WASHINGTON (CNN) -- Sen. Tim Johnson of South Dakota has made "significant progress" in his speech therapy, and is able to read and follow written commands as he continues his recovery from a brain hemorrhage and subsequent surgery in December, according to a statement issued by his office Thursday.

"Senator Johnson is using limited full sentences and is initiating conversation with his family and therapists," said Philip Marion, medical director for the Department of Rehabilitative Medicine at George Washington University Hospital.

As part of Johnson's therapy, he participated in a reading test on Wednesday, the statement said. "Simply put, he is reading," Marion said. "The test showed that he is comprehending written material and successfully following written commands.

"When it comes to rehabilitation, each patient's recovery is unique, but Senator Johnson continues to impress us with his progress during his daily therapy sessions."

Johnson is in the hospital's in-patient rehabilitation unit after being transferred from intensive care last month.