Das and Sirotin used an optical-imaging technique to measure the amount of oxygen in the blood and the rate of blood flow separately in the monkeys' visual cortices. They then compared the results with measurements of brain activity taken with electrodes inserted in the same area.
The electrode and blood measurements coincided when the monkeys were looking at a dot, as expected. But when they were expecting to see something, and nothing actually appeared, there was an increase in the flow of oxygen-rich blood to the visual cortex without a corresponding electrode signal.
"What caught us completely by surprise was that there was this huge anticipatory signal which appeared prior to us showing the visual stimulus," Das says. The purpose of this could be to supply cortical arteries in the visual region in time for the upcoming stimulus, he says.
The study was published in Nature.
This means that there could have been a lot of misinterpretation of the degree of brain activity correlated to certain events. I'm surprised that such a "controlled" study isn't done first before such a thing is used.
 Y.B. Sirotin and A. Das, Nature v.457, p.475 (2009).