"The present study compared the effects of caffeine on subjective arousal among introverts and extraverts.... caffeine improved the task performance of extraverts but overaroused introverts and thus impaired their performance."
The second part of your quote (which comes first) was previous research they were setting out to repeat, instead they conclude with "Results do not support the hypothesis that caffeine differentially affects extraverts and introverts".
"The present study compared the effects of caffeine on subjective arousal among introverts and extraverts.... caffeine improved the task performance of extraverts but overaroused introverts and thus impaired their performance."
http://www.ncbi.nlm.nih.gov/pubmed/10472512