Cognitive reserve: The warehouse within.

Dementia is characterized by progressive and mostly irreversible memory loss. Other neuropsychiatric disorders affect cognition in varying manner. Are all people affected with such disorders manifest clinically in similar manner or does our brain have some reserve to tolerate insults? Relevant researches over the last two decades were scrutinized to understand brain reserve, appreciate the conceptual change in the same over years, and how the same can be improved for better cognition and memory over the year. Literature evidence suggests that the cognitive reserve (CR) is a dynamic and functional concept. There is adequate evidence to suggest that enriched environment and various other measures are likely to improve CR across all age. Improving CR may delay or reverse the effects of aging or brain pathology.

Our life is defined by the memories we possess. Be it mother's affection, father's admonition, childhood friends, or college pranks; sweet or sour, these memories modify our personalities to what we are today. However, as Ben Johnson quotes “Memory, of all the powers of the mind, is the most delicate and frail.” One of the diseases which primarily affect these memories is dementia.

Dementia, a disease of the old age is characterized by progressive memory loss and associated behavior problems. Treatment primarily aims at reducing the speed of damage by pharmacological and psychological interventions. With better medical facilities, we have definitely added more years to life but what about more life to years!!! If we gloss over the statistics, dementia accounts for approximately 25 million people in the world, which doubles every 20 years. In developed country, it affects 15% of people less than 80 years and around 25% of people above 80 years of age.[1] In India, around 3.7 million are affected by this illness, of which 2% are less than 65 years of age. The rate doubles with every 5-year increase of age.[2] Can we do something to prevent ourselves from this disease? Can we take some preventive measures to avoid, halt, or arrest this decay of memory?

The vision somewhere began in 1988. Katzman et al. brought out a research article in the Annals of Neurology as to why some people develop Alzheimer's dementia and some do not. They studied 137 autopsied brains. Alzheimer's brain revealed characteristic plaque and tangles. However, came out a startling revelation that 10 of the autopsied brain had same level of plaques and tangles, but no symptoms whatsoever of Alzheimer's disease during their lifetime. What made them different? These brains were heavier and had more neurons per age.[3] This let out the research question that are some people more resilient or adaptive to brain pathology? Can one function normally despite neuronal changes? Do some people have more cognitive capacity than needed and when needed can we draw an extra reserve? Thus, came out the concept of cognitive reserve (CR).

COGNITIVE RESERVE

CR is a hypothesized capacity of mature adult brain to sustain the effects of disease or injury without manifesting clinically. It accounts for individual differences in the cognitive processes and neural networks which allow one to cope better than others with the brain damage.[4]

In earlier period, people entertained the concept of what is called as “brain reserve”.[56] This construct was a passive model and stated that each individual had a fixed “brain reserve capacity,” which does not change with time, experience, or training. It thus implied that an individual has a prefixed amount of brain damage that can be sustained before reaching threshold for clinical expression. Brain reserve capacity was then measured in brain size or the synaptic count, which was understood as a fixed component and thus a nonmodifiable factor.

Various researches since the landmark revelation point toward an active construct called “cognitive reserve.” This reserve is as active as other part of the brain during any activity; it is functional as it augments the functions of other neurons; and it is modifiable with time, experience, and training. This thus implied that even people with similar brain size can have different CRs.[7]

CR has two components: Neural reserve and neural compensation. Neural reserves are those networks which are less susceptible to damage, are more efficient, have greater capacity, and may be invoked while coping with increased task demand. It thus helps individual cope with brain pathology. Neural compensation is a process by which individual suffering from brain pathology use brain structures or networks not normally used by intact brain to compensate for brain damage. These are networks other than CR network. These compensations occur by two methods. In “compensation to improve performance,” the brain recruits additional brain areas which have resilient and healthy networks, i.e., like similar network on contralateral hemisphere. In “compensation to maintain performance,” the brain recruits any network. These are not as health and resilient as the prior. Thus, the performance is poor. Such compensations are more in older age.[8] In general, a better CR implies greater overall cognitive efficiency, greater proliferation of brain neurons, more connection between neurons, enhanced ability to compensate by recruiting generalized neurons for specific task at hand, and better ability to use alternative strategies to solve problems.[9]

Measures of cognitive reserve

The anatomical or structural measures of CR constitute hardware model. These are measured by brain volume, head circumference, synaptic count, dendritic branching, etc. The functional measure is akin to software model which focuses on process than structures. These measures are intelligence quotient (IQ) or premorbid IQ, level of activation in functional magnetic resonance imaging (fMRI), efficiency of neural network, active compensation by alternative/more extensive network after challenge, etc. Most of these measures are malleable with time and life experiences. Summations of these are taken as proxy equivalent of CR. These proxy measures are income, occupational attainment, educational attainment, and degree of literacy. Although educational attainment is more widely used proxy, the degree of literacy is a more sensitive measure.[8]

Where is that specific CR network? Recent findings suggest that these networks are located in frontal lobe. It was seen that a higher CR individual was able to activate this network while working on more difficult task, whereas lower CR individual was unable to tap this network. These networks were found more often in younger people.[10] Do these networks degrade during natural aging process? Can this degradation be slowed or halted?

How to improve the cognitive reserve

Various literatures suggest that enriched environment (EE), cognitively stimulating activities, cognitive training, and physical training are likely to increase our CR, thus making us more resilient to cognitive decline and damage.

Enriched environment: This term has been given to environmental conditions that facilitate enhanced sensory, cognitive, and motor stimulation. EE has been seen to improve experience-dependent microdevelopment. EE has been found to promote neurogenesis, synaptogenesis, and transgenerational transmission.[11]

Neurogenesis: Concept in neurology till last decade held that brain cells cannot reproduce. However, adequate evidences suggest that brain structures could generate new cells. Evidence is strongest for hippocampus and neocortex.[12] Short-term exposure to EE has led to fivefold increase in new neurons, substantial improvement in learning, exploratory behavior, and locomotor activity. These effects were seen even beyond the age of 65 years and potential exists throughout the lifespan.[13]

Synaptogenesis: Physical activity enhances growth of neurons and learning promotes new connection between the neurons. Neuronal aging diminished by active and challenging life even if starts later in life.[14]

EEs are meaningful social engagement and activities that provide a sense of mastery.[15] These are complex, highly structured enjoyable activity that provide opportunity for self-expression, e.g., charity, clubs, group games, voluntary work, and caring for sick.[16] A separate study on cohabitation revealed that single, widowed, or separated had three times higher risk for cognitive impairment and 7.67 higher risk for Alzheimer's disease vis a vis their married/cohabiting counterparts.[1]

Cognitively stimulating task: In a unique study, 6- to 12-year old learnt in company of older adults in an intergenerational Charter School Cleveland where older adults contributed by sharing knowledge and experience. Improvement in cognition and QOL was seen in these elderlies[17] A China-based study found gardening and other community activities protective for incident dementia.[18] Crossword puzzles led to better performance on complex cognitive tasks.[19] Learning magic was found to be protective.[20] A German study worked on the effects of juggling. A volume increase was found in visual cortex, nucleus accumbens, and hippocampus. Effect was temporary in later two.[21] Formal education, reading book, magazines, etc. led to increased cognitive activity, the practice of which was associated with reduction in risk for Alzheimer's dementia and slower rates of cognitive decline.[2223] Prospective cohort study from New York found that musical instruments, dancing, and leisure activity accounted for reduced risk for incident dementia and memory decline[24] La Rue recommendation (2008) suggests following for better memory: to carve out time for cognitively stimulating activities that one enjoys, to add some new challenging pursuits as time and energy allow, to engage in these pursuits several times a week, and to participate in social interaction.[12]

Cognitive training: Advanced cognitive training for independent and vital elderly (ACTIVE) study undertook training of older adults in areas of reasoning, memory, and processing speed. Generalized benefit was seen in cognition in adult 65 years and older. Improvement lasted at least 5 years with booster training.[25] In Experience Corps (EC) project, elders were taught literacy, mathematics, and conflict resolution in elementary school. Training showed improvement in executive function and memory[17] There are many cognitive training websites available, few of these include www.fitbrains.comnone, www.gamesforthebrain.comnone, www.mybraintrainer.comnone, www.luminosity.comnone, and www.sharpbrains.comnone. Many books are also available which elucidates the concept of CR and ways to improve it. These books are “The sharp brain guide to brain fitness—Alvaro and Elkhonon 2009,” “Magnificient mind at any age—Amen D G 2008,” “The mature mind: the positive power of ageing brain—Cohen G D 2005,” “Cognitive reserve: theory and application—Yakov Stern 2006,” etc.[12]

Physical training: Aerobic fitness training in older adult promoted significant increase in gray (cortical neurons) and white matter (connecting pathways) Physical and cognitive training combined had multiplier effect.[26]

Richards and Deary have brought out a life course model for better memory. Figure 1 shows the role of multiple factors in the same. The model lays emphasis on the role of genetics and the environment. It lays stress on the structural neuronal complexity as well as the dynamic functional processing capacity and efficacy. From the above, it is obvious that the CR has a role to play in almost all aspects of cognitive and at all stage of development.[27] Improving CR will not only improve the function capacity but also delay the expressions of cognitive disorders such as Alzheimer's disease significantly [Figure 2].

Figure 1

Life course model for better memory

Figure 2

Factors affecting cognitive reserve at various stages of human development

Role of cognitive reserve in other disorders

CRs have implications in various other disorders. Common neurological disorders include traumatic brain injury,[28] epilepsy,[29] multiple sclerosis,[30] Huntington's disease,[31] and Parkinson's disease.[32] Psychiatric disorders where CR was found to have positive role were schizophrenia,[33] affective disorders,[34] substance abuse,[35] sleep apnea-related cognitive deficits,[36] etc.

CONCLUSION

Available research evidences support the construct of CR structurally and functionally. Reserve is dynamic, interactive, and malleable. EE and cognitively and physically stimulating activities affect the CR positively in all stages of life and development. These reserves are determined by efficiency and capacity of existing brain networks and ability to enlist new compensatory networks and pure CR networks. CR has prophylactic and therapeutic implication in a wide range of neuropsychiatric conditions across all ages. Improving CR may delay or reverse the effects of aging or brain pathology.