Neural mechanisms of episodic memory formation

In order to remember what you had for breakfast today, you must rely on episodic memory, the memory for personal events situated within a spatiotemporal context. In this dissertation, I use electroencephalographic (EEG) recordings to measure the neural correlates of successful episodic memory formation. The recorded EEG signals simultaneously sample local field potentials throughout the brain, and can be analyzed in terms of specific time-varying oscillatory or spectral components of neural activity which are thought to reflect the concerted activity of neuronal populations. I collected EEG recordings while participants engage in free recall, an episodic memory task during which participants must study and then recall a list of items. In the first chapter, I compare the spectral correlates during encoding of items later remembered to those later forgotten using two separate recording modalities, scalp and intracranial EEG. I find that memory formation is characterized by broad low frequency spectral power decreases and high frequency power increases across both datasets, suggesting that scalp EEG can resolve high frequency activity (HFA) and that low frequency decreases in intracranial EEG are unlikely due to pathology. In the next chapter, I connect these HFA increases to memory-specific processes by comparing study items based on how they are re- called, not whether they are recalled. I find increased HFA in left lateral cortex and hippocampus during the encoding of subsequently clustered items, those items recalled consecutively with their study neighbors at test. The precise time course of these results suggests that context updating mechanisms and item-to-context associative mechanisms support successful memory formation. In the third chapter, I measure how the formation of these episodic associations is modulated by pre-existing semantic associations by including a semantic orienting task during the encoding interval. I find that semantic processing interferes with the formation of new, episodic memories. In the final chapter, I show that the memory benefit for emotionally valenced items is better explained by a contextual mechanism than an attentional mechanism. Together, my work supports the theory that contextual encoding associative mechanisms, reflected by HFA increases in the memory network, support memory formation