Nabil Hasshim

Cognitive control is the ability to allocate attention away from stimuli that are irrelevant to achieving a goal, towards stimuli that are. When conflict is anticipated, attention is biased in a global, top-down manner called proactive control and this effortful type of cognitive control is engaged before stimulus onset. The list-wise congruency proportion (LWPC) effect, where the Stroop congruency effect is reduced when there are more incongruent than congruent trials compared to vice versa, has been viewed as one of the prime signatures of this type of cognitive control. However, there has been recent debate about the extent to which this effect should be attributed to proactive control instead of alternative explanations such as simpler associative learning or reactive control. Thus, by using pupillometry (i.e., an indicator of cognitive effort), the present study investigated the extent to which LWPC effects result from effortful proactive control. Experiment 1 employed a classic proportion congruency manipulation, while Experiment 2 replaced congruent trials with neutral trials to control for potential effects of associative learning. While in line with past findings, proportion congruency effects were obtained in response times of both experiments and pupillometry showed both proportion congruency and Stroop effects after stimulus onset, no differences in pupil sizes were found during the preparatory phase. Therefore, these results do not support the idea that the observed LWPC effects are due to participants engaging in effortful proactive control.

The present study examined the extent to which a key marker of task conflict, negative facilitation, is modified by onset complexity. Negative facilitation, slower reaction times (RTs) to congruent stimuli than to non-lexical neutral stimuli in the Stroop task, is thought to reflect competition between the task sets of colour naming and word reading in the Stroop task (also known as task conflict). That is, it reflects competition between whole task sets, over and above any competition between specific responses associated with a stimulus. An alternative account of negative facilitation argues that it reflects the specific phonological processing differences between pronounceable (e.g., congruent) and non-pronounceable (e.g., xxxx) stimuli that are magnified by the specific task contexts that produce negative facilitation (a mostly non-lexical trial context). Here we used onset complexity to manipulate pronounceability of the irrelevant words in the Stroop task to test this alternative account. However, before applying manipulations that produce negative facilitation, we initially tested whether there was an effect of onset complexity on Stroop task performance. The results from Experiment(s) 1 (and 3) showed that complex onsets led to larger positive facilitation and congruency effects relative to simple onsets, but did not modify incongruent or neutral-word RTs. Experiment 2 directly tested whether onset complexity modifies negative facilitation and provided strong evidence for no effect of onset complexity, contrary to the alternative account predictions. The implications of the results for task conflict theory, selective attention, and phonological processing in the manual response Stroop task are discussed.