Modulation of Stimulus Driven Neuronal Oscillations by the Emotional and Motivational Significance of Visual Stimuli

Abstract

The aim of the present thesis was to investigate oscillatory cortical activity evoked by repetitive visual stimulation using stimuli with varying emotional content. It was not subject of the current work to infer feelings, thoughts or moods individuals had during affective stimulus depiction from neuromagnetic responses measured with magnetencephalography. As emotions were understood as phasic reactions of motivational systems to distinct biologically significant stimuli in the environment of an individual preparing it for action, the main question was rather how visual stimuli of high motivational significance for the organism were processed in sensory and association cortex. One of the main hypothesis was that stimuli with high motivational impact for the individual should lead to a facilitated processing in visual sensory brain regions such as extrastriate cortex in order to extract relevant information more effectively from the environment and to promote preparing for proper action.In order to introduce the main method of steady state stimulation a steady state visual evoked potentials study was presented in chapter 1, demonstrating two important features of the such signals; amplitude peak around the repetition frequency of the stimuli and a stable phase of the elicited waveforms over a longer time period. In chapter 2 basic principles of linear mixed models were described that were intensively used in chapters 4 and 5.The main finding of chapter 3 was that high arousing visual stimuli generate greater neuromagnetic responses in visual and parietal cortex. Using pictures from the International Affective Picture System (IAPS), it was demonstrated that arousing pleasant and unpleasant pictures engage fronto-parietal cortical networks and extrastriate cortex to a greater extent than neutral low arousing pictures. Concurrent measurement of heart rate change during picture depiction and recorded viewing times of the subjects assessed after the experimental session, indicated that the affective stimuli were actually of high motivational relevance. As fronto-parietal cortex is discussed as part of a general attention network, it is possible that such a network is engaged during affective picture viewing.In chapters 4 and 5, it was demonstrated that not only complex stimuli like pictures of the IAPS set can mediate high motivational relevance. Using delayed aversive Pavlovian conditioning paradigms, it was shown that simple gratings paired with an aversive noise generated more activity in visual sensory and parietal cortex than gratings never paired with the unconditioned stimulus. Interestingly, only subjects reacting with an heart rate increase in response to the reinforced conditioned stimulus during acquisition trials demonstrated the facilitated cortical neuromagnetic response. Heart rate increase to aversive stimuli have been considered as fear responses in the conditioning literature.To date, many researchers in the field of affective neuroscience have considered the amygdala as a neuronal core substrate for emotional evaluation of stimuli and autonomic reaction in response to them. Direct projections from the thalamus to the amygdala have been discussed as a fast route for fear relevant information circumventing slow detailed cortical processing. Efferents from the amygdala to various stages of visual and temporal cortex in turn, could be one possible way to facilitate visual processing of motivational relevant stimuli at early stages of visual stimulus processing. However, the present data also suggests that cortical attention mechanism possibly sharing the same neuronal substrate as attention systems discussed in spatial selective attention could enhance feature processing in visual cortex. A network comprising parietal and frontal cortex could be a candidate for a general attention system also engaged in motivated attention in emotion. The studies presented in the current thesis have shown the involvement of parietal cortex in processing of affective pictures as well as of aversively reinforced simple gratings coinciding with enhanced activity in visual cortex. Therefore, attentional processes during processing of affective stimuli should be investigated in future research more intensively in order to further elucidate brain mechanism of affective processing.