The office of U.S. Senator Tim Johnson says the South Dakota Democrat's overall medical condition has improved after emergency brain surgery two weeks ago, but he remains in critical condition.

A neurosurgeon at George Washington University Hospital in Washington said Thursday the senator is gradually being weaned from sedation. The doctors say Johnson is opening his eyes and responds to his wife.

Thursday was Senator Johnson's 60th birthday. His wife Barbara says he gives the family great hope.

Johnson underwent emergency surgery December 13 for bleeding in his brain. Democrats could lose control of the Senate if Johnson were unable to serve out his term.

Democrats will have a 51-49 majority when the new Senate convenes next month. If Johnson were unable to serve, South Dakota's governor would likely appoint a fellow Republican to the seat. This would result in a 50-50 split with Vice President Dick Cheney - the Senate president - casting any tie-breaking votes.

Some information for this report was provided by Reuters.