Read this: Recent years have seen increasing applications of genetic and genomic techniques to the study of the brain  and a focus on the roles of neurotrophic factors and physical activity in neuroplasticity. The most common subjects are mice, because of the availability of technical tools. It is now possible with relative ease to "knock out" or mutate a wide variety of genes, and then examine the effects on brain function. More sophisticated approaches are also being used: for example, using Cre-Lox recombination it is possible to activate or deactivate genes in specific parts of the brain, at specific times.

What is the most common test subjects for studying of the brain?
What is the answer? (If it cannot be answered, return "unanswerable")
mice