Dorsolateral Prefrontal Cortex and Stroop Performance: Tackling the Lateralization

Article excerpt

Neuroscience research has identified the involvement of the dorsolateral prefrontal cortex (DLPFC) in cognitive control. Questions remain, however, about its lateralization correlates during Stroop task performance, an experimental cornerstone on which a large amount of cognitive control research is based. After reviewing the literature, we find that three Stroop variants have been used in an attempt to uncover different aspects of cognitive control related to DLPFC involvement. In sum, rapid and sequential up-regulation of the attentional set seems to be related to the left DLPFC. These attentional adjustments are based on participants' expectancies regarding the conflicting nature of the upcoming trial, and not on the conflict itself. In contrast, the right DLPFC is associated with an overall up-regulation of the attentional set when attentional conflict is experienced.

Theoretical Background

Cognitive control can be conceptualized as the capacity to suppress prepotent but incorrect responses and the ability to filter out irrelevant information within a stimulus set (Botvinick, Braver, Barch, Carter, & Cohen, 2001). Experimental conflict situations that require cognitive control have been found in tasks that demand overriding of prepotent responses, such as in the Stroop (1935) paradigm. The Stroop interference effect is one of the most frequently used tasks in cognitive psychology, clinical neuropsychology, and cognitive neuroscience. In a Stroop color-naming task, participants have to name the ink color of a printed color word, and greater conflict occurs for incongruent (e.g., the word red in green ink) than for congruent (e.g., the word red in red ink) trials-in other words, the Stroop interference effect.

Neuroimaging techniques can be used to investigate the brain correlates of cognitive control during experiments. Such activation techniques include PET (positron emission tomography), fMRI (functional magnetic resonance imaging), and ERPs (event-related potentials). These neuroimaging techniques face the critical problem, though, of distinguishing cause from effect, because no interference with brain activity is provoked. As a noninvasive tool for stimulation of the human cerebral cortex, repetitive transcranial magnetic stimulation (rTMS) induces alterations of neuronal activity that have an effect on cognition; this is becoming a promising technique for investigating whether a particular area is essential for task performance.

Different neurocognitive studies have revealed that cognitive control is related to a specific cortico-subcortical circuit, including the anterior cingulate cortex (ACC) and the dorsolateral prefrontal cortex (DLPFC) (e.g., Barber & Carter, 2005; MacDonald, Cohen, Stenger, & Carter, 2000). According to the prominent conflict-monitoring hypothesis (Botvinick et al., 2001; Botvinick, Nystrom, Fissell, Carter, & Cohen, 1999), the occurrence of response conflict is signaled by the ACC, leading to recruitment in the DLPFC of more cognitive control for subsequent performance. This involvement of the DLPFC (Brodmann area [BA] 9/46) has been identified in neuroscience research with top-down regulatory processes of cognitive control. Overall, BA 9 includes the superior and inferior frontal gyrus, whereas BA 46 includes the middle frontal gyrus.

For a number of years, the correlates of top-down attentional control were thought to reflect the integrity of the left frontal cortex and the cortico-subcortical circuit (Swick & Jovanovic, 2002). However, recent literature has demonstrated a more heterogeneous picture of lateralization with respect to cognitive control and the DLPFC. Notably, the lateralization of control processes seems to be task-specific (Kerns et al., 2004; Miller & Cohen, 2001; Rubia et al., 2006). Since the Stroop interference effect remains a cornerstone on which the conflict-monitoring model is based, this review article focuses on lateralized processes of cognitive control based on different Stroop designs that have been used. …