LOS ANGELES — Arizona Rep. Gabrielle Giffords, whose condition was upgraded from serious to good this week, has begun an intense regimen of full-time rehabilitation after a successful move between Houston medical facilities.

Although she faces a long recovery, doctors said she has so far progressed at “lightning speed,” given the severity of her injury. In Houston, she has been interacting with hospital staff and following commands.

Giffords was shot in the head during an event with constituents outside a Tucson supermarket Jan. 8. Jared Lee Loughner, 22, has been charged in the rampage that killed six people and injured 13, including Giffords.

The congresswoman was flown last week from Tucson to Houston, where she was treated in the intensive care unit of Memorial Hermann Texas Medical Center. Her husband, astronaut Mark E. Kelly, chose the center’s Institute for Rehabilitation and Research because it is among the most highly rated brain injury recovery center in the nation and is close to his family.

Giffords requires assistance to sit up or stand. A team of therapists will work with her in the coming weeks to improve her speech and language functions and to rebuild her physical strength after weeks of immobility.

Kelly gave a thumbs-up sign to observers from the passenger seat of the ambulance while en route between hospitals Wednesday.

Doctors projected optimism at a press conference after the transfer. Dr. Dong Kim, director of Memorial Hermann’s neuroscience institute, said he and other staff members “have noted daily improvements in her neurological condition.”

Giffords began a course of physical, occupational and speech therapy in her room Wednesday afternoon. The main focus of therapy at this point is to improve her conditioning, strength and range of motion, said chief medical officer Gerard Francisco. As the therapy progresses, she should be able to sit and stand on her own, he said.

Although she has a breathing tube, Dr. Imoigele Aisuku said Giffords can breathe without assistance. Doctors said a valve will be put in place that will allow her to speak until the tube can be safely removed. The doctors skirted questions about the extent of Giffords’ ability to communicate, with Kim noting only that her “speech function” has been improving.

A tube to drain excess cerebrospinal fluid and prevent swelling in the brain was removed Monday.

Although Giffords’ seat was left empty from Tuesday’s State of the Union address, Republicans and Democrats abandoned their usual seating configuration and sat side by side in response to calls for more civil political discourse in the wake of the shooting. Most lawmakers wore white and black ribbons in honor of Giffords and the other Tucson shooting victims.

Kelly watched the State of the Union on the TV from his wife’s bedside, while holding her hand.