Julianne Fisher, a spokeswoman for the South Dakota Democrat, said Johnson won't be present in the first days of the new Congress next week but is continuing to improve. She said he is responsive to directions from his wife but has not yet spoken.
It's too early to tell how long recovery will take, Fisher said.
In a statement Thursday, Johnson's doctors said he remains in intensive care at George Washington University Hospital. They have released few new details about Johnson's condition and prognosis since the days after the Dec. 13 surgery to stop bleeding in his brain.
Dr. Vivek Deshmukh, head of Johnson's surgical team, said in a statement that the South Dakota senator's overall condition has improved and he is gradually being weaned off sedation to help his brain heal.
The statement said Johnson is expected to undergo more tests in coming days.
Johnson's wife, Barbara, said her husband "continues to give us great hope" and that two of the couple's three grown children were at the hospital to be with him on his birthday.
"While we were both looking forward to celebrating his 60th birthday with our family and friends, I know the celebration is just postponed," she said.
Johnson was diagnosed with arteriovenous malformation, a condition, often present from birth, that causes arteries and veins to grow abnormally large, become tangled and sometimes burst. He was rushed to the hospital after becoming disoriented on a call with reporters and had surgery hours later.
The senator's sudden illness raised questions about the Democrats' one-vote majority in the upcoming Senate session. South Dakota's Republican governor, Mike Rounds, would appoint a replacement if Johnson's seat were vacated by his death or resignation.
A Republican appointee would create a 50-50 tie and effectively allow the GOP to retain Senate control because of Vice President Dick Cheney's tie-breaking vote.
There is ample precedent for senators to continue to hold office while incapacitated.
Dr. Keith Siller, director of the Comprehensive Stroke Care Center at NYU Medical Center and assistant professor at the NYU School of Medicine, said it is unusual for a patient to be sedated after brain surgery for more than a few days.
"The two-week period is longer than I would be happy with," he said.