Dehaene and his group have published wonderful work on the neurobiology of mathematics. The research suggests that the brain is organized into distinct areas that consider numbers and number relationships, distinct from their verbal representations.
The paper below is a fascinating read, although technical. Based on varieties of evidence, unusual strokes in patients, epidemiological studies in students with math disabilities, and functional brain imaging, separate systems appear to exist different ways of performing basic math operations. This means that in the setting of disability or injury, difficulty in one area, may not necessarily mean difficulty in another.
For any teacher confronting the math struggles of a student, it would important to know that the activities of mental math, number comparison, estimation, and subtraction tend to cluster together, whereas exact calculation, rote memory of facts (stored in verbal memory), and multiplication form another cluster. It means that for a given student, it would be valuable to test which is the stronger route for processing, and then adjust the learning to that knowledge. So, some groups might take more readily to Mental Math, while Rote-Verbal-Story-Based apparoaches might form a different group.