Not sure, but I think you'd find it interesting that there are some humans out there who do not have an intuitive sense of any number past two. For example, people who are part of hunter gatherer tribes who, for example, only need to know that they have a lot of berries, not that they have exactly 7 or exactly 65; for their lifestyle, there is no need for counting. Their numbers are "1", "2", and "more than 2", and their language lacks the capability to express any other numerical idea.
Presumably this has an effect on the physiology of their brain. I'm pointing this out because maybe it suggests that something as specific as the numerical digits 0-9, which are represented with different symbols even across cultures that have all 10 digits, are too specific to elicit any sort of measurable difference in brain activity while reading one compared to the other. And that's the reason no studies have been done on them. Am totally unsure of this.