Notes and researches.
I'm using cognition here to refer to a sort of qualitative function of the brain and thought. Primarily, I am interested in how the mind thinks, learns, and remembers. I don't yet have a master structure for this; so until enough information accrues here these are to be rough notes.
This rather long word refers to the ability of the mind to rewire neurons to adapt a skill, learn a new language, etc. Neuroplasticity is then, effectively, the ability to learn and create. To me, that's the most important ability of the mind. Conventional wisdom has it that as a person ages, their mind becomes 'set' and they can not learn new things or change their habits. However, it seems that so long as a person keeps their mind engaged in learning, creating, and exploring, neuroplasticity can preserved*.
The lesson here is striking and grim: don't let your mind slip into tempting abyss of rote process and consistency or it might never escape.
Sensory input has a big impact on continued neuroplasticity. Much of the brain is dedicated to processing and correcting sensory input. It has surprising capacity to alter sensory signals to correct for error (see the Helmholtz Innsbruck experiments*). Keeping the body immersed in complex, often novel sensory experiences is an important 'passive' habit that preserves neuroplasticity*. Nature especially is effective at providing a complex but understandable environment that is always changing.
As the brain ages cognitive function generally slows. There are a variety of reasons for this, but the below can be 'treated', so to speak, with lifestyle choices.
Myelin is a fatty coating around 'axons' (links between neurons) that functions like a grounded shield around a signal cable. Production of myelin is dependent upon good dietary fats. Having a good supply of these fats in the diet is necessary to prevent the decline of myelin coatings and associated cognitive decline*. Sufficient Omega-3 fats and Vitamin B12 specifically are required for this.