Predicting future events, and their order, is important for efficient planning. We propose a neural mechanism to non-destructively translate the current state of memory into the future, so as to construct an ordered set of future predictions. This framework applies equally well to translations in time or in one-dimensional position. In a two-layer memory network that encodes the Laplace transform of the external input in real time, translation can be accomplished by modulating the weights between the layers. We propose that within each cycle of hippocampal theta oscillations, the memory state is swept through a range of translations to yield an ordered set of future predictions. We operationalize several neurobiological findings into phenomenological equations constraining translation. Combined with constraints based on physical principles requiring scale-invariance and coherence in translation across memory nodes, the proposition results in Weber-Fechner spacing for the representation of both past (memory) and future (prediction) timelines. The resulting expressions are consistent with findings from phase precession experiments in different regions of the hippocampus and reward systems in the ventral striatum. The model makes several experimental predictions that can be tested with existing technology.

In continuous recognition the recency effect manifests as a decrease in accuracy and a sublinear increase in response time (RT) with the lag of a repeated stimulus. The recency effect could result from the gradual weakening of mnemonic traces. Alternatively, the recency effect could result from a search through a compressed timeline of recent experience. These two hypotheses make very different predictions about the shape of response time distributions. Using highly-memorable pictures to mitigate changes in accuracy enabled a detailed examination of the effect of recency on retrieval dynamics. The recency at which pictures were repeated ranged over two orders of magnitude across three experiments. Analysis of the RT distributions showed that the time at which memories became accessible changed with the recency of the probe, as predicted by a serial search model suggesting that visual memories can be accessed by sequentially scanning along a compressed representation of the past.In recognition memory experiments participants must determine whether a probe stimulus has been previously experienced or not. As the recency of a repeated probe decreases, the accuracy of the judgment decreases and response time (RT) increases (Donkin

The classic finding from short-term relative JOR tasks is that correct response time (RT) depends on the lag to the more recent item but not to the less recent item (Hacker, 1980). For decades, researchers have argued that this finding is consistent with a self-terminating backward scanning model (Muter, 1979;Hacker, 1980;Hockley, 1984;McElree & Dosher, 1993). This finding has taken on new importance in light of recent proposal that many forms of memory depend on a compressed representation of the past (Howard, Shankar, Aue, & Criss, 2015). This paper replicates and extends the results of the classic papers. A Bayesian t-test showed substantial evidence for the null e↵ect of lag to the less recent item on correct RT. In addition, this paper reports that correct RT is a sub-linear function of lag to the more recent probe and replicates the classic finding that error RT depends on lag to the less recent probe. These findings place new constraints on models of short-term memory scanning.In a relative judgment of recency (JOR) task participants choose which of two probes from a list was presented more recently. Recency in this task is traditionally measured in units of lag, which is the number of time steps in the past at which the probe was presented. That is, if the last item in the list was presented as a probe, it would be associated with a lag of one. The classic finding from short-term relative JOR tasks is that correct response time (RT) depends on the lag to the more recent item but not to the less recent item (see Fig. 1, Muter, 1979;Hacker, 1980;Hockley, 1984). For decades, researchers have argued that this finding is consistent with a self-terminating backward scan (Muter, 1979;Hacker, 1980;Hockley, 1984;McElree & Dosher, 1993). Because the scan starts at the present and proceeds backwards in time, RTs show a recency e↵ect. Because the scan is self-terminating this naturally accounts for the finding that correct RTs do not depend on the lag of the less-

Several authors have suggested a deep symmetry between the psychological processes that underlie our ability to remember the past and make predictions about the future. The judgment of recency (JOR) task measures temporal order judgments for the past by presenting pairs of probe stimuli; participants choose the probe that was presented more recently. We performed a short-term relative JOR task and introduced a novel judgment of imminence (JOI) task to study temporal order judgments for the future. In the JOI task, participants were trained on a probabilistic sequence. During a test phase, the sequence was occasionally interrupted with pairs of probes. Participants chose the probe that they expected would be presented sooner. Replicating prior work, we found that in JOR the correct RT depended only on the recency of the more recent probe. This suggests that memory for the past was supported by a backward self-terminating search model operating on a temporally-organized representation of the past. Analogously, in the JOI task we find that correct RT depended only on the imminence of the more imminent probe. By analogy to the JOR results, this suggests a forward self-terminating search model operating on a temporally-organized representation of the future. Critically, in both JOR and JOI the increase in RT with recency/imminence was sublinear, suggesting that both the timeline for the future and the timeline of the past are compressed. These results place strong constraints on computational models for constructing the predicted future.

A growing body of evidence suggests that short-term memory does not only store the identity of recently experienced stimuli, but also information about when they were presented. This representation of 'what' happened 'when' constitutes a neural timeline of recent past. Behavioral results suggest that people can sequentially access memories for the recent past, as if they were stored along a timeline to which attention is sequentially directed. In the short-term judgment of recency (JOR) task, the time to choose between two probe items depends on the recency of the more recent probe but not on the recency of the more remote probe. This pattern of results suggests a backward self-terminating search model. We review recent neural evidence from the macaque lateral prefrontal cortex (lPFC) (Tiganj, Cromer, Roy, Miller, & Howard, in press) and behavioral evidence from human JOR task (Singh & Howard, 2017) bearing on this question. Notably, both lines of evidence suggest that the timeline is logarithmically compressed as predicted by Weber-Fechner scaling. Taken together, these findings provide an integrative perspective on temporal organization and neural underpinnings of short-term memory.