Neuroplasticity Osterhout, et al., 2008)). It seems that the

Neuroplasticity

The neuroscientific field has published many findings referring
to the idea that the brain is flexible and sensitive to experience (Guzmán-Vélez & Tranel, 2015; Osterhout, et al.,
2008)). It seems that the brain can be shaped and is constantly adapting
to our environment (Stern, 2009). Studies such as
those of Maguire, et al. (2000) have
shown neurological changes in response to environmental factors. They
discovered that London taxi drivers, of whom had extensive navigation
experience, had significantly larger posterior hippocampi, an area which indeed
is associated with memory and spatial navigation (Mandal, 2014),  compared to those who did not drive taxis.
Similarly, other experiences such as playing a musical instrument can have an impact
on the brain- a study by Hanna-Pladdy &
MacKay (2011) showing better nonverbal memory, naming and executive
processes. Taking this into account, we can assume that multilingualism, which
has required an individual to learn, use and manage multiple languages, has a
similar beneficial neurological effect. 

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Cogntiive reserve definition

Cognitive reserve
can be defined as ‘building up additional abilities to compensate for the
possibility of declining memory or thinking’ (Heerema, 2017).
It describes how some people are able to cope better and function relatively
normally, despite possessing higher degrees of neuropathology (or brain damage)
(Chasman, 2011).

In Stern’s (2009) study,
he highlighted two types of reserve. The first being brain reserve, known as
the passive model, whereby reserve relates to brain size and neuronal count (Stern, 2009).
Studies such as that of Katzman (1993) highlight that there does not seem to be
any “direct relationship between the degree of brain damage and the clinical
manifestation of that damage” (Stern , 2002,
abstract) since Katzman found 10 elderly people to have advanced Alzheimer’s
disease pathology in their brain on observation at death, despite being
considered cognitively normal throughout their life. It was speculated that
they did not show manifestation of brain degradation because their brains were
observed as larger than average. The second type of reserve Stern (2009) highlighted is an active model
known as cognitive reserve. This concept speculates that “the brain
actively attempts to cope with brain damage by using pre-existing cognitive
processes or by enlisting compensatory processes” (Stern, 2002 cited in Stern, 2009, p.2016)TP1 

CR types and factors

As mentioned, cognitive
reserve varies from person to person and this variation stems from genetic
differences as well as life experiences (Stern , 2002).
Such factors include education, occupational status and mental engagement (Stern, 2012).
Cognitive and mentally–stimulating
activities have often been concluded as providing the most significant
protection. Frequently engaging in cognitive activities such as playing chess,
reading books and volunteering could reduce risk of developing Alzheimer’s
Disease by 47% (Wilson, et al., 2002)) and could be “more
important than education in determining reserve” (Reed, et al., 2011, p.
615).
Additionally, linguistic ability has been shown to be a predictor of
asymptomatic AD (Tyas, et al., 2009) and overall
literacy rate has been associated with slower declines in memory, language
skills and executive function (Stern, 2009). With that
said, if utilising multiple languages can be deemed as a mentally stimulating
activity, then multilingualism should expect to see this protective effect.

CRTP2  and dementia

Although many of the
factors predisposing an individual to dementia are of biological origin (Corder, et al., 1993), there is evidence,
as mentioned above, that there are other factors of environmental origin that
can play a role in dementia onset.

Although as of yet
there is no research that clearly shows that cognitive reserve prevents the
development of dementia (Heerema, 2017),
many studies, which will be explored later, have suggested that there is some
kind of ‘buffer’ between developing dementia and not developing it. It has been
hypothesised that cognitive reserve provides a protective effect due to
enhancing neural plasticity, enriching brain vasculature and using alternative
brain regions as a compensatory method (Bialystok, et al., 2007). https://www.researchgate.net/publication/6672036_Bilingualism_as_protection_against_the_onset_of_symptoms_of_dementia

Association
between neuropathology and cognitive impairment

Some research has linked the amount of amyloid-B and
neurofibrillary tangles to the severity of cognitive impairment and the
likelihood of receiving an AD diagnosis (SantaCruz,
et al., 2011; Vemuri, et al., 2011), but other research has found not such a simple association, instead
finding levels of neuropathology associated with dementia, despite individuals
not showing dementia symptomology (Katzman,
et al., 1988; Mortimer, 1997). This seemingly supports the concept of
cognitive reserve, and that removing plaque from cortical tissues does not
improve AD symptoms or improve cognition (Holmes, et al., 2008)

A study by Lazarov, et al. (2005) found that transgenic AD
mice exposed to an enriched environment (i.e one with more toys and objects)
demonstrated a reduction in AD neuropathology levels, which suggests that
mental stimulation influences neural structures and protects against AD and
also back up the idea that environmental experiences play a role in brain
reserve (Hack, et al., 2012).