Fables Of The Reconstruction

A flurry of papers (well, OK, three) has been published recently on the relationship between how we remember the past and imagine the future. The third and final paper is another functional neuroimaging study (Addis et al., 2007).

Before we begin, let’s highlight a 2003 study by Okuda and colleagues. They were the first to compare the neural correlates of imagining past and future events. Common regions engaged by both forms of (re)construction included bilateral medial PFC, hippocampus, parahippocampal gyrus, and the left precuneus (Okuda et al., 2003). Looks fairly similar to the Bill Clinton study (Szpunar et al., 2007), except the latter lacked hippocampal activation and saw future > past for left precuneus.

So why is it big news now, when it’s not novel? That’s a question for sociologists to answer…

Anyway, Addis et al. did point out that the older study used PET and a blocked design where participants spoke freely about the past or the future during a 60 sec time interval (Okuda et al., 2003), meaning there was no temporal precision to link specific brain activations to specific events. So similar to Szpunar et al., the Addis experimental design consisted of giving cue words to prompt either remembered or imagined events. The control conditions, however, were quite different: semantic retrieval (think of two words related to the cue word and then use all three words in a sentence) and visual imagery (think of two objects related to the cue word, one larger and one smaller). In addition, they devised a rating scale for

the episodic specificity of events generated during scanning, and collected subjective ratings of the level of detail, emotionality, personal significance and field/observer perspective.

They also tried to separate the participants’ introspection about the past and future into a construction phase and an elaboration phase.

Fitting for such a complicated design, the results were complicated. First, there was a set of regions more active for event construction (past+future) than for control construction (semantic+imagery), shown in (a) below. Then there was another set of regions more active for event elaboration than for control elaboration, shown in (b) below. So these brain areas were hoppin’ to an equivalent extent for past and future.

Then the future and the past were compared directly. There seemed to be no regions that were more active for (supposedly) veridical memory than for imagination. Hmm. There were a bunch of areas with greater activity for prospective event construction (i.e., future) than for historical reconstruction (i.e., past), as seen in (a) below.

But what about memory retrieval??

In contrast to common past–future activity in the left hippocampus, the right hippocampus was differentially recruited by future event construction. This finding is notable, not only because others report right hippocampal activity to be common to both past and future events (Okuda et al., 2003) but also because it is surprising that future events engage a structure more than the very task it is thought to be crucial for: retrieval of past autobiographical events…

It does seem strange that no regions were more active for memory than for imagination. So memory doesn’t differ from fiction? At the very least, it didn’t result in greater brain activity than fiction, not in this particular study (an important point).

There was no evidence of any regions engaged uniquely by past events, not only in the PFC but across the entire brain. This outcome was unexpected in light of previous results (Okuda et al., 2003). Moreover, regions mediating retrieval processes (e.g., cue-specification, Fletcher et al., 1998) such right ventrolateral PFC (e.g., BA 47) should be engaged by a pure retrieval task (i.e., past events) more than a generation task (i.e., future events). More surprising was the finding that right BA47 showed more activity for future than past events, and that past events did not engage this region significantly more than control tasks.

I won’t try to summarize how the authors interpret the rest of their data, so you’ll have to read the abstract.

Addis DR, Wong AT, Schacter DL. (2007). Remembering the past and imagining the future: Common and distinct neural substrates during event construction and elaboration. Neuropsychologia 45: 1378-1385.

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 20 s 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 20 s 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.

Reference

Okuda J, Fujii T, Ohtake H, Tsukiura T, Tanji K, Suzuki K, et al. (2003). Thinking of the future and the past: The roles of the frontal pole and the medial temporal lobes. Neuroimage 19: 1369–1380.

Read Part 1 and Part 2.


The Reconstruction of Fables

Appendix A. Example of a specific future event generated by a pilot participant.

Future event (in 5 years; cue=dress)

My sister will be finishing . . . her undergraduate education, I imagine some neat place, Ivy league private school . . . it would be a very nice spring day and my mom and my dad will be there, my dad with the camcorder as usual, and my mom with the camera as usual. My sister will be in the crowd and they’d be calling everyone’s name . . . I can see her having a different hair style by then, maybe instead of straight, very curly with lots of volume. She would be wearing contacts by then and heels of course. And I can see myself sitting in some kind of sundress, like yellow, and under some trees . . . the reception either before or after and it would be really nice summer food, like salads and fruits, and maybe some sweets, and cold drinks that are chilled but have no ice. And my sister would be sitting off with her friends, you know, talking with them about graduating, and they’d probably get emotional.