Electrophysiological Evidence of Human Memory Formation

This dissertation involves a series of experiments that investigated the neurocognitive processes engaged in episodic association formation. Electroencephalograms were recorded from healthy, young adults while they memorized visually presented words or word pairs. Event-related potentials (ERPs) recorded during the study phase were examined based on participants’ subsequent memory performance. Study 1 demonstrated that association formation, but not item encoding, was reflected by ERP positivity that occurred late during the ERP trial over the parietal scalp region. Study 2 showed that encoding processes associated with subsequent paired-associate recall were qualitatively different between two retention intervals (RIs). Sustained ERP positivities were observed late during the ERP trial over the frontal and fronto-central scalp regions for the long and short RIs, respectively. The fronto-central positivity was investigated further in Study 3 using the distinction between primary memory (PM) and secondary memory (SM). Items retrieved from SM have been absent from consciousness, whereas items in PM have not. The observed fronto-central positivity was shown to reflect binding in a general sense (inter-item and intra-item associations). Study 4 demonstrated that associative binding strength is reflected by ERPs recorded late during the ERP trial over the right frontal and left centro-parietal scalp regions. Study 4 also enabled an investigation on the “negative recency effect” – items presented at the end of a series are remembered with the lowest probability on a delayed memory test, even though they are remembered best on an immediate memory test. The results suggest that at least some of the factors that contribute to this effect occur during retrieval. Collectively, the results demonstrate that the ERP methodology, combined with behavioural data, can be used to provide an objective measure of depth of processing used to encode information, and in doing so helps address one of the major criticisms of the levels-of-processing framework.Ph