This study was one of the key pieces of research in this area. Valentine & Sweet measured sustained attention in 19 meditators. They found that the meditators demonstrated superior performance in comparison with controls, and long-term meditators were superior to short-term meditators.
In addition they looked at the effects of an unexpected stimulus on meditators. Mindfulness meditators showed superior performance in comparison with concentrative meditators when the stimulus was unexpected but there was no difference between the two types of meditators when the stimulus was expected.
Reference: Valentine & Sweet, 1999, Mental Health, Religion and Culture