In this doctoral thesis, I mainly investigate the functional role of encoding-related pre-stimulus activity. At least two kinds of encoding-related pre-pre-stimulus ERP activity have been found (e.g. Galli et al., 2011; 2014; Gruber & Otten, 2010; Otten et al., 2006;
2010). These two kinds of pre-stimulus ERP activity are qualitatively different depending on the preparatory processes. Semantic preparation is reflected by a frontal negative-going pre-stimulus subsequent memory effect. A widespread positive-going pre-stimulus subsequent memory effect is thought to indicate motivational processes.
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Although there is evidence of qualitatively different pre-stimulus effects, it is currently unknown whether the same pre-stimulus effect can be invoked to different degrees. In other words, it is unknown whether encoding-related activity before event onset can be graded in nature.
In the previous studies, effects have been found in one condition but not another, depending on the type of task or stimulus indicated by a pre-stimulus cue. For example, in Otten et al. (2006), a task cue indicated whether the encoding task of an upcoming word was semantic or non-semantic in nature. A pre-stimulus frontal negative subsequent memory effect was only observed when the cue indicated a semantic encoding task. The absence of a pre-stimulus subsequent memory effect in the non-semantic condition might be explained by the lack of engaging semantic preparatory processes. A question of interest is whether, when semantic preparatory processes are needed in different conditions but to varying degrees, pre-stimulus activity will similarly vary in magnitude. If so, this would strengthen an interpretation of pre-stimulus activity as reflecting an active preparatory nature of pre-stimulus activity. In support of this idea, Gruber and Otten (2010) demonstrated that pre-stimulus activity can be strategically engaged in supporting encoding. In the study, pre-stimulus activity was only engaged to form a new memory representation when a high monetary reward was given. The high monetary reward condition was also associated with better memory performance. On the assumption that high monetary rewards encourage participants to use more efficient encoding-related processes, the findings suggest that engaging pre-stimulus activity that benefits encoding is, at least to some extent, under an individual’s control. If this interpretation is correct, individuals may be able to engage pre-stimulus processes to different degrees depending on the circumstances. In turn, this would be expected to lead to subsequent memory effects of different amplitudes (Otten & Rugg, 2005). Therefore, in this doctoral thesis, the research
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question is how amount of advance information influences encoding-related pre-stimulus activity to assess whether the preparatory processes that benefit encoding can be engaged to different degrees.
Pictures of daily objects were used as stimuli. First, pictures are found to have different subsequent memory effects from verbal materials (for a review, see Wagner et al., 1999). Pre-stimulus subsequent memory effects for pictures might also differentiate from those seen for verbal materials, which might suggest a perceptual preparatory process as pictures contain more perceptual details than verbal materials. To my knowledge, there have only been two EEG studies that investigated pre-stimulus encoding-related activity for pictures and their focuses are how emotional control contributed to encoding-related activity before emotional scenes (Galli et al., 2011;
2014). Second, compared to verbal materials, pictures hold more perceptual information than words. Therefore, the amount of perceptual information could be manipulated to see whether the neural correlations of preparation are accordingly varied. To dissociate encoding-related activity before and after an event efficiently, EEG was the neuroimaging technique for investigating the neural correlates of successful memory formation in the thesis, which is briefly introduced in Chapter 2.
This thesis addresses three main questions: first, is pre-stimulus encoding-related activity influenced by amount of advance information? Chapter 3 reports three ERP experiments to address this question. The first experiment used detailed grey-scale photos and perceptually impoverished line drawings as stimuli. Pre-stimulus cues indicated the amount of perceptual information to prepare for encoding. The letter ‘P’
signalled the presentation of a detailed grey-scale photo, an ‘O’ a perceptually impoverished outline, and an ‘X’ that either could occur. All advance information concerned the perceptual nature of a stimulus. If perceptual preparatory processes are under strategic control, participants may be able to use the advance information to
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strategically adjust encoding-related preparatory activity to be larger when more perceptual information is available to guide preparation (i.e. prior to a photo rather than impoverished outline). Bollinger et al. (2010) suggested that when a face was expected to be encoded, face-related regions (FFA) were functionally connected with a frontoparietal network of region. Therefore, it would be expected that anticipation to a perceptually rich picture would activate more perceptual preparatory processes than anticipation to a perceptually impoverished picture. When a pre-stimulus cue is non-informative about the amount of to-be-encoded perceptual information, the preparatory process might be insensitive to the amount of perceptual information. One possible expectation could be that participants might not be able to recruit preparatory activity for encoding in that condition.
As discussed above, when cognitive resources are limited, strategic control processes may be recruited to help allocate resources for optimising actions. To optimise encoding processes, more resources must be allocated for more preparation.
Less preparation needs less cognitive resources. Therefore, the degree in preparatory processes cannot only be manipulated by the amount of information to prepare, but also by the opportunity to prepare. To manipulate the opportunity available for preparation, different amounts of preparation time were used for preparing for encoding grey-scale photos in the second experiment. Advance information indicated the amount of preparation time. The letter ‘L’ indicated that participants had a relatively long preparation time (3 s). An ‘S’ indicated a preparatory interval of 1.5 s. The letter ‘N’
was a non-informative cue, which could be followed by either a 1.5 or 3 s interval. As to-be-encoded stimuli were always grey-scale photos, the same preparatory processes should be engaged in preparing for encoding the pictures in all three cases. The advance information was expected to guide preparatory activity to strategically engage in memory encoding. ‘L’ was expected to elicit larger encoding-related preparatory
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activity than ‘S’. Galli et al. (2013) suggested that sufficient cognitive resource is important for developing encoding-related pre-stimulus activity, which means preparation for memory encoding might depend on the opportunity to engage such pre-stimulus activity. Therefore, it would be expected that a long pre-pre-stimulus interval gave more opportunity to employ preparation for encoding an upcoming event. Differently from the non-informative cues used in Experiment 1, here, the non-informative cue condition would be informative once the short preparatory interval elapsed. Therefore, participants might also be able to prepare even if the cue is non-informative in the present case. Uncapher et al. (2011) found that pre-stimulus dorsal parietal cortex was activated only when a pre-stimulus cue validly signalled the location of an upcoming picture. More importantly, such activity was positively correlated with later encoding success on the objects that were presented after valid location cues. Different neural networks in ventral parietal cortex were involved in bottom-up attention in the invalid cue condition. This suggests that the pre-stimulus subsequent memory effects after a cue signalling the cue length might be qualitatively different from the pre-stimulus subsequent memory effects after a non-informative cue.
The final ERP experiment attempted to induce different preparatory brain states by manipulating advance information of pre-stimulus cues. The pre-stimulus cues indicated the Delayed Match-to-Sample (DMS) task participants needed to prepare. The conjunction DMS task is suggested to involve the hippocampus, which participants need to remember a combination of an object and its location in a short delay (Olson, Page, Moore, Chatterjee, & Verfaellie, 2006). In contrast, the feature DMS task that requires participants to remember only a location of an object does not involve the hippocampus. In half of the trials, a critical stimulus, a colour image of an object was presented 3 s into a 5 s preparatory interval. As mentioned above, pre-stimulus hippocampal activity can predict successful episodic memory encoding (e.g. Park &
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Rugg, 2010). If preparation for the conjunction DMS task can activate the hippocampus, it would be expected that a colour image of an object that was presented after the preparation was more likely to be remembered in a later recognition memory test than a colour image of an object that was presented after preparation for the feature DMS task.
The second question is how amount of advance information influences encoding-related theta and alpha oscillatory states prior to stimulus onset. The three EEG experiments were analysed in the time-frequency domain to address this question in Chapter 4. The third question is the relationship between encoding-related anticipatory processes and overall memory performance. Galli et al. (2013) demonstrated that encoding-related pre-stimulus activity is influenced by cognitive resources before an event. However, the overall memory performance was not affected by whether cognitive resources before an event were sufficient to engage pre-stimulus activity for encoding. Galli et al. (2013) suggested other memory stages such as consolidation or retrieval might compensate weaker memory representations during encoding. In Chapter 5, two behavioural experiments (Experiment 4 and 5) manipulated cognitive resources during retrieval to prevent the engagement of the compensatory mechanisms during retrieval. During encoding, the design was the same as the second ERP experiment. If longer preparation time enables more opportunity to engage pre-stimulus activity for encoding processes, then the memory representations after longer preparation time should be stronger than the memory representations after shorter preparation time. It would be expected if there was no sufficient cognitive resources to engage such compensatory mechanisms during retrieval, weaker memory representations were more likely to be unsuccessfully retrieved than stronger memory representations.
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