Investigation of the neural correlates of recognition memory using magnetoencephalography

Neuroimaging literature has identified several regions involved in encoding and recognition processes. A review of the literature illustrated considerable variations in the precise location and mechanisms of these processes, and it was these variations that were investigated in the studies in this thesis. Magnetoencephalography (MEG) was used as the neuroimaging tool and a preliminary study identified Synthetic Aperture Magnetometry (SAM) and not a traditional dipole fitting technique, as an appropriate tool for identifying the multiple cortical regions involved in recognition memory. It has been suggested that there is hemispheric asymmetry in encoding and recognition processes. There are two main hypotheses: the first suggesting that there is task-specificity, the second that this specificity is determined by stimulus modality. A series of experiments was completed with two main aims: first to produce consistent and complementary recognition memory data with MEG, and second to determine whether there exists any hemispheric asymmetry in recognition memory. The results obtained from five experiments demonstrated activation of prefrontal and middle temporal structures, which were consistent with those reported in previous neuroimaging studies. It was suggested that this diverse activation may be explained by the involvement of a semantic network during recognition memory processes. In support of this, a subsequent study involving a semantic encoding task demonstrated that category-specific differences in cortical activation also existed in the recognition memory phase. Controlling for the involvement of such semantic processes produced predominantly bilateral activation. It was suggested that the apparent hemispheric asymmetry findings reported in the literature may be due to the 'coarse' temporal analysis available with earlier imaging techniques, which over-simplified the networks reported by being unable to recognise the early complex processes associated with semantic processing which these MEG studies were able to identify. The importance of frequency-specific activations, specifically theta synchronisation and alpha desynchronisation, in memory processes was also investigated.

The effect of bilingual representation in prose reading on explicit and implicit memory

Chinese-English bilingual students were randomly assigned to three reading conditions: In the English-English (E-E) condition (n = 44), a text in English was read twice; in the English-Chinese (E-C) condition (n = 30), the English text was read first and its Chinese translation was read second; in the Chinese-English (C-E) condition (n = 30), the Chinese text was read first and English second. An expected explicit memory test on propositions in the format of sentence verification was given followed by an unexpected implicit memory test on unfamiliar word-forms.^ Analyses of covariance were conducted with explicit and implicit memory scores as the dependent variables, reading condition (bilingual versus monolingual) as the independent variable, and TOEFL reading score as the covariate.^ The results showed that the bilingual reading groups outperformed the monolingual reading group on explicit memory tested by sentence-verification but not on implicit memory tested by forced-choice word-identification, implying that bilingual representation facilitates explicit memory of propositional information but not implicit memory of lexical forms. The findings were interpreted as consistent with separate bilingual memory-storage models and the implications of such models in the study of cognitive structures were discussed in relationship to issues of dual coding theory, multiple memory systems, and the linguistic relativity philosophy. ^