There have been at least some studies indicating positive effects on cognition:
Supplementation with DHA-rich fish oil, in comparison with placebo, resulted in a significant increase in the concentrations of oxy-Hb and
total levels of Hb, indicative of increased cerebral blood flow.
pre-supplementation RBC DHA levels was predictive of baseline
performance (i.e., adjusted hit rate, AHR on 3-back) on the n-back
task (y = 0.19+0.07, r2 = 0.55, p = 0.009). In addition, subjects AHR
performance improved on 3-back post-supplementation (pre 0.65±0.27,
post 0.80±0.15, p = 0.04). The correlation between n-back performance,
and DHA levels are consistent with reports in which higher DHA levels
is related to improved cognitive performance.
Blood analyses showed that after Omega-3 supplementation the arachidonic acid/eicosapentaenoic acid ratio (AA/EPA) was strongly reduced. The mood profile was improved after Omega-3 with increased vigour and reduced anger, anxiety and depression states. This was associated with an effect on reactivity with a reduction of reaction time in the Go/No-Go and Sustained Attention tests.
- may increase cognitive performance
- may increase cerebral blood flow
- may increase mood & reaction time
Conclusion: conflicting results (see other answer) indicate more studies should be made