Imagining the future and remembering the past both involve
mental time travel. This commonality could indicate shared
mental processes, as held by the Constructive Episodic
Simulation Hypothesis (Schacter & Addis, 2008), or else
interactive processes that complement one another, a
possibility we call the Complementarity Hypothesis.
According to the Complementarity Hypothesis, future thoughts
are constructed from schemas making them episodically poor,
whereas past thoughts are constructed from schemas and direct
retrieval of memory traces, making them relatively
episodically rich. We tested these hypotheses using machine
learning to data mine mental operations in language, much as
a geologist can recover physical processes from the geological
record. People’s natural, unprompted talk on web blogs was
automatically analyzed for past, present, and future references
using a temporal orientation classifier. In Study 1, we found
that perceptual details were mentioned more often in past than
future talk, implying greater use of episodic processing in past
than future thinking. In Study 2, a neural network using
schemas generated from Latent Dirichlet Allocation better
predicted the content of references to the future than the past,
implying that constructive processes are more common in
future than past thinking. In Study 3, we used the results from
the two prior studies to construct an episodic-by-constructive
process space. We adapted techniques from fMRI analysis to
analyze this space for clusters of activity, as if the frequency of
past and future thinking were BOLD responses in cortical
space. We found that past and future thinking occupy highly
separable regions of processing space, supporting the
Complementarity Hypothesis.