result in increased error rates. For example, we might say that the average difference in error rates in incongruent and congruent Stroop trials was 15%.
Interference effects are relevant for basic cognitive psychology as they reveal how well we are able to focus on relevant information and ignore irrelevant information. They are also relevant in applied contexts, for example when designing user interfaces for appliances, machines or software. In addition, there is evidence that cognitive control is affected in developmental disorders (e.g. ADHD: Mullane et al., 2009; Onandia-Hinchado et al., 2021) and psychiatric conditions (e.g. schizophrenia: Lesh et al., 2011; Westerhausen et al., 2011).
16.3 Flanker task
Another well-known interference task is the flanker task. The classic demonstration of the flanker interference effect was reported by B. A. Eriksen & Eriksen (1974). In the visual flanker task (there is also an auditory version), a target is presented in the middle of screen. On both sides of the target are the flankers. The target is relevant, the flankers are irrelevant. One popular version of the flanker task uses letters. In the letter flanker task, you might receive the following set of instructions:
Based on these instructions, we can again create incongruent, congruent and neutral trials (see Figure 16.4).
The flanker effect refers to a slowing of RTs in incongruent relative to neutral and congruent trials and/or a decrease in accuracy in incongruent relative to neutral or congruent trials.
Note the following key characteristics of this task:
- The incongruent condition is incongruent because the two letters are mapped onto different responses. The fact that the target and flanking letters are different is in itself not sufficient (otherwise there could not be a neutral condition…). The key is that they are associated with different responses in the context of the task.
- The congruent condition is congruent because the two letters are mapped onto the same response. The fact that the target and flanking letters are identical will speed up the response, but is not a requirement for a congruent trial (e.g., you could map two letters onto the same response, present these two letters as target and flankers, respectively, and it would still be a congruent trial).
- The neutral condition is neutral because the flanking letters are not associated with a response in the context of the task.
Theoretical explanations of the flanker effect assume that we are not able to consistently focus our attention solely on the target and that we therefore also tend to process the flankers (C. W. Eriksen & St James, 1986; White et al., 2011). Cognitive control mechanisms are involved in focusing our attention on the target.
References
Cohen, J. D., Dunbar, K., & McClelland, J. L. (1990). On the control of automatic processes: A parallel distributed processing account of the Stroop effect. Psychological Review, 97(3), 332–361. https://doi.org/10.1037/0033-295X.97.3.332
Eriksen, B. A., & Eriksen, C. W. (1974). Effects of noise letters upon the identification of a target letter in a nonsearch task. Perception & Psychophysics, 16(1), 143–149. https://doi.org/10.3758/BF03203267
Eriksen, C. W., & St James, J. D. (1986). Visual attention within and around the field of focal attention: A zoom lens model. Perception & Psychophysics, 40(4), 225–240. https://doi.org/10.3758/BF03211502
Lesh, T. A., Niendam, T. A., Minzenberg, M. J., & Carter, C. S. (2011). Cognitive control deficits in schizophrenia: Mechanisms and meaning. Neuropsychopharmacology, 36(1), 316–338. https://doi.org/10.1038/npp.2010.156
Mullane, J. C., Corkum, P. V., Klein, R. M., & McLaughlin, E. (2009). Interference control in children with and without ADHD: A systematic review of flanker and Simon task performance. Child Neuropsychology, 15(4), 321–342. https://doi.org/10.1080/09297040802348028
Onandia-Hinchado, I., Pardo-Palenzuela, N., & Diaz-Orueta, U. (2021). Cognitive characterization of adult attention deficit hyperactivity disorder by domains: A systematic review. Journal of Neural Transmission, 128(7), 893–937. https://doi.org/10.1007/s00702-021-02302-6
Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology, 18(6), 643–662. https://doi.org/10.1037/h0054651
Westerhausen, R., Kompus, K., & Hugdahl, K. (2011). Impaired cognitive inhibition in schizophrenia: A meta-analysis of the Stroop interference effect. Schizophrenia Research, 133(1), 172–181. https://doi.org/10.1016/j.schres.2011.08.025
White, C. N., Ratcliff, R., & Starns, J. J. (2011). Diffusion models of the flanker task: Discrete versus gradual attentional selection. Cognitive Psychology, 63(4), 210–238. https://doi.org/10.1016/j.cogpsych.2011.08.001
Think of a trial as a stimulus-response cycle in an experiment. In the Stroop task, a trial begins when a colour word is presented. After a short time, the participant responds (e.g., by pressing a key on a keyboard). After the response, there might be a short break of a few 100 ms before the next stimulus comes up. This completes the trial. Overall, a trial in the Stroop task might take about 1 to 1.5 s. Clearly, this is very different from the meaning of the word “trial” in the context of a clinical study where “trial” refers to the complete study. Make sure to distinguish these two meanings! Also, be aware that “trial” might be auto-corrected to “trail” by some pieces of software. If this does happen, you might want to tell your software that it should not auto-correct “trial”.↩︎
“Pathways” should not be understood to imply neuronal pathways specific to word reading and colour naming. Instead, both of these functions will involve a network of brain areas and the neuronal tracts connecting them.↩︎