The Role of the Dorsolateral Prefrontal Cortex: Evidence from the Effects of Contextual Constraint in a Sentence Completion Task

Although the prefrontal cortices, in particular the dorsolateral prefrontal cortex (DLPFC), have been examined in numerous imaging and neuropsychological studies, it has proved difficult to assign a specific function to this brain region. The aim of this study was therefore to delineate the function of the DLPFC first by using positron emission tomography with a word generation task and second by comparing these findings with a series of different circumstances in which the DLPFC is activated in association with response selection. Six healthy volunteers were presented with a sentence completion task based on the Hayling test. In each of two conditions, (A) response initiation and (B) response suppression, volunteers saw a sentence with the final word omitted. In condition A they had to provide a word that fitted at the end of the sentence and in condition B they had to provide a word that did not fit. The corpus of sentences used varied systematically in their level of contextual constraint, ranging from low to high. With all levels of constraint combined, significant greater activation was observed in the left DLPFC (BA46/9) under the suppression condition and in the medial orbitofrontal cortex (BA11) under the initiation condition. Under the high-constraint condition with both tasks combined, significant right middle temporal activity (BA21) was observed, whereas under low constraint, the left DLPFC was significantly activated. An interaction of task and constraint revealed that the left DLPFC was significantly more active in the suppression task at all levels of constraint, but only under the low-constraint conditions in the initiation task. The reaction time analyses mirrored the pattern of activity observed, with slower reaction times under conditions of suppression and under conditions of initiation with low constraint. By comparing these findings with other studies of response selection, we argue that the most likely single cognitive function of the DLPFC is to specify a set of responses suitable for a given task and to bias these for selection (sculpting the response space). This function resembles the biasing of competition between stimuli in the model of Desimone and Duncan and is analogous to the component of the Supervisory Attentional System that modulates the contention scheduling system.