LAUSR

Ebbinghaus (1) showed, long ago, that forgetting follows a curvilinear trajectory, with information forgotten rapidly at first and more slowly with the continued passage of time. But why do we forget at all and why in just that way (instead of, for example, with the rate of forgetting remaining constant over time)? Setting aside pathological conditions like amnesia, common explanations for the ubiquitous phenomenon of forgetting include 1) pure decay (e.g., the processes required to maintain structural changes to the synapse after learning are subject to random error), 2) interference (e.g., other memorized information inhibits, competes with, or degrades to-be-remembered information), 3) context drift (the current context differs from the context at the time of learning, analogous to “state-dependent” memory), and 4) the adaptive inhibition of information that is still represented in the brain but is no longer needed (e.g., in Pavlovian conditioning, extinguished and seemingly forgotten memories can be restored with a single conditioning trial). One intriguing way to shed light on the theoretical mechanisms of forgetting is to investigate variables that “undo” forgetting, showing that what has been forgotten is not necessarily gone forever. In that regard, in PNAS, B€ auml et al. (2) report a counterintuitive pattern of results that provides compelling support for the idea that context drift plays a role in forgetting. Before delving into the findings, consider, first, some longstanding laws and concepts that help to frame the issue: 1) the power law of forgetting, 2) Jost’s law of forgetting, 3) Ribot’s law of retrograde amnesia, and 4) the notion that memories consolidate over time. As has long been known, the time course of forgetting is generally well characterized by a power law (3), according to which R1⁄4 a0ð1þ ktÞ , where R is the amount of information retained in long-term memory, t is the “retention interval” (i.e., how much time has passed since the information was encoded), and a0, b, and k are parameters. According to this equation, when t1⁄4 0 (i.e., immediately after learning), R1⁄4 a0, which means that a0 represents the “degree of learning.” In other words, it represents how much retrievable information was initially encoded into long-term memory, when the learning context was still in effect. The parameter b represents the rate of forgetting (the larger its value, the faster information is lost with the passage of time), and the parameter k is simply a scaling constant which can be set to one for convenience. When the tested retention intervals all substantially exceed zero, as is usually the case in studies of forgetting, a simpler approximation of this equation can be (and usually is) used such that R1⁄4 at , where a represents the degree of retention at t1⁄4 1 (essentially still capturing the degree of learning) and b still represents the rate of forgetting. A noteworthy feature of the power law of forgetting is that the proportional rate of decay slows with the passage of time (4). For example, retrievable information might decline by 40% in the first 24 h after learning, by another 20% in the next 24 h, by another 10% in the next 24 h, and so on (unlike the constant proportional rate of decay that would be observed if forgetting were exponential in form). This property is also enshrined in Jost’s (5) second law of forgetting, which states that, for two memories of the same strength but different ages, the older will decay more slowly than the younger. Ribot’s (6) law of retrograde amnesia provides a possible explanation of why that might be. This law states that, as memories age, they become more resistant to the effects of disruptive forces like brain damage, electroconvulsive shock, and (one might reasonably assume) interference caused by new learning. Such increasing resistance to disruption may reflect the fact that memories consolidate and therefore stabilize over time (7, 8). As Squire and Kandel (7) put it: “A memory that has become consolidated is robust and resistant to interference. In its initial stages, even memory that would otherwise persist is highly susceptible to disruption if, for example, an attempt is made to learn some other similar material” (p. 4). Conceivably, the consolidation of long-term potentiation in the hippocampus in the hours after learning (synaptic consolidation) and the later consolidation of memories in cortex (systems consolidation) both have the effect of stabilizing memories, making them resistant to Fig. 1. Results of experiment 1. Following retrieval practice for half the items (selective retrieval), recall of both the tested and untested items was enhanced relative to recall of the studied items. However, only the tested items exhibited a reduced rate of forgetting from that point forward.