Estudio, mediante potenciales evocados, de la interferencia de las dimensiones irrelevantes del estímulo en tareas tipo Simonefectos de la edad y del deterioro cognitivo ligero

Resumen

In the present dissertation, several cognitive tasks were designed, specifically stimulus-response compatibility (SRC) tasks, which were implemented in samples of healthy young, middle-aged, elderly participants and participants diagnosed of Mild Cognitive Impairment (MCI). The research aimed to obtain electrophysiological correlates of cognitive processes and changes in these electrophysiological correlates related with ageing and the MCI state. The Simon tasks are a type of SRC task in which participants respond to one feature of a lateralized stimulus (e.g., the stimulus colour) by pressing one of two buttons disposed in the same spatial arrangement that the presentation of the stimuli. The Reaction Time (RT) is slower when the stimulus position is spatially contralateral (and thus, incompatible) to the side of the required response. That slowing in RT is known as the Simon effect. Event-related brain potential (ERP) studies established the locus of the Simon effect in the response selection stage using the lateralized readiness potential (LRP), an ERP component that allows distinguishing between interference in perceptual and motor stages of processing. The ERP studies with Simon-type tasks usually presented the stimuli in a vertical arrangement to avoid the overlap of the LRP with the N2 posterior contralateral (N2pc) and the N2 central contralateral (N2cc). However, considering that N2pc is a correlate of visuospatial processing of lateralized stimuli and N2cc is involved in preventing the bias of responding towards the side where the attention is directed, N2pc and N2cc might play an important role in the Simon effect. Nonetheless, previous studies did not research on the N2cc modulation between experimental conditions and those studying the N2pc modulations are scarce and showed contradictory results. In the present research, three different SRC tasks (which required response to coloured stimuli) were designed. These tasks were implemented in a sample of young participants. The tasks differed in the source of the irrelevant dimension: stimulus position (SRC task based on the position ¿left or right-: SRC-p), arrow direction (SRC task based on the direction -left or right-: SRC-d), and both (SRC task based on the position and direction of an arrow: SRC-pd). The SRC-pd was also implemented in samples of healthy middle-aged and elderly participants, as well as in MCI participants. Regarding the basic electrophysiological research, a first study (Study 1) was conducted with the SRC-p task. In this study, a central stimulus was used to remove the common motor activity between this and the lateralized conditions. Therefore, it was possible studying N2pc and N2cc modulations without contamination by motor activity (LRP). The eLORETA analyses showed greater premotor activity in both lateralized conditions in comparison to the neutral condition. This activity was related with the N2cc component. Also, N2cc was larger in the incompatible than in the compatible condition, which was consistent with greater premotor activity for monitoring the selection of the correct response in the incompatible condition. N2pc was not modulated by the stimulus position suggesting that processes of visuospatial attention were not an interference locus, as demonstrated by previous studies. However, N2pc had been modulated by the Simon effect in a study where the participants responded to the direction of a lateralized arrow (Valle-Inclán, 1996, exp 2). Thus, interference from the stimulus position was accompanied by a conflict of spatial information (i.e., the arrow direction pointed to the opposite side regarding the hemifield where it was located). Consequently, effects could be attributed to position interference but also to perceptual conflict. In a second study (Study 2), the SRC-pd task was used for dissociating motor interference and perceptual conflict. As result of combining the two irrelevant dimensions (position and direction), the task resulted in the following conditions: Compatible Direction-Compatible Position (CDCP); Incompatible Direction-Compatible Position (IDCP); Compatible Direction-Incompatible Position (CDIP); and Incompatible Direction-Incompatible Position (IDIP). In IDCP and CDIP, the two irrelevant dimensions conveyed contradictory spatial information while in CDCP and IDIP both irrelevant dimensions carried congruent spatial information (i.e. the direction indicated was compatible with the stimulus position). The N2pc amplitude was smaller in IDCP and CDIP than in CDCP and IDIP, suggesting that N2pc was modulated by the perceptual conflict and not by motor interferences. A third study (Study 3) compared the interference from the stimulus position (in the SRC-p task) and arrow direction (in the SRC-d task) since behavioural evidence showed that the stimulus position was processed faster than the arrow direction whereas that ERP studies suggested similar interference loci. The results of the Study 3 replicated the existence of the previously mentioned behavioural differences between the SRC-p and the SRC-d task. The electrophysiological data showed a similar locus of interference in motor processes for both S-R incompatibilities; however, unlike of previous ERP studies, P3b (link between stimulus evaluation processes and response onset) was only modulated by the stimulus position. It was discussed that P3b modulations by incompatibility from the direction observed in previous studies were not unequivocally related with interference from the direction. In those studies a low proportion of incompatible trials, relative to neutral and/or compatible trials, were presented. Therefore, in it was not possible to attribute P3b modulations to the effect of the arrow direction or to the low proportion of presented incompatible trials (i.e., to the oddball design). A subsequent study (Study 4) focused on age-related changes in correlates of visuospatial and motor processes in samples of young, middle-aged and elderly participants. The motor execution stage (studied through response-locked LRP ¿LRP-r-) was progressively slowed with age whereas visuospatial processes (N2pc latency) were slower in middle-aged and elderly than in young participants but differences were not present between the middle-aged and the elderly groups. It supported a specific pattern of age-related cognitive slowing on each particular process. Also, the distributional analyses of the RT showed that the interference from the direction affected to faster responses in young participants, to slower responses in middle-aged participants, and did not affect in elderly participants. These results suggested that the processing of the direction of the arrow was disproportionately delayed in comparison to the processing of the arrow colour (which conditioned the time of emitting the response). Results from N2pc amplitude showed that the conflict of spatial information conveyed by the two irrelevant dimensions only affected to young participants, which was consistent with no processing of the arrow direction in middle-aged and elderly groups. Another study (Study 5) focused on modulations in ERP correlates of visuospatial (N2pc) and motor processes (LRP-r) in samples of healthy participants and participants diagnosed of amnestic mild cognitive impairment (aMCI). Although behavioural performance was similar between healthy adults and aMCI participants, N2pc was smaller in aMCI than in the control group, suggesting a reduction in the allocation of visuospatial attention to the target stimulus. Furthermore, LRP-r amplitude was smaller in aMCI than in the control group, which was consistent with recent studies that suggested a deficit in motor cortex in MCI participants. Interestingly, the LRP-r amplitude proved to be a good marker of aMCI (Area under curve: 0.86; sensitivity: 0.85; specificity: 0.92). In summary, the present research studied the modulation of ERP correlates of visuospatial, motor, and cognitive control processes in SRC tasks. In addition, it compared the interference loci between spatial interferences induced by stimulus position and by a symbolic signal (i.e., the direction pointed by a central arrow). Also, in the frame of the neuropsychological ageing, contributions were made to the cognitive slowing theory, and to the obtaining of aMCI biomarkers.