consolidation

Children learn. It’s what they do. And they build themselves over the years from wide-eyed baby to a person that walks and talks and can maybe fix your computer, so it’s no wonder that we have this idea that learning comes so much more easily to them than it does to us. But is it true?

There are two particular areas where children are said to excel: learning language, and learning skills.

Years ago I reported on a 2003 study that challenged the widespread view that young children learn language more easily than anyone older, in regard to vocabulary. Now a new study suggests that the idea doesn’t apply to grammar-learning either.

In the study, 24 Israeli students aged 8, 12, or 21, were given ten daily lessons in a made-up language. A rule in the language — not made explicit to the students — was that verbs were spelled and pronounced differently depending on whether they referred to an animate or inanimate object. In the lessons, the students were asked to listen to a list of correct noun-verb pairs, and then say the correct verb when given further nouns. Two months later, the students were tested on what they remembered.

The young adults were significantly faster at learning and more accurate than the other groups. Moreover, the 8-year-olds never succeeded in transferring the rule to new examples (even when they were given additional training, with the rule made more obvious), while most 12-year-olds and adults scored over 90%, with the adults doing best. It’s also noteworthy (given popular belief) that children's pronunciation was inferior to that of older subjects.

The findings point to the importance of explicit learning, as well as indicating that language skills are not reduced post-puberty, as has been suggested. So why does it seem more difficult for most adults to learn a new language? The problem may lie with interference from the native (or indeed any other) language.

I’ll get back to that. Let’s move on to the related question of procedural memory, or skill learning.

Here’s a study in which we learn something truly fascinating about interference. In the study, 74 young people (aged 9, 12, and 17) were trained on a finger-tapping task, then tested on the two following days. Some of the participants were further tested six weeks later. In a second experiment, 54 similarly-aged people had the same training, but also given an additional training session two hours later, during which the motor sequence to be learned was the reverse of that practiced in the initial session. They were then tested, 24 hours later, on the first sequence.

In the first experiment, all age-groups improved steadily during training, in both speed and accuracy, and showed jumps in performance when tested 24 hours later (such jumps are typical in procedural learning and are referred to as ‘off-line gains’; they are assumed to reflect memory consolidation).

These jumps were maintained or improved at 48 hours, and six weeks. The gains were the same for each age-group, but there was a clear difference between the groups in terms of their starting point, with the older ones performing noticeably better initially. Because the effect of practice was the same for all, the performance difference between each age-group was the same at each point in time.

It is worth emphasizing that performance six weeks after the experience was the same, and sometimes better, despite the lack of practice over that time.

So these results challenge the view that children have an advantage over adults in terms of learning skills, and also demonstrate that children improve “off-line” as adults do, indicating that they too have an effective consolidation phase in motor memory.

But the second experiment is the really interesting bit. You would expect, if you learned one sequence and then learned the reverse, that this would interfere badly with your memory for the first sequence. And so it did, for the 17-year-olds. But not for the 9- and 12-year-olds, who both showed a performance gain at 24 hours, as seen in the first experiment.

Moreover, the better the 17-year-olds became at the reverse sequence, the worse their performance on the initial sequence at the 24-hour test (as you’d expect) — but for the 12-year-olds, the better they were on the reverse sequence, the better they did on the first sequence at the 24-hour test.

What does this mean? Why didn’t interference occur in the pre-pubertal children?

It appears that the consolidation occurring in children is different in some way from that occurring in adults.

There are several possibilities. It may be that the consolidation process becomes, post-puberty, more selective. In the situation where there are several different experiences, priority is given to the more recent. It may also be that consolidation simply occurs faster in children.

One mechanism of change may occur through sleep. The structure of sleep changes during puberty, and we don’t yet know whether consolidation occurs during sleep in children as it does in adults. Another is competition for neural resources (transcription and protein synthesis related factors) during consolidation. It has been suggested that this “competitive maintenance” only fully matures at puberty.

On the other hand, it may have to do with the effects of experience. Interference only occurs when tasks overlap at some point. If children are representing the movement sequences in a more specific, less abstract, way than adults, the sequences may be less likely to use the same neurons (e.g. adults are learning a rule; children are learning two different ways of moving particular fingers). Accordingly, training on the reverse sequence provides additional training in the art of moving these fingers in this way, but doesn’t interfere because the pattern is not the same.

Interference is the bug-bear of learning. Interference may be the key to why learning gets harder the older we get — despite a number of advantages. So let’s explore this a little more.

Here’s a small study in which 14 young adults (average age 20) and 12 older adults (average age 58; range 55-70) learned a motor sequence task requiring them to press the appropriate button when they saw a blue dot appear in one of four positions on the screen. The training included several learnable sequences interspersed with random trials. Participants, however, were not informed of this. There were three blocks of trials during the first session (separated by a 1-2 minute rest), and a fourth block on the second session, 24 hours later.

As expected, younger adults were notably faster in their responses than the older group. Less expected was the fact that the older group showed markedly greater improvement on the learnable sequences than the younger group. However, on the second session, while the younger adults showed the expected off-line gain in performance, indicative of consolidation, the older adults performed at the same level as they had early in the first session.

It should be noted that the average reaction time of the older group in the very last session matched the reaction time of the younger group in the first sessions, demonstrating that, while we may slow down with age, we can counter that with training. The fact that the older adults were noticeably better at learning the sequences may reflect the increases in activation seen in motor regions in normal aging, possibly compensating for decreased activation and atrophy in the hippocampus.

But what’s interesting in this context is this lack of off-line gain.

The same thing was seen in another study comparing younger and older adults, which found that, while the older adults showed improvement in general skill on an implicit sequence-learning task after 12 hours, this improvement had disappeared at 24 hours. Nor was it seen at one week.

So why aren’t these memories being consolidated in the older adults?

(This is not to say that all benefit of the earlier training was lost — the improvement over the second session indicates that some memory was retained. So it may be — and is consistent with what we know about the effects of training in older adults — that more, and perhaps longer, training sessions are needed before older adults can properly consolidate new learning.)

Is this because we become slower to consolidate with age? This harks back to the idea that children suffer less interference because they can consolidate memories more swiftly.

Or perhaps it has to do with the greater interference attendant on the brains of older adults being more richly-connected. A computer model mimicked a decline in language learning as a function of the growth in connectivity in the neural network. This computational model suggests that once connectivity in the parts of the brain responsible for procedural memory slows, learning suffers increasingly from first-language interference.

It may be, of course, that both processes are going on. Greater interference, and slower consolidation.

It may also be that the adult brain becomes more selective in the making of long-term skill memory.

It may also be that these (and other) changes in the adult brain lead to more interaction between information-sets that are further apart (see my recent news item on preventing interference). Thus, if you learn something at ten in the morning, and something else at twelve, your brain can, and will, try to relate the two (which can be good or bad). A child’s brain can’t stretch to encompass that. They would need to be explicitly reminded of the first lesson.

I suspect that all these factors are important, and point to ways in which we should approach learning/teaching differently for pre-pubertal children, young adults, and older adults.

In the case of older adults, it is clear that we need to provide the optimal conditions for consolidation.

I have talked repeatedly about the value of spaced training, distributed training, interleaved training. So it’s interesting to note that studies have found that consolidation of motor memories occurs differently depending on whether training occurs in blocks (each sequence mastered before learning another one) or on a random schedule involving all sequences.

Off-line learning is better when motor skills are learned under a random practice schedule. While blocked practice produces better immediate learning, random practice produces better delayed learning. It appears that a random schedule generates activity across a broad network involving premotor, parietal, sensorimotor and subcortical regions, while learning under the blocked schedule is limited to a more confined area (specifically one particular part of the motor cortex).

This suggests that interleaved practice is even more important for older adults. Although it slows down initial learning (which, remember, was better for older adults compared to younger, so there’s leeway there!), spreading the load across a broader neural network is especially important for those who have some atrophy or impairment in specific regions (as often occurs with age).

Judicious resting during learning may also be of greater benefit for older adults. Consolidation occurs most famously during sleep (and let’s not forget how sleep changes in old age), and also occurs to a lesser level while awake, within a few hours of training. But there is also evidence that a boost in skill learning can occur after rests that only last a few minutes (or even seconds). This phenomenon is distinguished from consolidation (it’s called ‘reminiscence’), because the gains in performance don’t usually endure. However, while in some circumstances it may simply reflect recovery from mental or physical fatigue, in others it may have a more lasting effect.

 Evidence for this has come from learning in music. A particularly interesting study involved non-musicians learning a five-key sequence on a digital piano. It found that even 5-minute rests during learning could be beneficial, but only if they occurred at the right time.

In the study, the participants repeated the sequence as fast and accurately as they could during twelve 30-second blocks interspersed with 30-s pauses. A third of the participants had a 5 minute rest between the third and fourth block, while another third had the rest between the ninth and tenth block, and the remaining third had no rest at all. Everyone was re-tested the next day, around 12 hours after training.

Participants showed large improvements during training after either 5-minute rest. However it was only those who were given a rest early in the training that continued to show improvement throughout the training. That is, even though the late-rest group matched their performance on block 10, after this ‘jump’ their performance fell on blocks 11 and 12, while the performance of the early-rest group continued to climb after their jump (at block 4). This group also showed the greatest off-line gain. That is, their performance ‘jumped’ more than that of the other two groups when tested on the following day.

In other words, consolidation was affected by the timing of the rest.

Among the late-rest and no-rest groups, improvement during blocks 4-9 was not as rapid as it had been during the first three blocks. This is a typical pattern during motor learning. It may be, then, that resting early allows processes triggered by repetition to develop fully, rather than becoming attenuated through too much repetition. Thus resting early in practice may allow the faster rate of learning to continue for longer. This in turn results in greater repetition before practice ends, leading to a more stabilized (short-term consolidated) memory, and thus greater overnight (long-term) consolidation.

On the other hand, the short-lasting gain achieved by the late-rest group didn’t affect later learning, but did predict the extent to which performance improved after sleep.

Other improvements to learning may come from reducing interference, and taking cognizance of greater selectivity. In the realm of language learning, for example, it’s argued that successful long-term learning in adults is more and more dependent on explicit learning, declarative knowledge, and its automatization. It may be that, for adults learning a second language, greater importance should be placed on explicit comparison with the native language.

It also seems likely that immersion in the new language is more important for adult learners. The problem is that every time you return to your native language, you’re encouraging interference (something to which, as we have seen, children may be far less susceptible).

In sum, as we get older, interference becomes more of an issue. To counter this, we need to be more thoughtful about planning our learning.

 

For more about the recently reported research into the difference between children's and adults' language learning, see

http://www.newscientist.com/article/mg21128224.000-age-no-excuse-for-failing-to-learn-a-new-language.html

http://blogs.edweek.org/edweek/inside-school-research/2011/07/study_older_students_may_learn.html

On my walk today, I listened to a downloaded interview from the On Being website. The interview was with ‘vocal magician and conductor’ Bobby McFerrin, and something he said early on in the interview really caught my attention.

In response to a question about why he’d once (in his teens) contemplated joining a monastic order, he said that the quiet really appealed to him, and also ‘the discipline of the hours … there’s a rhythm to the day. I liked the fact that you stopped whatever you were doing at a particular time and you reminded yourself, you brought yourself back to your calling’.

Those words resonated with me, and they made me think of the Moslem habit of prayer. Of the idea of having specified times during the day when you stop your ‘ordinary’ life, and touch base, as it were, with something that is central to your being.

I don’t think you need to be a monk or a Moslem to find value in such an activity! Nor does the activity need to be overtly religious.

Because this idea struck another echo in me — some time ago I wrote a brief report on how even a short ‘quiet time’ can help you consolidate your memories. It strikes me that developing the habit of having fixed points in the day when (if at all possible) you engage in some regular activity that helps relax you and center your thoughts, would help maintain your focus during the day, and give you a mental space in which to consolidate any new information that has come your way.

Appropriate activities could include:

• meditating on your breath;
• performing a t’ai chi routine;
• observing nature;
• listening to certain types of music;
• singing/chanting some song/verse (e.g., the Psalms; the Iliad; the Tao te Ching)

Regarding the last two suggestions, as I reported in my book on mnemonics, there’s some evidence that reciting the Iliad has physiological effects on synchronizing heartbeat and breath that is beneficial for both mood and cognitive functioning. It’s speculated that the critical factor might be the hexametric pace (dum-diddy, dum-diddy, dum-diddy, dum-diddy, dum-diddy, dum-dum). Dactylic hexameter, the rhythm of classical epic, has a musical counterpart: 6/8 time.

Similarly, another small study found that singing Ave Maria in Latin, or chanting a yoga mantra, likewise affects brain blood flow, and the crucial factor appeared to be a rhythm that involved breathing at the rate of six breaths a minute.

Something to think about!

"Consolidation" is a term that is bandied about a lot in recent memory research. Here's my take on what it means.

Becoming a memory

Initially, information is thought to be encoded as patterns of neural activity — cells "talking" to each other. Later, the information is coded in more persistent molecular or structural formats (e.g., the formation of new synapses). It has been assumed that once this occurs, the memory is "fixed" — a permanent, unchanging, representation.

With new techniques, it has indeed become possible to observe these changes (you can see videos here). Researchers found that the changes to a cell that occurred in response to an initial stimulation lasted some three to five minutes and disappeared within five to 10 minutes. If the cell was stimulated four times over the course of an hour, however, the synapse would actually split and new synapses would form, producing a (presumably) permanent change.

Memory consolidation theory

The hypothesis that new memories consolidate slowly over time was proposed 100 years ago, and continues to guide memory research. In modern consolidation theory, it is assumed that new memories are initially 'labile' and sensitive to disruption before undergoing a series of processes (e.g., glutamate release, protein synthesis, neural growth and rearrangement) that render the memory representations progressively more stable. It is these processes that are generally referred to as “consolidation”.

Recently, however, the idea has been gaining support that stable representations can revert to a labile state on reactivation.

Memory as reconstruction

In a way, this is not surprising. We already have ample evidence that retrieval is a dynamic process during which new information merges with and modifies the existing representation — memory is now seen as reconstructive, rather than a simple replaying of stored information

Reconsolidation of memories

Researchers who have found evidence that supposedly stable representations have become labile again after reactivation, have called the process “reconsolidation”, and suggest that consolidation, rather than being a one-time event, occurs repeatedly every time the representation is activated.

This raises the question: does reconsolidation involve replacing the previously stable representation, or the establishment of a new representation, that coexists with the old?

Whether reconsolidation is the creating of a new representation, or the modifying of an old, is this something other than the reconstruction of memories as they are retrieved? In other words, is this recent research telling us something about consolidation (part of the encoding process), or something about reconstruction (part of the retrieval process)?

Hippocampus involved in memory consolidation

The principal player in memory consolidation research, in terms of brain regions, is the hippocampus. The hippocampus is involved in the recognition of place and the consolidation of contextual memories, and is part of a region called the medial temporal lobe (MTL), that also includes the perirhinal, parahippocampal,and entorhinal cortices. Lesions in the medial temporal lobe typically produce amnesia characterized by the disproportionate loss of recently acquired memories. This has been interpreted as evidence for a memory consolidation process.

Some research suggests that the hippocampus may participate only in consolidation processes lasting a few years. The entorhinal cortex, on the other hand, gives evidence of temporally graded changes extending up to 20 years, suggesting that it is this region that participates in memory consolidation over decades. The entorhinal cortex is damaged in the early stages of Alzheimer’s disease.

There is, however, some evidence that the hippocampus can be involved in older memories — perhaps when they are particularly vivid.

A recent idea that has been floated suggests that the entorhinal cortex, through which all information passes on its way to the hippocampus, handles “incremental learning” — learning that requires repeated experiences. “Episodic learning” — memories that are stored after only one occurrence — might be mainly stored in the hippocampus.

This may help explain the persistence of some vivid memories in the hippocampus. Memories of emotionally arousing events tend to be more vivid and to persist longer than do memories of neutral or trivial events, and are, moreover, more likely to require only a single experience.

Whether or not the hippocampus may retain some older memories, the evidence that some memories might be held in the hippocampus for several years, only to move on, as it were, to another region, is another challenge to a simple consolidation theory.

Memory more complex than we thought

So where does all this leave us? What is consolidation? Do memories reach a fixed state?

My own feeling is that, no, memories don't reach this fabled "cast in stone" state. Memories are subject to change every time they are activated (such activation doesn't have to bring the memory to your conscious awareness). But consolidation traditionally (and logically) refers to encoding processes. It is reasonable, and useful, to distinguish between:

• the initial encoding, the "working memory" state, when new information is held precariously in shifting patterns of neural activity,
• the later encoding processes, when the information is consolidated into a more permanent form with the growth of new connections between nerve cells,
• the (possibly much) later retrieval processes, when the information is retrieved in, most probably, a new context, and is activated anew

I think that "reconsolidation" is a retrieval process rather than part of the encoding processes, but of course, if you admit retrieval as involving a return to the active state and a modification of the original representation in line with new associations, then the differences between retrieval and encoding become less evident.

When you add to this the possibility that memories might "move" from one area of the brain to another after a certain period of time (although it is likely that the triggering factor is not time per se), then you cast into disarray the whole concept of memories becoming stable.

Perhaps our best approach is to see memory as a series of processes, and consolidation as an agreed-upon (and possibly arbitrary) subset of those processes.