Medical journal Psychophysiology announces the results of a study in the latest issue of their magazine suggesting action gaming has different effects on two types of cognitive activity: proactive and reactive attention. There is an abstract of the study here, and here is a chunk of the announcement outlining how their data "converges with other recent findings indicating that there is a relation between frequent video game playing and Attention Deficit Disorder." Word is:

Proactive attention can be thought of as a sort of “gearing up” mechanism. For instance, when players that are familiar with a particular game anticipate an action they need to take, such as getting a key or a pot of gold, in order to get to the next level. Reactive control is described as happening “just in time”, for example, when a monster suddenly appears that is about to thwart the player’s advantage or ability to get to the next level.
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The researchers found that the just-in-time form of control was similar in the two groups of gamers. In contrast, brain wave and behavioral measures of proactive attention (the “gearing up” mechanism”) were significantly diminished in the frequent video game players. These data reveal a reduction in brain activity and disruption of behavior associated with sustained attention ability related to video game experience, which converges with other recent findings indicating that there is a relation between frequent video game playing and ADD. This negative relationship between action games and proactive attention can be contrasted with the beneficial effects of these games on other aspects of visual processing. The research team is also exploring whether non-gamers who play action games produce the same results as those found in frequent players.