The neural basis of flexibility in semantic memory retrieval

At the heart of adaptive cognition is flexibility – the capacity to focus on particular mental representations and to utilise distinct processes in a way that is appropriate for current goals. This thesis aimed to investigate the neural basis of flexibility in semantic retrieval, using cortical thickness, intrinsic connectivity and task-based fMRI. Chapter 2 investigated the structural basis of controlled semantic retrieval. We found that participants who performed relatively well on tests of semantic control showed increased structural covariance between left posterior middle temporal gyrus and left anterior middle frontal gyrus. This study provides converging evidence for a distributed network underpinning semantic control. Chapter 3 explored whether memory-related regions within the default mode network (DMN) represent goal information during semantic cognition. Participants were asked to perform a semantic feature matching task in which they decided whether a probe and target word shared a feature indicated by a cue. We found DMN regions, angular gyrus and posterior cingulate cortex, represented goal information, consistent with the view that DMN supports controlled cognition. Chapter 4 investigated how human cortex is organised to produce a spectrum of cognition, from efficient memory-based decisions to more flexible novel patterns of thought. We asked participants to match words on the basis of a specific semantic feature, such as colour, while parametrically varying other features. This created a ‘psychological gradient’ varying from strong to weak convergence between long-term memories and evolving task demands. We found the neural response to the task varied in a gradual way along a previously-described connectivity gradient which captures transition along the cortical surface from perception and action, through executive control areas, to regions of DMN. The findings of this thesis provide evidence for the structural and functional basis of flexibility in semantic retrieval and point to organizational principles of this conceptual flexibility in the brain.