Continuing with the last part of an article published by the Harvard Health Publications on the causes of memory impairment.

Intellectual stimulation
Over time, people who don’t challenge their minds exhibit a greater degree of memory decline compared with people who remain mentally active. The landmark MacArthur Foundation Study on Successful Aging found that education level was the strongest predictor of mental capacity as people aged. A 2005 study in the Journal of Gerontology: Psychological Sciences reported that people who held jobs that involved complex work, such as speaking to, instructing, or negotiating with others, had a lower risk of dementia than people whose jobs were less intellectually demanding.
Most experts think it’s not the years of formal education or the type of occupation per se that benefits memory. Instead, a lifelong habit of learning and engaging in mentally challenging activities seems to keep the brain in shape.
Scientists believe that intellectual enrichment and learning stimulate the brain to make more connections, increasing the density of synapses. As a result, the “educated brain” may possess greater reserves of cognitive ability — a deeper well, so to speak — and be able to sustain more neuronal loss. Researchers refer to this enhancement of skills or abilities as cognitive reserve (see “Cognitive reserve: Engage your brain”).

Level of physical activity
Insufficient physical exercise also increases the likelihood of memory impairment as people age. The MacArthur study researchers were surprised to find that sedentary people had greater memory declines compared with people who were physically active on a regular basis. The active people didn’t necessarily work out at a gym, but they did engage in more routine physical activity such as climbing the stairs at home or taking walks in the neighborhood.
A Swedish study of 1,500 people ages 65 to 79, published in The Lancet Neurology in 2005, found that those who participated in a physical activity at least twice a week were only 40% as likely to develop Alzheimer’s disease and half as likely to develop any form of dementia compared with sedentary people (see “Exercise”).

Stress
Several studies link stress to memory loss. The key here is not the minor stresses of daily life, like deadline pressures or petty arguments, but rather an ongoing sense of extreme anxiety. Such severe stress may impair a person’s memory. Some people — for example, veterans of wars or others who experience profoundly traumatic events — may develop post-traumatic stress disorder (see “Post-traumatic stress disorder”).
Through animal research, scientists have discovered that neurons in the hippocampus are damaged by cortisol, a hormone released during stressful “fight or flight” situations. One study implicated cortisol in memory dysfunction in humans as well. In the study, a group of healthy adults received daily low-dose treatments of cortisol over the course of four consecutive days — about the amount that would be released under everyday stress. Another group received high doses over the same period of time. All participants took a test in which they listened to stories and then had to remember details about them immediately and 30 minutes later. People who had received the higher cortisol dose remembered less, both immediately and later, than people receiving the lower dose.
The memory dysfunction induced by this four-day experiment was reversible; once the cortisol wore off, the participants’ memories returned to normal. No one knows how many days, weeks, months, or years of high-level stress it takes to cause irreversible memory impairment. What we do know is that a stressful event will cause some people to react more intensely than others, and it is the resulting long-term stress reaction that does the damage. To that extent, people may be able to control the harm that stress does to the memory by trying to modify their exposure and responses to stressful life events.
Post-traumatic stress disorder
Extreme psychological trauma sometimes leads to post-traumatic stress disorder (PTSD), a condition in which intrusive memories of the traumatic event recur persistently and cause the individual to experience episodes of sustained, severe anxiety. Prolonged or frequently elevated levels of the stress hormone cortisol (as seen in PTSD) can harm brain structures vital to memory. Brain imaging studies reveal that the hippocampus may sustain damage or shrink in people with PTSD.Prompt, effective treatment can reverse the memory problems caused by PTSD. Treatment typically entails a combination of medication and psychotherapy. However, if cortisol damages the hippocampus and other brain structures, the resulting memory difficulties will persist.Scientists are now looking at ways to use medication to blunt painful memories at the time of the traumatic event, thereby preventing PTSD from developing in the first place. The high level of cortisol present in the body during a traumatic event has the effect of fortifying the emotions surrounding the memories of the incident. However, taking the beta-blocker drug propranolol (most commonly used to control high blood pressure) immediately after an emotionally disturbing event disrupts the release of adrenaline-related hormones, thereby undermining the formation of a traumatic memory. In a 2003 study in France, doctors in the emergency departments of two hospitals offered propranolol to victims of auto accidents or physical assaults who did not have serious physical injuries. The participants took the drug in the emergency room and continued with lesser doses for periods of two to three weeks afterward. At a two-month follow-up, these people had fewer symptoms of PTSD than did a similar group who didn’t receive medication. Although this was a small pilot study, the results raise interesting questions about the ability to alter the process of forming troubling memories. On the one hand, nipping PTSD in the bud could spare potential sufferers the pain of dealing with the condition down the road. However, the use of drugs to alter emotion and memory formation raises significant ethical and legal questions that have yet to be answered.

Sleep
People who don’t sleep well at night tend to be more forgetful than people who sleep soundly. A good night’s sleep is essential for consolidating memories. Although people vary in their need for sleep, six hours may be the minimum that you need to ensure adequate daytime alertness and memory function (see “Sleep and memory”).
Sleep may aid memory in another way as well — by decreasing levels of stress hormones. Stress hormones decline during the first few hours of sleep, which experts believe may free up the hippocampus to consolidate memories.
With age, people become more prone to sleep disorders. The most common sleep disorder is insomnia, which is difficulty falling asleep or staying asleep. Unfortunately, many medicines used in the treatment of insomnia can also impair memory and general cognitive function, and it’s best to avoid long-term use of sleep-inducing medications. Lifestyle changes are often helpful (see “Get a good night’s sleep”).
Another common disorder is obstructive sleep apnea, in which interrupted breathing can cause sleep to be fragmented by hundreds of “mini-awakenings” each night.
Sleep and memory
Why does sleep make a difference? One theory suggests that changes in brain activity during two phases of sleep — slow-wave sleep and rapid eye movement (REM) sleep — are important for memory consolidation. Research with rodents showed that during slow-wave sleep, the pattern of activity in the hippocampus was very similar to the pattern that appeared earlier when the animals were engaged in learning something. This finding suggests that during slow-wave sleep, the hippocampus “replays” the recently acquired information to strengthen the neuronal patterns that were activated during the acquisition phase. This replaying of the learning scenario is the key component of the consolidation process.An interesting study in the March 2007 Science elaborates on this theory. Research participants played a memory game called Concentration, in which they were asked to recall the locations of like cards. Each time a person matched a pair of cards correctly, he or she was exposed to the scent of roses. Later that night, the rose fragrance was reintroduced during slow-wave sleep for half of the participants. The next morning, these individuals demonstrated a greater ability to match the locations of previously learned pairs over the subjects who did not receive sleep-time exposure to the scent. These results can be explained, the researchers say, by the fact that the rose scent activated the hippocampus and provided contextual information that aided in the consolidation of the memory and completing the learning task.Research has also revealed that important changes take place in the brain during REM sleep, which is the time when dreaming occurs. In particular, there is increased activity in the cortex during dreams. Some scientists now think that dreaming is one of the ways in which the brain strengthens the neuronal pathways that encode memories.

Smoking
Smokers have a greater degree of age-related memory loss and other memory problems than nonsmokers. Research shows that they’re less adept at recalling names and faces. A 2003 British study reported that the verbal memory function of people ages 43 to 53 was weaker in the smokers than in the nonsmokers. A 2004 study published in Neurology tracked the memory of 9,209 people ages 65 and older annually for several years by giving them the Mini-Mental State Examination, a standard test of cognitive functions. Nearly everyone’s score dropped during this period, but the decline was five times greater for smokers than for people who never smoked. These conclusions were confirmed in a 2007 meta-analysis of 19 studies involving a total of more than 26,000 participants. Smokers showed a greater yearly decline in cognitive scores compared with nonsmokers. In addition, current smokers exhibited a higher risk of Alzheimer’s disease than did former smokers.

Substance abuse
People who use certain illicit drugs are likely to have problems with memory and related cognitive functions — not only while they are under the influence, but also for weeks after the drugs’ immediate effects wear off.
A 2006 study in Neurology found that people who used marijuana heavily for 10 or more years had substantially higher rates of memory impairment than did shorter-term heavy users or nonusers. Long-term users also scored significantly lower than the published norms on the verbal learning tests used in the study. Marijuana’s active chemical, THC, causes its psychoactive effect by attaching to receptors in brain regions vital for memory formation, including the hippocampus, amygdala, and cerebral cortex.
Memory impairment is also a side effect of habitual cocaine use. Cocaine users score lower than nonusers on tests of working memory and recall of long-term memories. Deficits are apparent on tests of verbal memory, even after users have abstained from cocaine for 45 days.
One of the worst things you can do to your memory is to use illicit drugs. It is unclear how much of the memory impairment can be restored by quitting, but at the very least, quitting can prevent further damage.

Toxic exposure
Lead, mercury, and other chemicals present in homes and workplaces can cause memory loss and poor concentration. Lead poisoning can result from drinking contaminated tap water and breathing in lead dust generated by the deterioration of lead paint in homes built before 1978, when it was outlawed. Carbon monoxide fumes leak from malfunctioning household furnaces and are spewed from automobile exhaust systems. Mercury and other toxic substances are found in some paints, dyes, and inks used in artwork. Still other sources of toxic exposure are pesticides used in home gardens and farms, darkroom chemicals, and chemicals used in metalwork and woodwork.

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