Spacing Effect

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Trying to learn two different things one after another is challenging. Almost always some of the information from the first topic or task gets lost. Why does this happen? A new study suggests the problem occurs when the two information-sets interact, and demonstrates that disrupting that interaction prevents interference. (The study is a little complicated, but bear with me, or skip to the bottom for my conclusions.)

In the study, young adults learned two memory tasks back-to-back: a list of words, and a finger-tapping motor skills task. Immediately afterwards, they received either sham stimulation or real transcranial magnetic stimulation to the dorsolateral prefrontal cortex or the primary motor cortex. Twelve hours later the same day, they were re-tested.

As expected from previous research, word recall (being the first-learned task) declined in the control condition (sham stimulation), and this decline correlated with initial skill in the motor task. That is, the better they were at the second task, the more they forgot from the first task. This same pattern occurred among those whose motor cortex had been stimulated. However, there was no significant decrease in word recall for those who had received TMS to the dorsolateral prefrontal cortex.

Learning of the motor skill didn't differ between the three groups, indicating that this effect wasn't due to a disruption of the second task. Rather, it seems that the two tasks were interacting, and TMS to the DLPFC disrupted that interaction. This hypothesis was supported when the motor learning task was replaced by a motor performance task, which shouldn’t interfere with the word-learning task (the motor performance task was almost identical to the motor learning task except that it didn’t have a repeating sequence that could be learned). In this situation, TMS to the DLPFC produced a decrease in word recall (as it did in the other conditions, and as it would after a word-learning task without any other task following).

In the second set of experiments, the order of the motor and word tasks was reversed. Similar results occurred, with this time stimulation to the motor cortex being the effective intervention. In this case, there was a significant increase in motor skill on re-testing — which is what normally happens when a motor skill is learned on its own, without interference from another task (see my blog post on Mempowered for more on this). The word-learning task was then replaced with a vowel-counting task, which produced a non-significant trend toward a decrease in motor skill learning when TMS was applied to the motor cortex.

The effect of TMS depends on the activity in the region at the time of application. In this case, TMS was applied to the primary motor cortex and the DLPFC in the right hemisphere, because the right hemisphere is thought to be involved in integrating different types of information. The timing of the stimulation was critical: not during learning, and long before testing. The timing was designed to maximize any effects on interference between the two tasks.

The effect in this case mimics that of sleep — sleeping between tasks reduces interference between them. It’s suggested that both TMS and sleep reduce interference by reducing the communication between the prefrontal cortex and the mediotemporal lobe (of which the hippocampus is a part).

Here’s the problem: we're consolidating one set of memories while encoding another. So, we can do both at the same time, but as with any multitasking, one task is going to be done better than the other. Unsurprisingly, encoding appears to have priority over consolidation.

So something needs to regulate the activity of these two concurrent processes. Maybe something looks for commonalities between two actions occurring at the same time — this is, after all, what we’re programmed to do: we link things that occur together in space and time. So why shouldn’t that occur at this level too? Something’s just happened, and now something else is happening, and chances are they’re connected. So something in our brain works on that.

If the two events/sets of information are connected, that’s a good thing. If they’re not, we get interference, and loss of data.

So when we apply TMS to the prefrontal cortex, that integrating processor is perhaps disrupted.

The situation may be a little different where the motor task is followed by the word-list, because motor skill consolidation (during wakefulness at least) may not depend on the hippocampus (although declarative encoding does). However, the primary motor cortex may act as a bridge between motor skills and declarative memories (think of how we gesture when we explain something), and so it may this region that provides a place where the two types of information can interact (and thus interfere with each other).

In other words, the important thing appears to be whether consolidation of the first task occurs in a region where the two sets of information can interact. If it does, and assuming you don’t want the two information-sets to interact, then you want to disrupt that interaction.

Applying TMS is not, of course, a practical strategy for most of us! But the findings do suggest an approach to reducing interference. Sleep is one way, and even brief 20-minute naps have been shown to help learning. An intriguing speculation (I just throw this out) is that meditation might act similarly (rather like a sorbet between courses, clearing the palate).

Failing a way to disrupt the interaction, you might take this as a warning that it’s best to give your brain time to consolidate one lot of information before embarking on an unrelated set — even if it's in what appears to be a completely unrelated domain. This is particularly so as we get older, because consolidation appears to take longer as we age. For children, on the other hand, this is not such a worry. (See my blog post on Mempowered for more on this.)

[2338] Cohen, D. A., & Robertson E. M.
(2011).  Preventing interference between different memory tasks.
Nat Neurosci. 14(8), 953 - 955.

In the first study, undergraduates studied English-Lithuanian word pairs, which were displayed on a screen one by one for 10 seconds. After studying the list, the students practiced retrieving the English words — they had 8 seconds to type in the English word as each Lithuanian word appeared, and those that were correct went to the end of the list to be asked again, and those wrong had to be restudied. Each item was pre-assigned a "criterion level" from one to five — the number of times it needed to be correctly recalled during practice.

In the first experiment, participants took one of four recall tests and one of three recognition tests after a 2-day delay. In the second experiment, in order to eliminate the reminder effect of the recall test, participants were only given a recognition test, after a 1-week delay.

Both experiments found that higher criterion levels led to better memory. More importantly, through the variety of tests, they showed that this occurred on all three kinds of memory tested: associative memory; target memory; cue memory. That is, practicing retrieval of the English word didn’t just improve memory for that word (the target), but also for the Lithuanian word (the cue), and the pairing (association).

While this may seem self-evident to some, it has been thought that only the information being retrieved is strengthened by retrieval practice. The results also emphasize that it is the correct retrieval of the information that improves memory, not the number of times the information is studied.

In a related study, 533 students learned conceptual material via retrieval practice across three experiments. Criterion levels varied from one to four correct retrievals in the initial session. Items also varied in how many subsequent sessions they were exposed to. In one to five testing/relearning sessions, the items were practiced until they were correctly recalled once. Memory was tested one to four months later.

It was found that the number of times items were correctly retrieved on the initial session had a strong initial effect, but this weakened as relearning increased. Relearning had pronounced effects on long-term retention with a relatively minimal cost in terms of additional practice trials.

On the basis of their findings, the researchers recommend that students practice recalling concepts to an initial criterion of three correct recalls and then relearn them three times at widely spaced intervals.

[2457] Vaughn, K. E., & Rawson K. A.
(2011).  Diagnosing Criterion-Level Effects on Memory.
Psychological Science.

Rawson, K.A. & Dunlosky, J. 2011. Optimizing schedules of retrieval practice for durable and efficient learning: How much is enough? Journal of Experimental Psychology: General, Jun 27, 2011, No Pagination Specified. doi: 10.1037/a0023956

I’ve spoken often about the spacing effect — that it’s better to spread out your learning than have it all massed in a block. A study in which mice were trained on an eye movement task (the task allowed precise measurement of learning in the brain) compared learning durability after massed training or training spread over various spaced intervals (2.5 hours to 8 days, with 30 minute to one day intervals). In the case of massed training, the learning achieved at the end of training disappeared within 24 hours. However learning gained in spaced training did not.

Moreover, when a region in the cerebellum connected to motor nuclei involved in eye movement (the flocculus) was anesthetized, the learning achieved from one hour of massed training was eliminated, while learning achieved from an hour of training spaced out over four hours was unaffected. This suggests that the memories had been transferred out of the flocculus (to the vestibular nuclei) within four hours.

However, when protein synthesis in the flocculus was blocked, learning from spaced training was impaired, while learning from massed training was not. This suggests that proteins synthesized in the flocculus play a vital part in the transfer to the vestibular nuclei.

I’ve talked about the importance of retrieval practice at length, so I’m pleased to report on the latest study to confirm its value. Indeed, this study demonstrates that practicing retrieval is a more effective strategy than elaborative studying.

In two studies, a total of 200 students studied texts on topics from different science disciplines. One group engaged in elaborative studying by creating concept maps. The second group read the texts, then put the material away and practiced recalling the concepts from the text. Both groups performed at about the same level on a test at the end of the study period. However, when the students were tested again a week later, the group that studied by practicing retrieval performed 50% better than the group that studied by creating concept maps.

The test involved understanding as well as memory, with some of the questions asking them to draw connections between things that weren't explicitly stated in the material.

The study also confirms that most students are poor at judging the success of their study habits. Asked to predict which technique would produce better results, most thought that concept mapping would be superior.

The findings should certainly not be taken as a slur on concept mapping, which is a study strategy of proven effectiveness. Moreover, while concept mapping can be used solely as an elaborative study method (as it was in these experiments), it can also be used as a retrieval practice technique.

An intriguing new study has found that people are more likely to remember specific information if the pattern of activity in their brain is similar each time they study that information. The findings are said to challenge the long-held belief that people retain information more effectively when they study it several times under different contexts, thus giving their brains multiple cues to remember it. However, although I believe this finding adds to our understanding of how to study effectively, I don’t think it challenges the multiple-context evidence.

The finding was possible because of a new approach to studying brain activity, which was used in three experiments involving students at Beijing Normal University. In the first, 24 participants were shown 120 faces, each one shown four times, at variable intervals between the repetitions. They were tested on their recognition (using a set of 240 faces), and how confident they were in their decision, one hour later. Subsequent voxel-by-voxel analysis of 20 brain regions revealed that the similarity of the patterns of brain activity in nine of those regions for each repetition of a specific face was significantly associated with recognition.

In the second experiment, 22 participants carried out a semantic judgment task on 180 familiar words (deciding whether they were concrete or abstract). Each word was repeated three times, again at variable intervals. The participants were tested on their recall of the words six hours later, and then tested for recognition. Fifteen brain regions showed a higher level of pattern similarity across repetitions for recalled items, but not for forgotten items.

In the third experiment, 22 participants performed a different semantic judgment task (living vs non-living) on 60 words. To prevent further encoding, they were also required to perform a visual orientation judgment task for 8 seconds after each semantic judgment. They were given a recall test 30 minutes after the session. Seven of the brain regions showed a significantly higher level of pattern similarity for recalled items.

It's interesting to observe how differences in the pattern of activity occurred when studying the same information only minutes apart — a difference that is presumed to be triggered by context (anything from the previous item to environmental stimuli or passing thoughts). Why do I suggest that this finding, which emphasizes the importance of same-context, doesn’t challenge the evidence for multiple-context? I think it’s an issue of scope.

The finding shows us two important things: that context changes constantly; that repetition is made stronger the closer context is matched. Nevertheless, this study doesn’t bear on the question of long-term recall. The argument has never been that multiple contexts make a memory trace stronger; it has been that it provides more paths to recall — something that becomes of increasing importance the longer the time between encoding and recall.

A new study explains why variable practice improves your memory of most skills better than practice focused on a single task. The study compared skill learning between those asked to practice one particular challenging arm movement, and those who practiced the movement with other related tasks in a variable practice structure. Using magnetic stimulation applied to different parts of the brain after training (which interferes with memory consolidation), it was found that interference to the dorsolateral prefrontal cortex, but not to the primary motor cortex, affected skill learning for those engaged in variable practice, whereas interference to the motor cortex, but not to the prefrontal cortex, affected learning in those engaged in constant practice.

These findings indicate that variable practice involves working memory (which happens in the prefrontal cortex) rather than motor memory, and that the need to re-engage with the task each time underlies the better learning produced by variable practice (which involves repeatedly switching between tasks). The experiment also helps set a time frame for this consolidation — interference four hours after training had no effect.

Loss of memory and problems with judgment in dementia patients can cause difficulties in relation to eating and nutrition; these problems in turn can lead to poor quality of life, pressure ulcers and infections. A study used two different step-by-step training programs to help dementia patients regain eating skills. Three institutions, involving 85 patients, were assigned to one of three programs: spaced retrieval training; Montessori-based training; control. Training consisted of three 30-40 min sessions per week, for 8 weeks. Both training programs resulted in significantly improved feeding skills, however the Montessori group needed more physical and verbal assistance. Nutritional status was significantly higher in the spaced-retrieval group compared to the control.

Lin, L., Huang, Y., Su, S., Watson, R., Tsai, B. W., & Wu, S. (2010). Using spaced retrieval and Montessori-based activities in improving eating ability for residents with dementia. International Journal of Geriatric Psychiatry, 9999(9999), n/a. doi: 10.1002/gps.2433.

It has long been known that spacing practice (reviewing learning or practicing a skill at spaced intervals) is far more effective than massed practice (in one heavy session). It is also well-known that people commonly over-estimate the value of massed practice, and tend not to give due recognition to the value of spacing practice, despite the fact that most memory improvement and study programs advise it.

Many learning strategies require extensive training. The advantage of spaced practice is that it does not. Experience with it may also result in better self-appraisal about how well information has been learned.

In this study, a class of 708 students were given instruction sheets on which was written a 7-digit number purporting to be a phone number. The students were instructed to memorize the number and told their recall would be tested later in the term. Half the class were told to memorize the number however they usually would. The other half were told to post the number where they would see it, and look at it once or twice a day for a week. They were told this would be an effective way of learning the number.

A significantly greater number of students from the spaced-practice group remembered the number correctly two weeks later (72.7% compared to 61% of the control group). According to the questions they answered, some 11.6% of the spaced-practice group in fact did all their studying in a single session, and only 46.4% studied the number on 3 or more days. Some 18% of the control group also studied the number on 3 or more days. In other words, being in the spaced-practice group doesn't necessarily mean spaced practice was used, nor does being a member of the control group mean that spaced-practice wasn't used.

Clearly, simple instructions to use spaced practice improve memory, but equally clearly, many people are not necessarily going to follow those instructions, for whatever reason.

Landauer, T.K. & Ross, B.H. (1977). Can simple instructions to use spaced practice improve ability to remember a fact? An experimental test using telephone numbers. Bulletin of the Psychonomic Society, 10, 215-218.

Older news items (pre-2010) brought over from the old website

The smart way to study

A large internet study has clarified the optimal timing for spacing out your learning. The very systematic study found much larger benefits to spacing your review of material than has been seen in earlier research when shorter intervals have been used. Given a fixed amount of study time, the optimal gap improved recall by 64% and recognition by 26%. Basically, the study found that if you want to remember just for a week, the optimal gap was one day; for remembering for a month, it was 11 days; for 2 months (70 days) it was 3 weeks, and similarly for remembering for a year. Extrapolating, it seems likely that if you’re wanting to remember information for several years, you should review it over several months. (You can read more about this study in my article on the most effective way of spacing your learning).

[872] Cepeda, N. J., Vul E., Rohrer D., Wixted J. T., & Pashler H.
(2008).  Spacing effects in learning: a temporal ridgeline of optimal retention.
Psychological Science: A Journal of the American Psychological Society / APS. 19(11), 1095 - 1102.

Cramming doesn't work in the long term

Thinking back on how much you remember from your schooldays, it’s apparent to most of us that despite all the time spent in school, we’ve forgotten most of what we learned. A new study points to what we were doing wrong. The study looked at overlearning, which is the term for continuing to study after you’ve apparently learned it. Students went through a list of new words either five times (getting a perfect score no more than once) or ten times (getting it perfect at least three times). A week later, students who did the extra drilling performed better when tested, but four weeks later there was no difference. The results suggest that overlearning in a single session is wasted effort. However, when the material was studied in two separate sessions, and the break between sessions was at least a month, students did much better. Although the experiments involved rote learning, the researchers have also found similar effects with more abstract learning, like math.

[878] Rohrer, D., & Pashler H.
(2007).  Increasing Retention Without Increasing Study Time.
Current Directions in Psychological Science. 16(4), 183 - 186.

Practicing skills in concentrated blocks not the most efficient way

While practicing several different skills in separate, concentrated blocks leads to better performance during practice, it appears that this approach is not the best method of learning for long-term retention. The temporary improvement in performance that results from blocked practice hinders learning because it allows people to overestimate how well they have learned a skill. For long-term retention, it appears that contextual-interference practice (practicing skills that are mixed with other tasks) results in better learning. This may be because such practice requires people to repeatedly retrieve the motor program corresponding to each task (repeated retrieval is a major factor in making stored memories easier to access). Such practice also requires the person to differentiate the skills in terms of their similarities and differences, which may be assumed to result in a better mental conceptualization of those skills. The fact that blocked practice leads to better short-term performance but poorer long-term learning "has great potential to fool teachers, trainers and instructors as well as students and trainees themselves."

[1167] Simon, D. A., & Bjork R. A.
(2001).  Metacognition in Motor Learning.
Journal of Experimental Psychology: Learning, Memory, and Cognition. 27(4), 907 - 912.