Research ReportThinking about the future versus the past in personal and non-personal contexts
Introduction
The idea that our ability to contemplate our futures is necessarily linked with remembering our pasts (Ingvar, 1985, Tulving, 1983, Tulving, 2005), which has long been indicated by neuropsychological data (Klein et al., 2002, Rosenbaum et al., 2005, Tulving, 1985), has recently received empirical support from a number of neuroimaging studies (Addis et al., 2007, Botzung et al., 2008, Okuda et al., 2003, Szpunar et al., 2007). Diverse areas of the brain that are involved in retrieving autobiographical episodic memories, or memories of events involving oneself that occurred in the past, were also shown to be responsive when thinking about possible personal events that could take place in the future.
Attempts to explain the considerable overlap between the brain regions involved in both types of episodic thinking as well as certain types of mental simulation has resulted in broader conceptualizations concerning this network's global function as necessitating self-projection (Buckner and Carroll, 2007), mental scene construction (Hassabis et al., 2007a, Hassabis and Maguire, 2007) constructive simulation (Schacter and Addis, 2007, Schacter et al., 2007) or proactive associative processing (Bar, 2007, Bar et al., 2007). Although the extensive activation overlap during episodic past and future thinking suggests the involvement of several common mental operations, only few efforts have been directed at understanding the functional differences between the many brain regions (D'Argembeau et al., 2008, Hassabis et al., 2007a).
One approach to glean functional differences would be to focus on factors that are common to episodic future and past thinking and vary them selectively within a different comparison variable. The recall of episodic memories can be conceptually broken down into many component processes such as self-processing, a subjective sense of time, narrative structure, retrieval of multimodal details, and a feeling of familiarity (Hassabis et al., 2007a). Of these, two factors that lie at the root of both episodic past and future thinking, are that they are inherently and explicitly self-referential, and that both entail the (re)construction of the personal event in question. What is meant by construction is that during episodic future thinking, constructive simulation of hypothetical events enables us to pre-experience the future, whereas during episodic past thinking constructive simulation comes into play when we remember personal events that we have experienced in the past (Gilbert and Wilson, 2007). As such, our ability to mentally simulate episodic past or future events is possible because episodic memory is constructed rather than reproduced (Schacter et al., 1998, Schacter and Addis, 2007), which is why it tends to be liable to distortions and errors (Schacter and Slotnick, 2004).
In an effort to differentiate between areas that are involved in the constructive aspects of episodic thinking from its inherently self-referential aspects, we created a novel event-related fMRI experimental design where episodic (or personal) past and future thinking were investigated together with semantic (or non-personal) past and future thinking. Semantic memory refers to memory of fact based world knowledge that is not bound to a specific learning event. As human beings, we not only have the ability to think about our personal pasts and possible futures, we are also able to contemplate past world events and make informed guesses about future happenings in the world. The contemporary debate on global climate change is an ideal example of our capacity to engage in theoretical issues about what could happen to our planet in the future, regardless of whether we will be around to experience it firsthand. So the inclusion of semantic thinking into the discussion on prospection is not only timely, it also allows for a better characterization of the functions of different brain regions involved in episodic thinking.
We would predict, for instance, that brain areas involved in the constructive and flexible recombination of representations from memory should be highly activated in the case of episodic past, episodic future and semantic future thinking.1 This is because semantic future thinking relative to the semantic past thinking would also involve accessing and manipulating a wider extent of representations that need to be weighed and integrated in order to predict the likelihood of the occurrence of a hypothetical world event. After all, generating new content necessarily requires having to combine and recombine existing elements in memory (Schacter and Addis, 2007, Suddendorf and Corballis, 2007).
These brain areas should be dissociable from other regions, that are specifically involved in self-referential processing as such regions would be only highly activated for the two episodic thinking conditions and not the semantic thinking conditions. In fact, one neuropsychological study has demonstrated a double dissociation in these declarative memory domains to encompass both future and past thinking such that the patient's capacity to engage in semantic past and future thinking was preserved, but his episodic past and future thinking were severely affected (Klein et al., 2002). The literature thus far indicates that the anterior medial prefrontal cortex (BA 10) plays a role both in the components of self-referential processing (Hassabis et al., 2007a) as well as constructive processes (Addis et al., 2007). The design of present study which allows for the disentanglement of these two processes should render it possible to shed more light on the precise function of this area.
Furthermore, uncovering which brain regions are generally responsive to representations that refer to the future, the past, or semantic content, and which are preferentially engaged by the single conditions would also help ascertain other specific functional contributions of the network of regions that includes the hippocampal formation, the lateral posterior parietal cortex, and the posterior cingulate cortex. Areas that are most strongly activated during both episodic future and semantic future thinking, for instance, could be regarded as being involved in more open-ended or divergent retrieval processes, as there is no objective correct or incorrect answer associated with the response in these conditions. By the same token, more convergent retrieval processes could be said to be operating in the areas most strongly involved during episodic past and semantic past thinking, as there is an objective true/false response associated with these conditions. Also, the functional differences between the areas involved in episodic future versus episodic past thinking could also be more clearly interpreted depending on the pattern associated with the semantic thinking conditions in the implicated brain regions. The latter would help indicate which combination of self-referential, constructive and/or open- versus close-ended retrieval components is at play.
The experimental design (Fig. 1) thus comprised of four experimental conditions (personal future, non-personal future, personal past and non-personal past). Stimuli used both in the personal past and future thinking conditions referred to events related to oneself, whereas stimuli in the non-personal past and future thinking condition referred to happenings in the external world. We employ the terms “personal” and “impersonal” in place of “episodic” and “semantic” respectively. This is because unlike in previous paradigms, where retrieval success of a particular episodic memory was checked in relation to the phenomenology and imagery associated with the recall (Addis et al., 2007, Botzung et al., 2008), the current speeded paradigm does not stress this facet of episodic memory retrieval. Our aim was to investigate thinking about the future versus the past in both semantic and episodic domains. Both domains are not comparable in terms of associated imagery because semantic memory per definition has far less associated multimodal detail as it refers to fact knowledge that is not cued to a given time or place. Given the study's objective, the experimental trials for both semantic and episodic thinking were made comparable to avoid problems associated with contrasting conditions that contain dissimilar stimulus events within a trial or have fMRI trials of differing lengths. This of course does not rule out the possibility that episodic thinking relative to semantic thinking could automatically trigger, even in a speeded paradigm with short trial durations, richer visual imagery and the retrieval of greater volume of content.
In order to discount possible unspecific effects arising from behavioral differences, such as perceived difficulty of the conditions, an unrelated control condition was also included where participants were required to make accuracy judgments about the coding of their response keys. A variation of this control task was used in a previous study (Abraham et al., 2008a).
Section snippets
Behavioral findings
Participants took longer to respond to statements referring to non-personal information compared to personal information (Main effect: context type; F1, 19 = 46.62, P < .001) and to statements referring to the future relative to those referring to the past (Main effect: time period; F1, 19 = 12.5, P = .002). Longer reaction times were associated with the non-personal future condition relative to the non-personal past condition (t19 = 2.82, P = .01), and the personal future condition relative to the
Discussion
Neuroimaging studies have investigated mental time travel with reference to episodic memory and prospection or thinking about one's own personal past or personal future. The aim of the present study was to further elucidate the functional roles of the brain regions that are activated when we engage in mental time travel by introducing the semantic memory or non-personal domain into this context. As a first step, similarities between the experimental conditions relative to the unrelated control
Participants
After excluding two participants due to severe imaging movement artifacts, the final sample included 20 right-handed healthy volunteers (10 female; mean age: 26; age range: 22–30) with normal or corrected-to-normal visual acuity. All participants were native German speakers with no history of neurological or psychiatric illness. None were taking medication at the time of measurement and all gave informed consent before participation. The experimental standards were approved by the local ethics
Acknowledgments
The authors would like to thank Uta Wolfensteller and Andreja Bubic for their valuable feedback on the manuscript. We are also grateful to Andrea Gast-Sandmann for her assistance with the figures and Sylvia Mössinger for her assistance during the pilot studies.
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