Research studies

Depression
The benefits of FISH OIL have been found in this Japanese study of over 2000 men and women 40 years plus
Cross-sectional association between serum concentrations of n-3 long chain PUFA and depressive symptoms: results in Japanese community dwellers (2015)

Cognitive Decline
1. The Nun Study, is a well known study conducted in 1996. A group of nuns in their 20's were asked to write their autobiographies and some 50 years later their cognitive function was assessed. It was found that the written work that had low grammatical complexity and short punchy sentences were associated with cognitive decline in later years.
Linguistic ability in early life and cognitive function and Alzheimer's disease in Late Life. Findings from the Nun Study. (1996)

2. This report describes a novel, comprehensive and personalised programme for Alzheimer's disease which involves multiple modalities. Case studies are presented with the positive results. The personalised programme can include items such as DIET, LIFESTYLE establishing the root cause of the condition which can be diet related promoting inflammation, or imbalance in blood sugar as well as other things. Very interesting paper.
Reversal of cognitive decline: a novel therapeutic approach (2014)

3. Review article describing the effects of insulin resistance when undertaking a CALORIE RESTRICTIVE DIET. Insulin decreases when food is decreased and this helps with cognitive function and ageing. This type of diet commonly called an intermittent fasting diet, can entail either cutting down calories for 1-2 days per week, or allowing a great number of hours between meals such as between evening meal and breakfast about 14-16 hours or perhaps just taking healthy juices or smoothies for 1 day.
Mechanisms of Brain Aging Regulation by Insulin: Implications for Neurodegeneration in Late-Onset Alzheimer's Disease (2011)

4. Synape function is dependent on a number of factors such as insulin, gherkin and other hormones as well as MAGNESIUM, however it has to be magnesium L-Theronate which is able to penetrate brain tissue and activate the receptors. Other types of magnesium will stay in the gut, this brief article by Dr Perlmutter explains this in his article
This study shows the benefits of Magnesium L-Threonate in learning abilities and memory in rates. 
Enhancement of Memory & Learning by Elevating Brain Magnesium (2009)

5. Review article describing the evidence regarding insulin deficiency and insulin resistance as mediators for Alzheimer's Disease type neurodegeneration.
Alzheimer's Disease is Type 3 Diabetes - Evidence Reviewed (2008)

6. Review article linking the mechanistic pathways of Diabetes and Alzheimer's Disease
How does diabetes accelerate Alzheimer's disease progression (2010)

Neurodegenerative Diseases
Review article highlighting several studies in which the use of spices can help neuroprotection, could this be one of the reasons why conditions such as Alzheimers's disease are lower in Asian countries. The spices include CURCUMIN (from Turmeric), RED PEPPER, BLACK PEPPER, LICORICE, CLOVE, GARLIC, CORIANDER, CINNAMON all target inflammatory pathways, providing a protective feature.
Neuroprotection by Spice-Derived Nutraceuticals: You Are What You Eat! (2011)

Meditation
The study looked at the brain waves during and after meditating amongst 10 Buddhist monks and 10 novice meditators. It was found that 2 of the novice meditators and all of the monks had an increase in gamma waves, which is associated with deep meditation. When the meditating finished the novices' brain waves turned to normal whist the monks still remained in the gamma state. The monks had been meditating for many years and this shows that triggering the neutron activity repeatedly for many years allows for creation of neural pathways or plasticity. 
Neural correlates of attentional expertise in long-term meditation practitioners (2007)

Exercise
1. Studies (meta-analysis) showing the reduced risk of dementia associated with midlife exercise.
Physical exercise as a preventive or disease-modifying treatment of dementia and brain raging (2011)

2. In this randomised control study of 120 adults with Dementia (60 test group and 60 control group), showed that hippocampus volume increased in size by 2%, along with  7.7% increase in oxygen consumption over a period of a year.
Exercise training increases size of hippocampus and improves memory (2011)