The chronic problem forced the rookie from Florida to miss the Vikings' game against the Cincinnati Bengals last week. Minnesota coach Brad Childress said Harvin participated in a walk-through with the team on Wednesday morning, but he wasn't at the afternoon practice.
Harvin is seeing a specialist for his condition, which Childress called "debilitating." The first-round draft pick is second on the team with 681 yards and six touchdowns receiving headed into Sunday's game at Carolina.
Harvin is also second in the NFL in kickoff returns with a 29-yard average and has brought back two for scores.
"It's kind of an abstract thing right now," Childress said. "We know that he gets headaches. We don't know the whys and what-fors."
Minnesota's injury report was slightly smaller Wednesday than last week's long list, with nine other players limited in practice. It included safety Tyrell Johnson, who missed the Bengals game because of a concussion. Center John Sullivan was also listed with a concussion.