Encoding failure or losing what was stored?
The key distinction is between a memory that never gets properly formed and one that is formed but then fades unusually fast. Encoding is the initial process of taking in information; consolidation is the process that stabilizes a memory after encoding. If either fails, a person may forget because the memory was never solidly stored. Faster long-term decay, by contrast, means the memory was stored but is lost more quickly than usual.
A study that directly addressed this in amnestic mild cognitive impairment used a lag-based continuous recognition task, which tests whether a person can recognize an item after varying delays. The finding was rapid forgetting of information that had been encoded, pointing to accelerated loss of stored memories rather than only a failure to encode them in the first place [4]. That is direct evidence against the idea that encoding and consolidation failure alone explain forgetting in MCI.
Why consolidation still matters
Other work frames forgetting in MCI as a consolidation problem. A study of accelerated long-term forgetting in MCI patients and normal controls treated ALF as a memory consolidation deficit, and its results are consistent with memory decline in MCI involving consolidation difficulties [1]. However, that study did not directly compare encoding or consolidation failure with faster long-term decay, so it supports consolidation as a factor without ruling out rapid decay.
A computational phenotyping study also used a model of memory consolidation and forgetting to detect MCI and monitor memory dysfunction remotely [3]. Because the model includes both consolidation and forgetting, the study is compatible with either mechanism contributing, but it was not designed to decide between them. These findings keep consolidation in the picture without showing that it is the only or main cause of forgetting.
What working memory and brain structure add
Working memory studies in MCI have examined whether forgetting contributes to impaired performance by varying the retention interval—the time between seeing something and having to recall it [2]. That design can reveal faster loss over short delays, but the study did not explicitly compare encoding or consolidation failure with long-term decay, so it does not settle the long-term question. It does suggest that forgetting processes are relevant to memory deficits in MCI, not just initial encoding.
Hippocampal imaging across the MCI-to-Alzheimer's continuum has linked structural changes to memory performance, including measures of long-term retention and forgetting [5]. This is consistent with memory decline having a biological basis in the hippocampus, but it does not identify whether the underlying mechanism is failed encoding, failed consolidation, or faster decay. Structural evidence therefore does not resolve the mechanism question on its own.
So what should a curious reader take away?
The most direct evidence available indicates that forgetting in MCI can involve rapid loss of memories that were successfully encoded, not only a failure to encode or consolidate them [4]. At the same time, studies framed around consolidation problems [1] and computational models that include both consolidation and forgetting [3] suggest that consolidation difficulties may also play a role. The evidence does not support a single simple answer that forgetting is caused entirely by encoding and consolidation failure rather than faster long-term decay.
A reasonable reading is that both mechanisms may contribute, with the balance varying by task and by person. The direct comparison study found rapid forgetting in amnestic MCI [4], while other studies are consistent with consolidation deficits but did not test the alternatives head-to-head [1][3]. For a general reader, the practical point is that forgetting in MCI is not just about information never getting in; it can also be about stored information being lost too quickly.
Sources used in this answer
Accelerated long‐term forgetting‐ALF in mild cognitive impairment: a sensitive tool for subtil memory problems
Accelerated long-term forgetting in MCI is consistent with a memory consolidation problem, but the study did not directly compare consolidation failure with faster long-term decay.
Working memory in mild cognitive impairment and Alzheimer's disease: contribution of forgetting and predictive value of complex span tasks.
Working memory deficits in MCI involve retention-interval effects that suggest forgetting contributes, but the study did not explicitly compare encoding/consolidation failure with long-term decay.
Reliable detection and continuous monitoring of memory dysfunction in mild cognitive impairment and healthy aging through adaptive computational phenotyping
A computational model of memory consolidation and forgetting can detect MCI and monitor memory dysfunction, but it does not directly test whether encoding/consolidation failure or faster decay explains forgetting.
Pattern separation and pattern completion in Alzheimer's disease: evidence of rapid forgetting in amnestic mild cognitive impairment
In amnestic MCI, a lag-based recognition task found evidence of rapid forgetting of encoded information, indicating that forgetting is not solely due to encoding failure.
MRI asymmetry index of hippocampal subfields increases through the continuum from the mild cognitive impairment to the Alzheimer's disease
Hippocampal subfield asymmetry increases along the MCI-to-Alzheimer's continuum and is associated with memory performance, including long-term retention and forgetting measures, but does not identify the mechanism of forgetting.