Date Published: December 9, 2016
Publisher: Public Library of Science
Author(s): Angharad N. Williams, Lisa H. Evans, Jane E. Herron, Edward L. Wilding, Philip Allen.
It has been proposed that people employ a common set of sustained operations (retrieval mode) when preparing to remember different kinds of episodic information. In two experiments, however, there was no evidence for the pattern of brain activity commonly assumed to index these operations. In both experiments event-related potentials (ERPs) were recorded time-locked to alternating preparatory cues signalling that participants should prepare for different retrieval tasks. One cue signalled episodic retrieval: remember the location where the object was presented in a prior study phase. The other signalled semantic retrieval: identify the location where the object is most commonly found (Experiment 1) or identify the typical size of the object (Experiment 2). In both experiments, only two trials of the same task were completed in succession. This enabled ERP contrasts between ‘repeat’ trials (the cue on the preceding trial signalled the same retrieval task), and ‘switch’ trials (the cue differed from the preceding trial). There were differences between the ERPs elicited by the preparatory task cues in Experiment 1 only: these were evident only on switch trials and comprised more positive-going activity over right-frontal scalp for the semantic than for the episodic task. These findings diverge from previous outcomes where the activity differentiating cues signalling preparation for episodic or semantic retrieval has been restricted to right-frontal scalp sites, comprising more positive-going activity for the episodic than for the semantic task. While these findings are consistent with the view that there is not a common set of operations engaged when people prepare to remember different kinds of episodic information, an alternative account is offered here, which is that these outcomes are a consequence of structural and temporal components of the experiment designs.
Tulving  defined retrieval mode as a cognitive set, entry to which ensures that subsequent experiences will be treated as cues for retrieval of information from episodic memory. When he introduced retrieval mode he described it as a generic set that will be engaged irrespective of what kind of episodic content people might prepare to recover , and he also commented on what he saw as difficulties in testing for its existence and properties. This was perhaps a slightly narrow perspective, in so far as task-switching manipulations and real-time measures of neural activity were tools that were then available, and could be employed either jointly or in isolation to investigate cognitive states and their sequelae. It was none the less the case that interest in the concept of retrieval mode, its neural basis and its functional properties was developed substantively in the early- to mid-nineties when Positron Emission Tomography (PET) was employed in studies of human memory [3–9].
This experiment was designed to test the proposal that a greater relative positivity at right-frontal scalp sites on switch trials would emerge in designs where participants switched between episodic and non-episodic tasks in which the kinds of information to be processed were similar. This prediction was based on the findings and subsequent interpretation offered by Evans, Williams  in an experiment where participants switched between a retrieval task requiring judgments about study location, and a task requiring judgments about where on screen test words were shown. ERPs elicited by cues signalling which task to prepare to complete diverged only on switch trials, where those associated with the episodic cue were more positive-going over right-frontal scalp. In the experiment reported here judgments about location were again required in both tasks. In the episodic task, the location manipulation was whether objects had appeared inside or outside a representation of a building in a prior study phase. In the semantic task the requirement was to denote the location in which objects were typically found.
This experiment was conducted with a view to understanding the failure to replicate previous findings in Experiment 1. A strong interpretation of the findings in Experiment 1 is that, contrary to previous claims, there is not a generic index of retrieval mode that is observed during preparation for episodic retrieval. This view is perhaps premature. Experiment 2 was designed to explore correlates of preparatory retrieval processing in the episodic task used in Experiment 1 contrasted with a second task used in the majority of prior studies: one that requires semantic memory judgments for a kind of content that differs substantively from that required in the episodic task.
All elements were the same as Experiment 1 with the following exceptions. The substantive change was the replacement of the semantic location task with a semantic size judgment task. The changes to the design and procedure as well as other numerical differences across the experiments are described below. EEG acquisition procedures were identical.
There was no evidence of robust preparatory retrieval processing in Experiment 2 on switch or repeat trials. The findings therefore correspond with those from Experiment 1, in so far as there is, in neither case, evidence for activity at right-frontal scalp sites that has the same direction and time course as that identified in several previous studies and linked to the process of retrieval mode [13–15]. This outcome is again consistent with the view that ERPs do not index a generic signature of retrieval mode, with a somewhat stronger corollary being that there is not a common process engaged during preparation for all kinds of episodic task. The General Discussion below, however, contains a consideration of factors that might render this claim a little premature.
The most striking element of the findings in both experiments is the absence, on switch and repeat trials, of a modulation with the polarity, temporal and spatial characteristics of that which has been associated previously with the process of retrieval mode. In a number of previous studies divergences between preparatory activity associated with cues to prepare for episodic or semantic retrieval were observed on repeat trials only [10, 12, 13]. This divergence comprised a sustained greater relative positivity associated with cues signalling preparation for episodic rather than for semantic retrieval. A morphologically similar modulation with the same polarity was reported by Evans, Williams , but in their experiment the effect was present on switch rather than on repeat trials. Experiment 1 in this report was designed to test the assumption that the switch trial onset of the differences according to cue-type reported by Evans, Williams  was a consequence of the fact that, in a departure from previous work, both their tasks required a focus on the same kind of content (spatial information). This has not been the case in previous studies [10, 12, 13]. This design element was also incorporated into Experiment 1 here, with one task requiring semantic knowledge about locations, and other memory for the locations in which items had previously been shown in the task.