Here's an interesting update on the dietary fatty acid issue, as it pertains to mood disorders and neuroplasticity:
This article, published in Nature Neuroscience (January 30, 2011), is an example of some good research being done by a group at the University of Bordeaux in France. They demonstrate substantial negative neurophysiological changes in mice as a result of an omega-3 deficient diet. It is interesting to note that the brain's endocanniboid system is specifically affected by omega-3 deficiency, according to this research.
This is further evidence supporting the importance of attending to a healthy diet, in maintaining optimal mental health. Omega-3 fatty acids are one element of a healthy diet. While omega-6 fatty acids are also needed in the diet, these lipids behave to some degree competitively with omega-3. Therefore, the ratio of dietary omega-3 to omega-6 is is very important. Western diets tend to have an unhealthy ratio of these lipids, due to excessive omega-6.
An ongoing issue of debate has to do with whether plant sources of omega-3 (primarily ALA) are as useful as fish sources (DHA and EPA). Existing evidence shows that DHA and EPA are more important. ALA can be converted in the body to DHA and EPA, but the efficiency of this may vary from person to person.
Wikipedia has a nice review of this subject: http://en.wikipedia.org/wiki/Omega-3_fatty_acid
but some of the sources are less than ideal.
It is interesting to consider that the DHA/EPA issue is not a "micronutrient" issue. They could be considered "macronutrients." The solid mass of the brain consists mostly of lipids (60-80 % of the non-aqueous mass); DHA and EPA make up over 10% of this lipid mass, which is a very high concentration.
Here's a link to a paper which quantifies the high fractions of omega-3 lipids in brain mass:
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=921064 --this paper also showed that dietary changes substantially altered the proportion of omega-3 lipids in brain tissue