There's probably been orders of magnitude more time spent analyzing rodent brains, simply because there is an effectively unlimited quantity available and an extremely limited quantity of human brains available. Essentially all invasive human brain studies are preceded by animal model studies (usually starting with rodents before proceeding to "higher" animals if that's called for), but conversely not all animal model studies result in a human follow up. Meaning a hell of a lot more rodent brains get put under the microscope. So in general you would expect to find them in rodents first, if the probability of serendipitous discovery is purely a function of time spent conducting unrelated analyses.