Jonah Lehrer shows how navigating a city street depletes the brain. He applies this research to the "is Google making us stupid" debate:
The lesson, I think, is that everything is a cognitive tradeoff. The city street forces us to exert top-down attention, and that leads to measurable decreases in mental function. On the other hand, the internet, it is argued, encourages a constant state of multitasking and distraction, and that leads to an intellectual shallowness, as we lose the ability to focus for extended tracts of time. My hunch is that the online world will, before long, come to seem as inevitable and necessary as the metropolis. Why? Because the value it provides far outweighs the cognitive costs (which may or may not exist.) And this is why I'm wary when the brain/mind becomes the main criterion for discussing the value of the internet. Such an approach is roughly equivalent to thinking about cities solely in the narrow terms of a few attentional lab tests.