Working memory influences on auditory novelty processing in the human brain

Resumo

Novel events in the context generate an involuntary orienting response which may have a different impact on the performance of ongoing tasks depending on the specific situation in which humans are immersed. The five studies that form this thesis studied several factors that can modulate responses towards novel sounds and the specific impact that these sounds have on behaviour. This issue was approached measuring the performance of healthy, young individuals on simple visual tasks while they attempted to ignore irrelevant sounds. The auditory stimulation consisted in all cases of a stream of repetitive, equal tones. Occasionally, and in an unpredictable manner, one of these repetitive tones was replaced by an environmental novel sound. In five studies, three different techniques were used to measure brain responses related to novel sounds: event-related potentials (ERP), magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI).In different studies, involuntary orienting towards novel sounds resulted in a disruption or a facilitation of visual task performance. The results of the present studies indicate that novel sounds generate a combined alerting and orienting response. The impact that novel sounds have on behaviour may thus depend on the relative contribution of the alerting and orienting components, which can be determined by the demand of the visual task. However, the same sequence of brain responses was elicited in the present studies by novel sounds irrespective of their behavioural outcomes.We investigated the modulation of brain responses elicited by novel sounds when participants had to perform visual tasks that involved the maintenance of information in working memory (WM). Generally, the results show that in conditions of WM load, the orienting of attention towards novel sounds is diminished. The neurophysiological mechanism underlying diminished orienting was the inhibition of brain responses in auditory cortex and also in inferior frontal cortex, both areas that were involved in the processing of novel sounds. Moreover, the processing of novel stimuli in these areas was not completely abolished, but rather it was attenuated at a late phase, circa 300 ms after the occurrence of the novel sound. At this stage, preattentive change detection processes had been completed in these same areas.Finally, we showed that interference by novel sounds can also be modulated by cognitive control mechanisms. We investigated brain responses to auditory stimuli presented immediately after participants had been distracted by a novel sound. The results showed that, following distraction, sensory responses related to task-relevant, visual information were enhanced in visual cortex, while auditory processing areas were inhibited. The results of these studies have provided further insight into the cerebral mechanisms of attention control, demonstrating specific interactions and mutual modulations between endogenous and exogenous attention.