Brain mechanisms of executive control: Event-related potentials and oscillations in schizophrenic patients and healthy controls during task-switching

Abstract

[eng] The brain's ability to represent, maintain and update contextual (task-set) information enables us to alternate successfully between different tasks. Task-set reconfiguration is required when task demands change, as goal directed behaviour has to be adjusted to the new task. Task-cueing paradigms are widely used to investigate the underlying processes which include attentional shifting from one task to the other (task-set shifting), the retrieval of goals and rules as well as the inhibition of the previously relevant (but now irrelevant) task-set.

In the first study, different aspects regarding the extraction of cue information and its importance for task-switching processes were investigated. The main focus of interest were the electrophysiological modulations regarding changes in sensory cues which (1) are or are not related to a switch in task, or even (2) may not carry any task-relevant information in some trials. The process of early (task-relevant) change detection is important in order to prepare the upcoming task. The results suggest that cue-locked event-related potentials (ERPs) are not modulated due to purely sensory changes, but rather any such ERP modulations reflect a task-related process. At least three sub-processes were identified: (1) a fast detection of task-relevant changes (as early as 180 ms), (2) a process of cue-response mapping, and (3) the reloading/updating of stimulus-response mappings. Target-locked analyses revealed that non-informative cue switches do not affect task preparation in any significant way.

The second study consisted of the analysis of the data from study I in the time-frequency domain in order to elucidate power changes in the alpha and theta bands. We independently manipulated (1) anticipatory task preparation by manipulating the informativeness of cues about the upcoming task, and (2) the exogenous contribution to endogenous task-set switching. This design enabled us to examine the relative contribution from exogenous cue changes upon endogenous task-set reconfiguration effects, and whether those effects depend on the presence of foreknowledge about the upcoming task.

The data confirmed strong generic preparation benefits as visible in behavioural performance and both frequency bands. Task switching effects in the alpha band could be related to both goal shifting and rule activation while task switch effects in the theta band seemed related to initial task-set reconfiguration rather than task-set implementation. Strong oscillatory modulations for cue switch trials suggested an “incongruent cue-task transition” effect. Importantly, no significant effects for “task-neutral” cue switches were found.

The third study aimed to investigate higher order cognitive impairments in schizophrenia. Previously, such type of deficits has been conceptualized as failures in executive control and contextual processing. Alternatively, disturbances in high-order cognitive functions might also be due to more delimited deficits, especially in lower-level stages of contextual processing. Moreover, early processing stages do not involve only mere sensory processing, but rather reflect interacting sensory and cognitive mechanisms. Therefore, it was deemed necessary to explore the patients' task-switching abilities by manipulating sensory updating and task-set updating orthogonally to examine the interplay between bottom-up and top-down control processes. Our results suggested that the observed impairments in task-switching behaviour in schizophrenic patients were not specifically related to anticipatory set-shifting, but derived from disrupted early sensory processes of both cue- and target-locked information, as well as from a deficit in the implementation of task-set representations at target onset in the presence of irrelevant and conflicting information.

[spa] Esta tesis tuvo como objetivo investigar los procesos de cambio de tarea en pacientes esquizofrénicos y controles sanos mediante un paradigma de cambio de tarea con indicadores explícitos. El estudio I examinó el uso de conocimiento previo durante el cambio de tarea, manipulando la actualización sensorial y el control ejecutivo en ensayos con indicadores informativos y no-informativos. Los cambios sensoriales en el estímulo indicador modularon los potenciales evocados (PEs) asociados al indicador sólo cuando contuvieron información conceptual sobre la tarea. La preparación para la tarea facilitó una re-asignación estímulo-respuesta ante el estímulo diana, suscitando respuestas más rápidas en todos los ensayos repetitivos. Los resultados apoyan la hipótesis de una detección rápida de los cambios sensoriales (del indicador) cuando éstos son relevantes para la tarea, y son contrarios a los beneficios por repetición de tarea debidos a una preparación (priming) perceptiva por repetición.

El segundo estudio examinó las modulaciones en las oscilaciones neuronales en las bandas alfa y theta de los datos del estudio I. Los resultados también revelaron importantes beneficios en la ejecución conductual. En los ensayos de cambio de tarea durante la actualización y la implementación de la tarea, el ritmo alfa estuvo relacionado con ambos procesos evocados por el indicador sensorial y el estímulo diana, respectivamente. Por el contrario, el ritmo theta estuvo más relacionado con las etapas iniciales de la preparación de tarea. Estos dos estudios refuerzan la hipótesis de que las modulaciones en la actividad cerebral asociadas a un cambio del indicador de tarea no son provocadas por un cambio sensorial simple, sino que están relacionadas con un cambio de meta de orden superior.

En el tercer estudio un protocolo de cambio de tarea con indicador explícito fue administrado a un grupo de pacientes esquizofrénicos y comparado con una muestra control sana. Los resultados sugieren que las alteraciones observadas en la conducta de cambio de tarea no estuvieron relacionadas específicamente con una reconfiguración estímulo-respuesta de orden superior, sino a un déficit en la implementación de dicha representación configuracional de la tarea durante la presentación del estímulo diana en presencia de información irrelevante y conflictiva.