Neuro-cognitive architecture of executive functions: A latent variable analysis

Executive functions refer to high-level cognitive processes that, by operating on lower-level mental processes, flexibly regulate and control our thoughts and goal-directed behavior. Despite their crucial role, the study of the nature and organization of executive functions still faces inherent difficulties. Moreover, most executive function models put under test until now are brain-free models: they are defined and discussed without assumptions regarding the neural bases of executive functions. By using a latent variable approach, here we tested a brain-centered model of executive function organization proposing that two distinct domain-general executive functions, namely, criterion setting and monitoring, may be dissociable both functionally and anatomically, with a left vs. right hemispheric preference of prefrontal cortex and related neural networks, respectively. To this end, we tested a sample of healthy participants on a battery of computerized tasks assessing criterion setting and monitoring processes and involving diverse task domains, including the verbal and visuospatial ones, which are well-known to be lateralized. By doing this, we were able to specifically assess the influence of these task domains on the organization of executive functions and to directly contrast a process-based model of EF organization versus both a purely domain-based model and a process-based, but domain-dependent one. The results of confirmatory factor analyses showed that a purely process-based model reliably provided a better fit to the observed data as compared to alternative models, supporting the specific theoretical model that fractionates a subset of executive functions into criterion setting and monitoring with hemispheric specializations emerging regardless of the task domain

MEG coherence imaging in dyslexia: Activation of working memory pathways

The aims of this dissertation are to 1) review the genetic, neurodevelopmental, structural, and functional brain imaging studies that are the foundations of our understanding of dyslexia and 2) investigate the pattern of activation and functional connectivity of neuronal networks critical in working memory in dyslexics by means of magnetoenchephalographic (MEG) coherence imaging. Dyslexics showed an early onset of activation in the precentral gyrus and the superior frontal gyrus, which differed from controls where activation was initiated in posterior cortical regions (supramarginal gyrus and superior temporal gyrus). Further, dyslexics showed lower normalized amplitudes of activation in the right superior temporal gyrus and right middle temporal gyrus than controls during a spatial working memory (SWM) task. In contrast, during a verbal working memory (VWM) task, dyslexics showed lower normalized amplitudes in the right insular cortex and right superior temporal gyrus and higher, likely compensatory, activation in the right fusiform gyrus, left parahippocampal gyrus, and left precentral gyrus. Dyslexics performing a SWM task showed significantly reduced MEG coherence and lower 1) right frontal connectivity, 2) right fronto-temporal connectivity, 3) left and right frontal connectivity, 4) left temporal and right frontal connectivity, and 5) left occipital and right frontal connectivity. MEG coherence by frequency band showed lower mean coherences in dyslexics than in controls at each frequency range and when the bands were combined during the SWM task. In contrast, during the VWM task, dyslexics showed a higher coherence in the low frequency range (1-15 Hz) and lower coherence in the high gamma frequency range (30-45 Hz) than controls. Logistic regression of the coherence by group membership was significant, with an overall predictive success of 84.4% (88.9% for controls and 77.8% for dyslexics). Coherence between the right lateral orbitofrontal gyrus and right middle orbitofrontal gyrus paired region substantially contributed to group membership. These findings deepen our understanding of the underlying pathophysiology of dyslexia, highlighting the importance of working memory circuits and prefrontal cortical dysregulation in this disorder. These results have far-reaching ramifications not only for prevention and early diagnosis, but also for the development of effective, evidence-based treatments and interventions

Fronto-parietal homotopy in resting-state functional connectivity predicts task-switching performance

Homotopic functional connectivity reflects the degree of synchrony in spontaneous activity between homologous voxels in the two hemispheres. Previous studies have associated increased brain homotopy and decreased white matter integrity with performance decrements on different cognitive tasks across the life-span. Here, we correlated functional homotopy, both at the whole-brain level and specifically in fronto-parietal network nodes, with task-switching performance in young adults. Cue-to-target intervals (CTI: 300 vs. 1200 ms) were manipulated on a trial-by-trial basis to modulate cognitive demands and strategic control. We found that mixing costs, a measure of task-set maintenance and monitoring, were significantly correlated to homotopy in different nodes of the fronto-parietal network depending on CTI. In particular, mixing costs for short CTI trials were smaller with lower homotopy in the superior frontal gyrus, whereas mixing costs for long CTI trials were smaller with lower homotopy in the supramarginal gyrus. These results were specific to the fronto-parietal network, as similar voxel-wise analyses within a control language network did not yield significant correlations with behavior. These findings extend previous literature on the relationship between homotopy and cognitive performance to task-switching, and show a dissociable role of homotopy in different fronto-parietal nodes depending on task-demands

The role of the ventrolateral frontal cortex in inhibitory oculomotor control

It has been proposed that the inferior/ventrolateral frontal cortex plays a critical role in the inhibitory control of action during cognitive tasks.However, the contribution of this region to the control of eye movements has not been clearly established.Here, we describe the performance of a group of 23 frontal lobe damaged patients in an oculomotor rule switching task for which the association between a centrally presented visual cue and the direction of a saccade could change from trial to trial. A subset of 16 patients also completed the standard antisaccade task.Ventrolateral damage was found to be a significant predictor of errors in both tasks. Analysis of the rate at which patients corrected errors in the rule switching task also revealed an important dissociation between left and right hemisphere damaged patients.Whilst patients with left ventrolateral damage usually corrected response errors with secondary saccades, those with right hemisphere lesions often failed to do so. The results suggest that the inferior frontal cortex forms part of a wider frontal network mediating inhibitory control over stimulus elicited eye movements. The critical role played by the right ventrolateral region in cognitive tasks may arise due to an additional functional specialization for the monitoring and updating of task rules

EXPLAINING LATERALITY

Working with multi-species allometric relations and drawing on mammalian theorist Denenberg’s works, I provide an explanatory theory of the mammalian dual-brain as no prior account has

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