The role and mechanisms of top-down optimisation of perception

According to the predictive coding approach to perception, the brain uses predictions based on previous experience to optimise perception, by allocating more computational resources to important or unexpected stimuli. Overall, predictions allow faster and more accurate recognition, but occasionally, when the prediction is incorrect, it may lead to a misperception. The aim of this thesis was to investigate the influence of top - down processes on perceptual decisions. I utilised misperceptions as a signature of those top - down influences and Signal Detection Theory to assess their size, type and direction. I used Electroencephalography to determine the stage of information processing at which different types of predictions influence sensory processing.The empirical studies are clustered around Topic 1: Influence of Predictions on Perception, Topic 2: Types of Predictions and Topic 3: Value as Modulator of Perception.Studies clustered in Topic 1 analysed and quantified the influence of predictions on perceptual decisions and showed that misperceptions can be triggered by wrong predictions only in very specific circumstances. In particular, misperceptions occurred only if there was some degree of correspondence between the wrong prediction and the sensory input. Otherwise, predictions were easily rejected, increasing the overall accuracy. I also demonstrated that misperceptions were most likely to happen in a window on the continuum of input quality where the stimulus - related uncertainty was highest. Topic 2 comprised experiments investigating different types of predictions and their interaction. Behavioural (but not EEG) results revealed interference between passive and active expectations. The early event related (ERP) components N1 and P2, as well as the P300, were all modulated by expectations. Expected events either increased or decreased the P300 amplitude, depending on whether the expected item was predictable and thus ignored, or awaited and thus flagged for further processing. This suggests that P300 might be an index of top - down resource allocation. Experiments within Topic 3 studied the influence of values, as examples of executive processes, on perceptual decisions, using either natural or acquired high - value stimuli. The results suggested that the process of recognition is adjusted in a top - down manner to account for the cost and benefit values related to different outcomes. The trade - off between processing time and accuracy is not fixed, but can be adjusted to optimise recognition in the task at hand. Furthermore, value can change the focus of perception, resulting in different elements of the sensory input being amplified or ignored. Overall, these results showed that misperceptions are 'intelligent mistakes' - a by - product of a top-down, prediction - based optimisation strategy that decreases the computational load, while increasing accuracy and improving the allocation of computational resources.