The work I did for my Master thesis was just accepted in the Frontiers Journal (Frontiers in Human Neursocience).
I originally designed this experiment as a follow-up to a previous study I'd analyzed in the lab, which examined the network of brain cells involved in the Stroop task [When the name of a colour (e.g., "blue," "green," or "red") is printed in a colour not denoted by the name (e.g., the word "red" printed in blue ink instead of red ink), naming that colour is more difficult hence taking longer and more prone to errors].
The prefrontal cortex (PFC) has been implicated in higher order cognitive control of behaviour. Sometimes such control is executed through suppression of an unwanted response in order to avoid conflict which would otherwise occur as in situations when two simultaneously competing processes lead to different behavioural outcomes.
The published study used emotional words and emotional faces to compare the two well-learned automatic behaviours of word reading and recognition of facial expressions. So the STROOP EFFECT in this study is defined as the conflict produced by the inconsistency between the facial expression and the superimposed word referring to that expression (i.e. Happy face superimposed by the word SAD).
In this emotional Stroop paradigm, words were processed faster than face expressions with incongruent trials (faces with expressions different than the written emotional word across them) yielding longer reaction times (RT, the time it took for them to respond) and larger number of errors compared to the congruent trials (same face expression and superimposed emotional word). This is in fact counter-intuitive as we would expect reporting facial expressions to be faster since it is an instinctive behaviour. In contrast, word reading which is an over-learned behaviour is processed faster in our brains suggesting the formation of stronger neural associations.
Using fMRI (functional Magnetic Resonance Imaging), we were able to identify the brain regions involved in the inhibition of the two distinct automatic behaviours. The Stroop effect activated the anterior and inferior regions of the mPFC, namely the anterior cingulate cortex (ACC), inferior frontal gyrus (IFG) as well as the superior frontal gyrus. The left IFG showed particular sensitivity to face expressions as we observed significant hemodynamic signal (change in blood flow) differences between all three expressions of happy, neutral, and sad. Hence not only is there a suppression mechanism accounting for the higher signal intensity the incongruent trials but the particular face expressions presented within those trials also have an effect on this region’s activation. It is possible that by attending to the facial expressions of the presented faces in the experiment, participants experienced those affective states by making inferences from the emotional faces. The presence of such experience is to be expected especially since half of the faces viewed by the participants in this study were of familiar individuals with whom they would have had personal acquaintances. The involvement of IFG in emotional empathy is evident through the lack of such ability in patients with lesions in this region (Shamay-Tsoory et al., 2009).
The findings in this study are the first to show such distinction between the processing of words and facial expressions both in our brains as well as in our behaviours. Further implications of our results are that dominant behaviours such as reading and recognition of face expressions are stimulus-dependent and perhaps hierarchical, hence recruiting distinct regions of the mPFC.
You can also find our study featured on York's Media website AND the Science Daily website:
http://news.yorku.ca/2010/12/22/york-u-study-pinpoints-part-of-brain-that-suppresses-automatic-responses/
http://www.sciencedaily.com/releases/2010/12/101222112112.htm
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