Brain size makes a difference when it comes to memory. The problem is that the brain shrinks as we age. If we could prevent that, we could also help prevent memory loss as we get older. That is what the reviewed research investigated (Daiello LA, et.al. 2014).
The participants were assessed with neuropsychological tests and brain magnetic resonance imaging every 6 months. The participants that used fish oil supplements during the follow up had a significantly better outcome on the neuropsychological tests which indicated better cognition. Not only that, but the fish oil use was also associated with less atrophy (brain loss) in one or more brain regions related to memory. The active ingredients of omega 3 fish oil is EPA and DHA, so if you are going to take fish oil, you want to use a product that has high amounts of EPA, DHA.
Daiello LA1, Gongvatana A2, Dunsiger S3, Cohen RA2, Ott BR4; Alzheimer's Disease Neuroimaging Initiative. Association of fish oil supplement use with preservation of brain volume and cognitive function. Alzheimer's Dement. 2014 Jun 18. pii: S1552-5260(14)00079-X. doi: 10.1016/j.jalz.2014.02.005.