Dr. Michael Lemole said her eyes are opening more frequently and she can carry out more complex sequences of activity in response to commands and on her own.
Giffords was shot in the head Saturday in an attack in Tucson that wounded 12 others and killed six people.
She remained in critical condition with a breathing tube still in, though Trauma surgeon Dr. Peter Rhee said it could be removed within a day.
(Scroll down to see video of CBS medical correspondent Dr. Jon LaPook discuss what he learned after speaking with Dr. Michael Lemole.)
Giffords' husband, astronaut Mark Kelly, posted a message on Twitter Friday thanking people for the messages of support and said "GG," as he referred to her, was "improving each day."
Just four days after receiving the devastating injury, President Barack Obama told a stunned nation Wednesday that Giffordsfor the first time.
Few people who take a bullet to the brain -- survive such a devastating wound.
Earlier, veteran trauma surgeon Dr. Peter Rhee said she has a "101 percent" chance of surviving.
Kelly recalled the moment Giffords first woke up in an interview with CNN.
"At first I thought she was trying to strangle me," he joked, describing how tightly she grabbed him at first.
Kelly had asked his wife to touch his wedding ring that night, said House Minority Leader Nancy Pelosi, who was also in the room, "and she touches his ring, then she grabs his whole watch and wrist and then the doctor was just so excited, he said, 'You don't understand ... this is amazing what she is doing right now and beyond our greatest hopes.'"
On Thursday, doctors have sat Giffords up on the side of the bed and dangled her feet over the edge. They are backing off on medications.
Lemole, called it "a major milestone," and said the congresswoman was clearly responding to the gathering of friends and family.
After five days of pushing for caution, Lemole said: "We're wise to acknowledge miracles."