EDEN PRAIRIE, Minn. -- Shortly after Vikings center John Sullivan walked to the sideline Sunday in Buffalo, after taking a knee to the head from Brandon Spikes on a play in which Sullivan tried to cut-block the Bills linebacker, the dizziness and double vision that initially made Sullivan think he'd suffered his fifth concussion started to go away.

Sullivan sat on the Minnesota Vikings' bench with athletic trainer Eric Sugarman, and Sullivan said he felt good enough to go back in the game on the Vikings' drive as they moved toward the Bills' 20. Sugarman resisted, saying he wanted to conduct one more round of testing in the locker room.

"I got into a little bit of an argument with Sugs, because after a few seconds, I was feeling fine sitting on the bench," Sullivan said. "On the way in, I suffered another spell of dizziness. It was not a catastrophic hit by any means. It was just, in order to go back in the game, I was going to have to lie to our doctors and to the neurologist, and I wasn't going to do that."

An important truth about concussions, and the messiness of managing them in real time, can be found in Sullivan's comments Friday: For all the advances the league has made in testing -- and for how stringent the treatment protocol has become -- there's still no black-and-white test to diagnose a concussion. That means there's still some element of the treatment process that relies on a player's cooperation. If Sullivan indeed passed his baseline test in the locker room and had been dishonest about lingering symptoms, he might have been able to go back in the game. It's especially in cases like these, where the effects of a concussion might not be as immediately apparent, that a player's honesty is integral to the process.

"It's tough, because you don't want to tell a guy how to live their life, but we have a responsibility to be honest about what we're going through with those hits," Sullivan said. "It's difficult to do any studies if guys aren't being truthful about what they're going through. You feel like you have a duty to yourself, and your family, to tell the truth. At the same time, even if you are suffering symptoms, you feel like you're letting your teammates down, because you're not out there. I felt horrible that I wasn't able to play the rest of the game, but I had to be honest."

Sullivan said he passed the ImPACT test Tuesday and was able to exercise without symptoms. He lifted weights and did on-field work Wednesday, and participated in noncontact drills Thursday. As he talked to reporters Friday afternoon, he said he felt "totally normal."

The next concussion could be right around the corner, as it could be for any player, and Sullivan admitted it's possible he's had more. But, he added, "I feel like my future isn't in doubt.

"It's not so much about the number, specifically. If you were to have five concussions where your symptoms subsided in a matter of days, and you don't even miss a game, is that as serious as one concussion that keeps you out for six months?" Sullivan said. "The other thing is, you can talk about the number, but if I had lied my way back onto the field, I'd still be at four. What point is there in getting caught up in the specific number? It just means I was honest five times."