LAKE FOREST, Ill. (CBS) — U.S. Sen. Mark Kirk (R-Ill.) is described as alert and responsive after another operation to relieve swelling in his brain, after he suffered a stroke last weekend.

Northwestern Memorial Hospital neurosurgeon Dr. Richard Fessler said Kirk remained in serious, but stable, condition as of Thursday morning, with no change in his prognosis.

The surgery performed Wednesday involved removing two small pieces of tissue that had been rendered non-functional by the stroke.

On Thursday morning, Kirk was “alert, responsive and gave us the thumbs up on request,” Fessler said in a statement.

On Friday, Kirk’s best friend, U.S. Sen. Joe Manchin (D-W.V.) will fly in from West Virginia to pay Kirk a visit. During President Barack Obama’s State of the Union Address Tuesday night, Manchin kept an empty seat next to him to honor the absent Kirk.

Kirk called his personal physician, Dr. Jay Alexander, after feeling light-headed and noticing a change in his vision while driving.

Alexander ordered a CT angiogram, which revealed a dissection, or tear in an artery supplying blood to Kirk’s brain.

By 3 p.m. Saturday, Kirk was admitted to Lake Forest, and placed on bed rest and blood thinners. Six hours later, it became clear he was having a stroke.

Kirk has been hospitalized since, but Fessler said Tuesday that the senator was “doing better than expected” and even asked for his Blackberry within two days of first having the stroke.

Doctors have said they don’t know when he might be cleared to go back to work.