Doctors are cautiously optimistic about the condition of US Congresswoman Gabrielle Giffords, who was shot when she was greeting constituents at a grocery store in Tucson, Arizona.

The 40-year-old politician is in a drug-induced coma and is being awakened periodically so doctors can see how she is progressing. Doctors say she was responsive on Sunday morning to simple commands, a positive sign although she is still in a critical condition.

The gunshot went through the left side of Giffords' head and did not pass through both hemispheres. The congresswoman now faces swelling, a grave danger. Surgeons have removed about half of her skull on the left side to alleviate the pressure.

Six people died and 13 others were injured in the killing spree on Saturday. Giffords is the only patient who remains in a critical condition.

US President Barack Obama has called on Americans to hold a moment of silence on Monday to honor the innocent victims.