Effects of task and material on hemispheric lateralisation of nonverbal memory

The role of the right temporal lobe in memory function is unresolved. The material specificity model has successfully guided the detection of left temporal lobe pathology using verbal memory tests, but the detection of right temporal lobe pathology using nonverbal memory tests has been unreliable. Considering factors beyond material type per se could improve prediction of right hemispheric pathology. This thesis investigated the factors associated with right-lateralisation of memory function including i) the type of nonverbal stimulus used (i.e., abstract designs, faces, spatial arrays), ii) differences in task-related processing (e.g., encoding versus retrieval), and iii) potential lower-level stimulus confounds (i.e., memory versus perceptual processing). In a comprehensive meta-analysis of studies employing nonverbal memory testing of temporal lobe epilepsy patients (k = 152), memory for faces or spatial stimuli had superior detection of right-sided pathology than memory for abstract designs. By comparison, task demands including learning type (single versus repeated stimulus presentation) and the delay before testing memory (short versus long) had negligible effect. Following the meta-analysis two empirical papers compared the effects of material (verbal, spatial) and processing (encoding, retrieval) on lateralisation using event-related potentials (ERPs) and changes in electroencephalographic power (EEG). ERP measures showed right-lateralisation for spatial learning while processing type affected lateralisation only in the anterior region of the brain (encoding: left; retrieval: right). In the next two empirical papers using the same measures, the ERP results revealed that spatial memory contributed to right-lateralised brain activity over and above spatial perceptual processing. In both experiments EEG measures were less sensitive to the effects than were ERPs. The main findings of this thesis were that right-lateralisation of nonverbal memory is most reliably affected by the type of material, but with important contributions of task-related processing (encoding, retrieval) in the anterior brain regions, and that spatial memory affects right-lateralisation over and above the lateralising influences of perceptual processing. The findings stand to enhance understanding of right hemispheric memory functions.