drain more of our mental resources while we are reading
In the abstract, the study cited points to "dual-task effects of fulfilling the assignment and working with the computer resulting in a higher cognitive workload".
Although this load may decrease with people getting more and more familiar with computers (and computers getting more and more intuitive), it is also the case that more distractors are available too.
This reminds me of a webinar hosted by Hypothesis in which Amanda Licastro mentioned Cathy Davidson's book "Now you see it" to talk about "Productive Multitasking". In the view that Amanda presented (at least what I understood of what she said), multitasking and distractions are unavoidable, but we can canalize them productively through web annotation, for example -instead of switching to Facebook and disconnect from what we were reading.


