"They did improve after they got a caffeine drink, but only up to the level the non-users had achieved without caffeine. When the non-users were given caffeine

"They did improve after they got a caffeine drink, but only up to the level the non-users had achieved without caffeine. When the non-users were given caffeine to drink their reaction times increased but they also became more jittery and anxious." Seems like an important part of the quote.