New insights in human memory interference and consolidation

Abstract

Learning new facts and skills in succession can be frustrating because no sooner has new knowledge been acquired than its retention is being jeopardized by learning another set of skills or facts. Interference between memories has recently provided important new insights into the neural and psychological systems responsible for memory processing. For example, interference not only occurs between the same types of memories, but can also occur between different types of memories, which has important implications for our understanding of memory organization. Converging evidence has begun to reveal that the brain produces interference independently from other aspects of memory processing, which suggests that interference may have an important but previously overlooked function. A memory's initial susceptibility to interference and subsequent resistance to interference after its acquisition has revealed that memories continue to be processed 'off-line' during consolidation. Recent work has demonstrated that off-line processing is not limited to just the stabilization of a memory, which was once the defining characteristic of consolidation; instead, off-line processing can have a rich diversity of effects, from enhancing performance to making hidden rules explicit. Off-line processing also occurs after memory retrieval when memories are destabilized and then subsequently restabalized during reconsolidation. Studies are beginning to reveal the function of reconsolidation, its mechanistic relationship to consolidation and its potential as a therapeutic target for the modification of memories.

Figure 1

Memory interference. (A) Immediately following its acquisition or retrieval a memory (M) is susceptible to interference, and so a disruptive technique such as applying TMS (coil) over a particular brain area can impair recall. (B) By contrast, after acquisition a memory is consolidated, and similarly following its retrieval a memory is reconsolidated making the memory resistant to interference. So, applying TMS several hours (for example +6 hours) after acquisition or retrieval has little or no effect upon recall because by this time the memory has been stabilized and is not longer susceptible to interference.

Figure 2

Examples of memory organization in the human brain. (A) There may be independent procedural (P; for example, a learning motor skill) and declarative (D; for example, learning a word-list) memory systems within the human brain. For example, impairment in declarative learning can occur without any impairment in procedural learning [9]. (B) Alternatively, there may be an interaction between different types of memories. Functional imaging work has shown that there is a correlation between activity within the procedural (the striatum) and declarative memory systems (the MTL, [13]). So, there is a functional coupling between memory systems with activity within one memory system affecting the other, and so it could support interactions between different memories. (C) There may only be a partial overlap between declarative and procedural processing, which would explain word-list learning impairing only a component, but not all of a motor skill [16]. (D) Brain areas can control the interference between memories. When the brain state is altered artificially (blur); for example, by applying TMS to there is no longer interference of a motor skill by a word-list memory (blurred line; [22]). The organization of human memories may also shift naturally depending upon brain state; for example, interference occurs between memories during wakefulness but not during sleep [15,17,21].

Figure 3

Memory interference and the prefrontal cortex. (A) Participants learnt a spatial location task while being exposed to a rose petal odor. The odor was subsequently used to reactivate the spatial memory when participants were either awake or asleep. Immediately after reactivation participants performed an interference task. (B) There was a decrease in memory recall and activation of the DLPFC in the wake group; whereas, there was an increase in recall and little activation of the DLPFC in the sleep group. Thus, for a memory to be susceptible to interference — as shown by a decrease in memory recall — the DLPFC has to be activated. (C) In another set of experiments, participants learnt a word-list (red box; Recall₁) and a motor skill (blue box) in quick succession, sham TMS or real TMS was applied to the DLPFC, and participants recall for the word-list was retested (Recall₂). (D) Interference from the motor skill task caused a substantial decrease in word-recall (Recall₂ – Recall₁) in the sham group. In contrast, this expected decrease in word-recall did not occur following DLPFC stimulation. (E) The correlation between memory interference ( Word recall) and initial motor skill, which was present following sham stimulation, was abolished by applying real stimulation to the DLPFC. Thus, disrupting the function of DLPFC by applying stimulation prevented interference between the tasks.