So you're worried about a representative sample? This is -- again -- only indirectly related to sample size. If the sample was randomly selected and the stati

So you're worried about a representative sample? This is -- again -- only indirectly related to sample size. If the sample was randomly selected and the statistical power is great enough, then any correlation it's missing is marginal; this is a feature, not a bug! You're not entirely wrong insofar as brain-imagery studies rarely conduct random samples, but increasing your sample size won't fix that.