Working memory (WM) enables individuals to maintain information about multiple items for a brief period, guiding goal-directed behaviours. One key feature of WM is its capacity limit. Although the neural basis of WM capacity is often studied by examining individual differences, capacity may also vary depending on the properties of the items being remembered for a given individual. My dissertation investigates how different memory materials (letters, regular shapes or abstract shapes) and presentation modes (simultaneous or sequential) influence WM performance and oscillatory activities using electroencephalography (EEG) and magnetoencephalography (MEG) in delayed response tasks with high and low WM load.
In Study I, the behavioural results showed that WM capacity for complex objects (i.e., abstract shapes) reached a plateau at a lower set size, compared to simple objects (i.e., English letters and regular shapes). Our EEG data was consistent with behavioural measures in capacity, showing a larger load-dependent (high versus low WM load) decrease in posterior alpha (8-13 Hz) power for simple objects than complex objects during WM maintenance. These findings suggest that alpha oscillations track WM capacity in a content-specific manner, reflecting not only the number of items but also their complexity.
In Study II, the EEG results demonstrated a larger attenuation of posterior alpha power during WM maintenance when stimuli were presented simultaneously during encoding compared to sequential presentations. Additionally, the behavioural recency effect was found in neural activity during WM retrieval for the sequential presentations. These findings suggest that the mode of stimulus presentation during encoding continues to affect processing during subsequent WM maintenance and retrieval.
In Study III, our behavioural and MEG results at the sensor level replicated the findings from Study I. Consistent with capacity measures, we found a more pronounced load-dependent (high versus low WM load) decrease in alpha (8-13 Hz) power and an increase in gamma (50-100 Hz) power for simple objects (i.e., letters) than complex objects (i.e., abstract shapes) during WM maintenance. Furthermore, we identified the sources of load-dependent alpha power decrease in the occipital regions and gamma power increase in the right occipital and parietal regions for letters.
In conclusion, our results suggest that WM performance can be modulated by the properties of the memoranda and the modes of stimulus presentation. We also propose that oscillations in the alpha and gamma bands are sensitive to WM processes, playing critical roles in the underlying mechanisms of WM capacity.