The monitoring role of right lateral prefrontal cortex: evidence from variable foreperiod and source memory tasks

The main purpose of this research project was to investigate the monitoring function of the right dorsolateral prefrontal cortex using different tasks in two domains. To that purpose, the architecture of the cognitive processes required to perform each task was extracted by means of different approaches of functional dissociation. A variable foreperiod (FP) task was initially adopted. In such a task, simple/choice RTs are required while FPs of different duration vary on a trial-by-trial basis equiprobably in a rectangular distribution but randomly. As a result, the conditional probability is higher later in the FP range and RT is faster as the FP increases. This is the variable FP effect, which a recent neuropsychological study shows to be impaired in right lateral prefrontal patients. Another phenomenon usually obtained with such a paradigm is that of the sequential effects: RT becomes slower as the FP on the preceding trial gets longer. Contrasting views in the literature propose either multi-process strategic accounts, or a single-process conditioning account. In the project, these alternative theories were tested using behavioural studies on adults and children. The findings of these studies were not fully compatible with the previous views. A composite dual-process account, which shares some aspects with the previous accounts, was put forward and discussed. On this account, sequential effects are due to automatic processes acting on the arousal level, whereas the FP effect is due to a strategic process monitoring the conditional probability of stimulus occurrence. Results of two TMS experiments confirm that the right dorsolateral prefrontal cortex is responsible for the FP effect, but not for the sequential effects. A neuropsychological study on tumor patients further corroborates this finding and suggests that left premotor areas are more likely to be the locus of the sequential effects. In order to test whether the explicit temporal judgment has an influence on the nature of the FP phenomena, a series of behavioural experiments was conducted using a modified version of the variable FP paradigm. In the basic task, explicit judgments about the FP length were required. No modulation of the FP phenomena was obtained. However, a new stimulus-response compatibility effect was found: RT was faster when short and long FPs had to be responded to with left and right response-keys, respectively, than with the opposite stimulus-response mapping. This effect suggests that elapsing time is represented, in some circumstances, by means of spatial coordinates. Control experiments enable us to reject accounts based on hand/hemispheric asymmetries, but not accounts based on more categorical factors such as the linguistic markedness of the words used to label the stimuli and the responses. The last part of the project aimed at extending results about a monitoring role (intended in a broad sense) of the right prefrontal cortex to a domain different from non-specific preparation. Two experiments in the source memory domain were run recording ERPs in the retrieval phase. The results show that prefrontal ERPs are not modulated by retrieval success, but by retrieval confidence, with low-confidence responses being associated with more positive waves than high-confidence ones, bilaterally, in the anterior prefrontal sites. Moreover, prefrontal waves were asymmetrically more positive in the right than in the left scalp regions, independently of confidence and accuracy. On the basis of these results, we could reject accounts linked to retrieval success. The results are instead interpreted in terms of different prefrontally-located monitoring processes in source memory retrieval. Overall, the project represents an instantiation of the fractionation approach recently adopted to study the supervisory functions of the prefrontal cortex. This approach was used here in order to understand the differential role of a particular prefrontal area (i.e., the right dorsolateral prefrontal cortex) in a rather specific function (i.e., monitoring). This goal was developed in synchrony with the attainment of a better functional description of the tasks employed.