Behavioural and Neural Investigation of the Subjective Experience of Remembering
The ability to recollect personal events in vivid multisensory detail from a first-person perspective is thought to be critical to the subjective experience of episodic memory. This PhD thesis comprises three experiments investigating these memory qualities and their parietal neural bases in different populations. The experiment presented in Chapter 2 used transcranial magnetic stimulation (TMS) in healthy younger adults and a novel associative memory task that manipulated stimulus modality to causally test whether the left angular gyrus (AnG) supports multimodal feature integration during episodic and semantic memory retrieval. Left AnG stimulation was found to selectively modulate response times in multimodal versus unimodal trials of both episodic associative recognition and semantic relatedness tasks, indicating AnG involvement in multiple forms of multimodal declarative memory retrieval. However, this stimulation effect was unexpectedly facilitatory rather than inhibitory, and no objective or subjective measures of episodic memory were modulated by AnG stimulation.
The experiment in Chapter 3 investigated the parietal neuroanatomical correlates of first-person versus third-person episodic recall in normal ageing using voxel-based morphometry and a custom 3D object location memory task that manipulated visual perspective during both encoding and retrieval. Compared to healthy younger adults, older adults showed a general deficit in first-person recall of object locations, irrespective of the original encoding perspective. Third-person recall was also impaired with age to a lesser extent and only when objects were encoded from the same third-person perspective. In older adults, left AnG and precuneus grey matter volume positively correlated with the adoption of a first-person recall perspective, but not first-person recall more generally. These parietal volumes were further shown to decline with age.
Finally, the experiment in Chapter 4 investigated whether individuals with self-reported mental imagery deficits (i.e., aphantasics) are impaired at first-person episodic recall. This was tested using a modified version of the task used in the previous chapter so that both object and spatial memory features, studied in first or third person, could be assessed via subjective vividness ratings and objective feature reproduction tasks. The visual perspective of spatial memory recall was additionally varied between the same and alternative studied perspectives to test whether aphantasics are impaired at manipulating visuospatial representations. Despite globally lower vividness ratings relative to controls, aphantasics unexpectedly showed no deficits in object or spatial memory in either visual perspective. Together, these results further understanding of parietal contributions to memory and suggest investigation of those with atypical imagery as a promising line of future inquiry into the factors necessary for its subjective reliving.