hippocampus

means "sea horse", and is named for its shape. It is one of the oldest parts of the brain, and is buried deep inside, within the limbic lobe. The hippocampus is important for the forming, and perhaps long-term storage, of associative and episodic memories. Specifically, the hippocampus has been implicated in (among other things) the encoding of face-name associations, the retrieval of face-name associations, the encoding of events, the recall of personal memories in response to smells. It may also be involved in the processes by which memories are consolidated during sleep.

Walking counteracts brain atrophy in older adults

February, 2011
  • Walking 40 minutes a day three days a week prevented ‘normal’ atrophy in the brains of older adults.

Another study has come out proclaiming the cognitive benefits of walking for older adults. Previously sedentary adults aged 55-80 who walked around a track for 40 minutes on three days a week for a year increased the size of their hippocampus, as well as their level of BDNF. Those assigned to a stretching routine showed no such growth. There were 120 participants in the study.

The growth of around 2% contrasts with the average loss of 1.4% hippocampal tissue in the stretching group — an amount of atrophy considered “normal” with age. Although both groups improved their performance on a computerized spatial memory test, the walkers improved more.

The findings are consistent with a number of animal studies showing aerobic exercise increases neurogenesis and BDNF in the hippocampus, and human studies pointing to a lower risk of cognitive decline and dementia in those who walk regularly.

Reference: 

[2097] Erickson, K. I., Voss M. W., Prakash R S., Basak C., Szabo A., Chaddock L., et al.
(Submitted).  Exercise training increases size of hippocampus and improves memory.
Proceedings of the National Academy of Sciences.

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Mindfulness meditation training changes brain structure in 8 weeks

February, 2011

After 8 weeks practicing mindfulness meditation, measurable changes occurred in brain regions associated with memory and emotion.

Brain images of 16 participants in an 8-week mindfulness meditation program, taken two weeks before and after the program, have found measurable changes in brain regions associated with memory, sense of self, empathy and stress. Specifically, they showed increased grey-matter density in the left hippocampus, posterior cingulate cortex, temporo-parietal junction, and cerebellum, as well as decreased grey-matter density in the amygdala. Similar brain scans of a control group of non-meditators (those on a waiting list for the program) showed no such changes over time.

Although a number of studies have found differences in the brains of experienced meditators and those who don’t practice meditation, this is the first to demonstrate that those differences are actually produced by meditation.

The Mindfulness-Based Stress Reduction program involved weekly meetings that included practice of mindfulness meditation and audio recordings for guided meditation practice. Participants reported spending an average of 27 minutes each day practicing mindfulness exercises.

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Predicting memory loss in healthy older adults

February, 2011

Having the ‘Alzheimer’s gene’ and showing reduced brain activity during a mental task combined to correctly predict future cognitive decline in 80% of healthy elders.

In a study in which 78 healthy elders were given 5 different tests and then tested for cognitive performance 18 months later, two tests combined to correctly predict nearly 80% of those who developed significant cognitive decline. These tests were a blood test to identify presence of the ‘Alzheimer’s gene’ (APOE4), and a 5-minute fMRI imaging scan showing brain activity during mental tasks.

The gene test in itself correctly classified 61.5% of participants (aged 65-88; mean age 73), showing what a strong risk factor this is, but when taken with activity on the fMRI test, the two together correctly classified 78.9% of participants. Age, years of education, gender and family history of dementia were not accurate predictors of future cognitive decline. A smaller hippocampus was also associated with a greater risk of cognitive decline.

These two tests are readily available and not time-consuming, and may be useful in identifying those at risk of MCI and dementia.

Reference: 

Woodard, J.L.  et al. 2010. Prediction of Cognitive Decline in Healthy Older Adults using fMRI. Journal of Alzheimer’s Disease, 21 (3), 871-885.

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How taking an active role in learning enhances memory

January, 2011

Being actively involved improves learning significantly, and new research shows that the hippocampus is at the heart of this process.

We know active learning is better than passive learning, but for the first time a study gives us some idea of how that works. Participants in the imaging study were asked to memorize an array of objects and their exact locations in a grid on a computer screen. Only one object was visible at a time. Those in the "active study” group used a computer mouse to guide the window revealing the objects, while those in the “passive study” group watched a replay of the window movements recorded in a previous trial by an active subject. They were then tested by having to place the items in their correct positions. After a trial, the active and passive subjects switched roles and repeated the task with a new array of objects.

The active learners learned the task significantly better than the passive learners. Better spatial recall correlated with higher and better coordinated activity in the hippocampus, dorsolateral prefrontal cortex, and cerebellum, while better item recognition correlated with higher activity in the inferior parietal lobe, parahippocampal cortex and hippocampus.

The critical role of the hippocampus was supported when the experiment was replicated with those who had damage to this region — for them, there was no benefit in actively controlling the viewing window.

This is something of a surprise to researchers. Although the hippocampus plays a crucial role in memory, it has been thought of as a passive participant in the learning process. This finding suggests that it is actually part of an active network that controls behavior dynamically.

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What happens after traumatic brain injury occurs?

December, 2010

Findings from a rat study show how TBI can begin a process that continues to deform the brain long after the original injury.

A rat study using powerful imaging techniques has revealed how an injured brain continues to change long after the original trauma. Widespread decreases in brain functioning over a period of months were seen in specific brain regions, in particular the hippocampus, amygdala, and ipsilateral cortex, even when these were remote from the site of direct trauma and unaccompanied by signs of injury.

The findings indicate that there is a time window during which intervention could reduce these processes and protect against some of the disabling consequences of TBI.

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Chronic jet lag has long-lasting effects on cognition

December, 2010

A hamster study indicates that chronic jet lag changes the brain in ways that cause long-lasting memory and learning problems.

Twice a week for four weeks, female hamsters were subjected to six-hour time shifts equivalent to a New York-to-Paris airplane flight. Cognitive tests taken during the last two weeks of jet lag and a month after recovery from it revealed difficulty learning simple tasks that control hamsters achieved easily. Furthermore, the jet-lagged hamsters had only half the number of new neurons in the hippocampus that the control hamsters had.

The findings support earlier research indicating that chronic jet lag impairs memory and learning and reduces the size of the temporal lobe, and points to the loss of brain tissue as being due to reduced neurogenesis in the hippocampus. Although further research is needed to clarify this, indications are that the problem is not so much fewer neurons being created, but fewer new cells maturing into working cells, or perhaps new cells dying prematurely.

Hamsters are excellent subjects for circadian rhythm research because their rhythms are so precise.

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More evidence for the cognitive benefit of treating sleep apnea

December, 2010

Another study has come out showing the benefits of CPAP treatment for cognitive impairment caused by obstructive sleep apnea.

Comparison of 17 people with severe obstructive sleep apnea (OSA) with 15 age-matched controls has revealed that those with OSA had reduced gray matter in several brain regions, most particularly in the left parahippocampal gyrus and the left posterior parietal cortex, as well as the entorhinal cortex and the right superior frontal gyrus. These areas were associated with deficits in abstract reasoning and executive function. Deficits in the left posterior parietal cortex were also associated with daytime sleepiness.

Happily, however, three months of treatment with continuous positive airway pressure (CPAP), produced a significant increase in gray matter in these regions, which was associated with significant improvement in cognitive function. The researchers suggest that the hippocampus, being especially sensitive to hypoxia and innervation of small vessels, is the region most strongly and quickly affected by hypoxic episodes.

The findings point to the importance of diagnosing and treating OSA.

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Sleep reorganizes your memories

December, 2010

New studies show how sleep sculpts your memories, emphasizing what’s important and connecting it to other memories in your brain.

The role of sleep in consolidating memory is now well-established, but recent research suggests that sleep also reorganizes memories, picking out the emotional details and reconfiguring the memories to help you produce new and creative ideas. In an experiment in which participants were shown scenes of negative or neutral objects at either 9am or 9pm and tested 12 hours later, those tested on the same day tended to forget the negative scenes entirely, while those who had a night’s sleep tended to remember the negative objects but not their neutral backgrounds.

Follow-up experiments showed the same selective consolidation of emotional elements to a lesser degree after a 90-minute daytime nap, and to a greater degree after a 24-hour or even several-month delay (as long as sleep directly followed encoding).

These findings suggest that processes that occur during sleep increase the likelihood that our emotional responses to experiences will become central to our memories of them. Moreover, additional nights of sleep may continue to modify the memory.

In a different approach, another recent study has found that when volunteers were taught new words in the evening, then tested immediately, before spending the night in the sleep lab and being retested in the morning, they could remember more words in the morning than they did immediately after learning them, and they could recognize them faster. In comparison, a control group who were trained in the morning and re-tested in the evening showed no such improvement on the second test.

Deep sleep (slow-wave sleep) rather than rapid eye movement (REM) sleep or light sleep appeared to be the important phase for strengthening the new memories. Moreover, those who experienced more sleep spindles overnight were more successful in connecting the new words to the rest of the words in their mental lexicon, suggesting that the new words were communicated from the hippocampus to the neocortex during sleep. Sleep spindles are brief but intense bursts of brain activity that reflect information transfer between the hippocampus and the neocortex.

The findings confirm the role of sleep in reorganizing new memories, and demonstrate the importance of spindle activity in the process.

Taken together, these studies point to sleep being more important to memory than has been thought. The past decade has seen a wealth of studies establishing the role of sleep in consolidating procedural (skill) memory, but these findings demonstrate a deeper, wider, and more ongoing process. The findings also emphasize the malleability of memory, and the extent to which they are constructed (not copied) and reconstructed.

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How the Alzheimer’s gene works; implications for treatment

November, 2010

Research with genetically engineered mice shows why the apoE4 gene is so strongly associated with Alzheimer’s, and points to strategies for countering its effects.

Carriers of the so-called ‘Alzheimer’s gene’ (apoE4) comprise 65% of all Alzheimer's cases. A new study helps us understand why that’s true. Genetically engineered mice reveal that apoE4 is associated with the loss of GABAergic interneurons in the hippocampus. This is consistent with low levels of GABA (produced by these neurons) typically found in Alzheimer’s brains. This loss was associated with cognitive impairment in the absence of amyloid beta accumulation, demonstrating it is an independent factor in the development of this disease.

The relationship with the other major characteristic of the Alzheimer’s brain, tau tangles, was not independent. When the mice’s tau protein was genetically eliminated, the mice stopped losing GABAergic interneurons, and did not develop cognitive deficits. Previous research has shown that suppressing tau protein can also prevent amyloid beta from causing memory deficits.

Excitingly, daily injections of pentobarbital, a compound that enhances GABA action, restored cognitive function in the mice.

The findings suggest that increasing GABA signaling and reducing tau are potential strategies to treat or prevent apoE4-related Alzheimer's disease.

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Distinguishing between working memory and long-term memory

November, 2010

A study with four brain-damaged people challenges the idea that the hippocampus is the hub of spatial and relational processing for short-term as well as long-term memory.

Because people with damage to their hippocampus are sometimes impaired at remembering spatial information even over extremely short periods of time, it has been thought that the hippocampus is crucial for spatial information irrespective of whether the task is a working memory or a long-term memory task. This is in contrast to other types of information. In general, the hippocampus (and related structures in the mediotemporal lobe) is assumed to be involved in long-term memory, not working memory.

However, a new study involving four patients with damage to their mediotemporal lobes, has found that they were perfectly capable of remembering for one second the relative positions of three or fewer objects on a table — but incapable of remembering more. That is, as soon as the limits of working memory were reached, their performance collapsed. It appears, therefore, that there is, indeed, a fundamental distinction between working memory and long-term memory across the board, including the area of spatial information and spatial-objection relations.

The findings also underscore how little working memory is really capable of on its own (although absolutely vital for what it does!) — in real life, long-term memory and working memory work in tandem.

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