The memory-related PAC that was exogenously induced in older adults was estimated to arise from ventral stream regions nearest the temporal cortex (Fig. 3a). Despite the limitations of source reconstruction, the detection of neural generators in the temporal location is consistent with evidence from magnetoencephalography (MEG) 18, 30, neuroimaging 31, 32, human intracranial EEG 33, 34, and nonhuman primate single-unit recordings 35, 36, implicating areas of the ventral visual pathway in the working memory processing of natural objects. For example, using human intracranial EEG, areas of the temporal cortex and medial temporal lobe have been shown to interact via phase synchronization to support working memory maintenance 34, and it is possible that the theta-gamma PAC measured in the present study captured cortical interactions in this general region. Moreover, the PAC effect was left lateralized in electrode and source space. Previous studies using synchronization measures recorded by MEG 18, 30 and intracranial EEG 34 have found similar results, suggesting PAC and phase synchronization may be more prominent in the left hemisphere of temporal areas during working memory tasks with complex objects.
