Remembering the past and imagining the future: common and distinct neural substrates during event construction and elaboration

Abstract

People can consciously re-experience past events and pre-experience possible future events. This fMRI study examined the neural regions mediating the construction and elaboration of past and future events. Participants were cued with a noun for 20s and instructed to construct a past or future event within a specified time period (week, year, 5-20 years). Once participants had the event in mind, they made a button press and for the remainder of the 20s elaborated on the event. Importantly, all events generated were episodic and did not differ on a number of phenomenological qualities (detail, emotionality, personal significance, field/observer perspective). Conjunction analyses indicated the left hippocampus was commonly engaged by past and future event construction, along with posterior visuospatial regions, but considerable neural differentiation was also observed during the construction phase. Future events recruited regions involved in prospective thinking and generation processes, specifically right frontopolar cortex and left ventrolateral prefrontal cortex, respectively. Furthermore, future event construction uniquely engaged the right hippocampus, possibly as a response to the novelty of these events. In contrast to the construction phase, elaboration was characterized by remarkable overlap in regions comprising the autobiographical memory retrieval network, attributable to the common processes engaged during elaboration, including self-referential processing, contextual and episodic imagery. This striking neural overlap is consistent with findings that amnesic patients exhibit deficits in both past and future thinking, and confirms that the episodic system contributes importantly to imagining the future.

Figure 1

Neural regions commonly engaged during the (a) construction and (b) elaboration of past and future events relative to the control task. For all regions significant in these conjunction analyses, past and future events were engaged more than the control task at a threshold of p < .001 uncorrected (p < .005 for hippocampal regions of interest). Percent signal change data associated with each of these conditions was extracted from the peak voxels of these clusters (see Table 2 for coordinates) and are plotted. Note that future events activated left inferior parietal lobule significantly more than past events (p = .045), and there was a trend towards the left hippocampus being significantly more active during past relative to future events (p = .058). BA = Brodmann area.

Figure 2

Sagittal slice (x = −4) illustrating the striking commonalities in medial left prefrontal and parietal activity during the elaboration of (a) past and (b) future events (relative to the control tasks) at a threshold of p < .001, uncorrected (shown at p < .005, uncorrected).

Figure 3

Neural regions showing significant increases in activity during the (a) construction and (b) elaboration of future relative to that of past events. All regions were significant at p < .001, uncorrected (p < .005, uncorrected, for hippocampal and right frontopolar regions of interest). Percent signal change data associated with each of these conditions was extracted from the peak voxels of these clusters (see Table 3 for coordinates) and are plotted. Data from control (imagery and semantic) tasks is also provided. BA = Brodmann area.

Figure 4

Activations associated with the (a) construction of past and future events relative to their elaboration, and conversely the (b) elaboration of past and future events relative to their construction. All regions were significant at p < .001, uncorrected. Percent signal change data associated with each of these conditions was extracted from the peak voxels of these clusters (see Table 4 for coordinates) and are plotted. BA = Brodmann area.