Speciesism and the Argument from Misfortune

Is there a morally relevant difference between a brain-damaged human being and a nonhuman animal at the same cognitive and emotional level to justify, say, performing medical experiments on the animal but not the human being? Some hold that the misfortune of the human being allows us to distinguish between them. I consider the nature of misfortunate and argue that an appeal to misfortune fails to distinguish between the human being and the nonhuman animal when the treatment at issue is equally morally serious, since the source of the limitation taken advantage of by performing the medical experiment, whether misfortunate or natural vulnerability, is irrelevant.