Prior knowledge and present events in the brain

Sammanfattning: Our experience the world is dependent on both the surroundings and the brain. In other words, perception is a synthesis of incoming signals, internal state, and previous knowledge rather than a mere reflection of the environment. Prior knowledge can be engendered from cues in the present context, from previous experiences, or in a wider sense by evolutionary processes effected in the organization of the brain. Knowing what is going to happen modulates the extent to which aversiveness induces stress and anxiety. Predictability is also of importance since it may improve the accuracy and speed of sensory processing as well as behavioral responses. As for evolutionary relevant threats, a perceptual and behavioral system has been organized around a fear module generating instant responses even outside of awareness. Accordingly, brain regions with altered activities and functions complement each other in constituting a system that allows for perceptions, reactions and behavioral planning at different time scales. In this thesis, we investigate the influence of prediction on perception of somatosensory and aversive events and the differential neural processing of phobic and fear-relevant stimuli, as measured by functional magnetic imaging (fMRI) and positron emission tomography (PET). The network of activation and deactivation during anticipation of an expected somatosensory stimulus was found to be similar to that engaged during the actual sensory stimulation. These overlapping patterns of change furthers the idea that predictions are subserved by a neuronal network similar to that which subserves the processing of the actual sensory input. It also infers that anticipation may invoke a tonic top-down modulation of neural activity and in this way favor selective processing of relevant information. Correspondingly, predictive cues of painful stimuli that allowed for correct temporal estimation enhanced activity in relevant sensorydiscriminative processing areas. Absence of cues predicting painful stimuli augmented the negative experience as well as neuronal activity in areas associated with the affective component of pain processing, such as the anterior insula, the caudal anterior cingulate cortex, and the orbitofrontal cortex. This context also prompted increased activity in the posterior parietal cortex and lateral prefrontal cortex that we attribute to enhanced alertness and sustained attention during unpredictability. An affective cortical network along with the amygdala and the periaqueductal gray was also active in response to phobic relative to fear relevant (but nonfeared) stimuli. However, with non-conscious stimulus presentation only the amygdala was activated both to the phobic and fear relevant stimuli. With time to decide that the fear relevant stimuli in effect were not dangerous the dorsolateral prefrontal cortex was engaged, a region included in a system that has been identified in strategic selection of behavior and in inhibiting environmentally cued responses. These results thus reflect fast perceptual pathways favoring rapid responses to threat, top-down direction of attention contributing to enhancement of relevant information processing along with attenuation of anxiety and finally, activity in regions mediating goal-directed perception and behavior.