Human novelty assessment systems, positron emission tomographic evidence

grantor: University of TorontoOne of the most fundamental and adaptive mental faculties any organism can possess is the ability to differentiate novel things from familiar things. Novelty assessment is the name given to the collection of neural processes which supports this ability. The goal of this thesis was to examine the workings of novelty assessment using positron emission tomography (PET). This was done in two parts--a multi-study analysis which examined recognition of novel and familiar words and pictures across five PET studies and an original study which examined encoding of novel and familiar words under semantic and non-semantic encoding instructions. The multi-study analysis examined novelty assessment across five PET studies of recognition memory. The objective was to determine whether a common novelty assessment system operates within the brain. The results revealed a common pattern of brain activation across four of the five studies. This common pattern distinguished between regions whose activity was greatest when subjects recognized novel items (novelty activations) and regions whose activity was greatest when subjects recognized familiar items (familiarity activations). Novelty activations were observed within the right hippocampus, bilateral middle and superior temporal gyri, and left prefrontal cortex. The findings of the multi-study analysis suggested that a task-, modality-, and stimulus-independent novelty assessment network may operate within the brain. Next, an original PET study was carded out to examine semantic and non-semantic encoding of visually and auditorily presented novel and familiar words. The objective of this experiment was to examine implicit novelty assessment. The main findings from this study suggested that implicit novelty assessment modulated the activity of task-specific (semantic and non-semantic encoding) brain networks rather than relying on a dedicated brain network. These findings suggest that explicit and implicit novelty assessment may rely on different underlying neural mechanisms. In the former case, the results of the multi-study analysis suggested that novelty assessment may be implemented as a dedicated brain network, whereas in the latter case, the results of the novelty-encoding experiment suggested that novelty assessment may modulate task-specific activations. These results are discussed in relation to current views about the neural mechanisms underlying novelty assessment.Ph.D