Electrophysiological studies of memory for pictures and words

The retrieval of information from explicit memory has been associated with three temporally and topographically dissociable ERP correlates, an early bilateral and late right frontal component as well as a left parietal effect. The three studies in this thesis investigated that material-specificity of these ERP old/new effects to address the question whether the ERP correlates of memory retrieval vary according to the nature of the information that is retrieved. The studies compared the ERP correlates for retrieval of pictures and words, when presented in the same modality at study and test and when presented across modalities (i.e., words at study - pictures at test or vice versa) in the two phases. Results provided little evidence for the engagement of different neural generators for the retrieval of the different types of stimuli. The topographic differences that emerged, suggested the engagement of a common set of generators activated at different levels and with a different time-course, depending on the type of encoding stimulus and retrieval cue. However, the results did provide further evidence regarding the functional interpretations of the established old/new effects. All experimental conditions revealed a left parietal effect which was largest when the encoding stimuli were presented in pictorial form. Given that the rich perceptual information inherent in pictures leads to a greater incidence of episodic retrieval for pictures (picture superiority effect) the current finding provides further support for the functional interpretation of this effect as the neural correlate of recollection. All experimental conditions also revealed a late right frontal effect which showed no differences in magnitude or topography between the different conditions. This insensitivity to the type of encoded material and the type of retrieval cue supports the interpretation of the effect as the neural correlate of post-retrieval monitoring processes. New functional properties of the early bilateral frontal effect emerged. The effect was only present when stimuli were presented in the same modality at study and test. Thus, the effect appears to be highly sensitive to the perceptual similarity of an item between study and test, which suggests a functional correlation to perceptual data-driven processes, proposed to be one of the bases of familiarity-driven recognition

Dissociating recollection from familiarity: electrophysiological evidence that familiarity for faces is associated with a posterior old/new effect

In recognition memory research, a tension exists between dual-process and single-process models of episodic retrieval. Dual-process models propose that ‘familiarity’ assessment and the ‘recollection’ of contextual information are independent processes, while single process models claim that one common process supports retrieval. Event-related potentials (ERPs) have been used to show dissociations between the mid frontal and the left parietal ERP old/new effects, which have been associated with familiarity and recollection, respectively. While much ERP evidence favours dual-process theory, Yovel and Paller (2004) used faces as retrieval cues to demonstrate that posterior old/new effects index both familiarity and recollection, a finding consistent with single process models. Here we present evidence supporting Yovel and Paller’s claim that a posterior old/new effect indexes familiarity for faces, along with a novel finding that recollection is associated with an anterior old/new effect. Importantly, and in contrast to Yovel and Paller, the old/new effects associated with familiarity and recollection were topographically dissociable, consistent with a dual process view of recognition memory. The neural correlates of familiarity and recollection identified here for faces appear to be different from those typically observed, suggesting that the ERP old/new effects associated with episodic recognition are not the same under all circumstances

Separating content-specific retrieval from post-retrieval processing

According to cortical reinstatement accounts, neural processes engaged at the time of encoding are re-engaged at the time of memory retrieval. The temporal precision of event-related potentials (ERPs) has been exploited to assess this possibility, and in this study ERPs were acquired while people made memory judgments to visually presented words encoded in two different ways. There were reliable differences between the scalp distributions of the signatures of successful retrieval of different contents from 300 to 1100 ms after stimulus presentation. Moreover, the scalp distributions of these content-sensitive effects changed during this period. These findings are, to our knowledge, the first demonstration in one study that ERPs reflect content-specific processing in two separable ways: first, via reinstatement, and second, via downstream processes that operate on recovered information in the service of memory judgments

The impact of change in stimulus format on the electrophysiological indices of recognition

The present study investigated the impact of changes in stimulus format between study and test on the event-related potential (ERP) correlates of recognition. To this end subjects studied the names and pictures of common objects. At retrieval, subjects were presented with old items for which the presentational format had changed (from picture to name or vice versa), old items that were shown in the same format as at study, and new items. ERPs revealed three temporally, spatially and functionally dissociable memory-related effects: an early bilateral frontal effect, which was sensitive to the perceptual similarity between study and test stimulus; a left temporo-parietal effect, which was sensitive to the type of stimulus that was encoded; and a late frontal effect, which was largest over right electrode sites and appeared to be sensitive to the type of test stimulus. The results provide further evidence for a functional dissociation between the early bilateral frontal and the later temporo-parietal old/new effect, suggesting an interpretation in terms of processes contributing to a perceptually-driven familiarity or novelty assessment and recollection, respectively