This paper grew out of an interesting collaboration with some physicians at the University of California, San Francisco and San Francisco General Hospital, initially through a meeting between Dr. Geoffrey Manley, Dr. Robert Knight, and I. Dr. Manley has recently published several papers on the clinical benefits of performing a decompressive hemicraniectomy on people who have had some kind of head trauma. To give a little bit of a background, a decompressive hemicraniectomy is a surgical procedure in which the surgeon actually removes a large part of the skull (see the picture at the right) after someone has had head trauma that has caused the pressure inside the skull to increase. This can happen in a few ways, but basically, because the head is an enclosed system, if the brain swells arteries can get pressed closed. This can cut off the blood supply to different brain areas. The swelling can also cause the brain to press down onto the brainstem which can lead to coma or death.
These folks go without a big piece of their skull for several months. If you're paying attention, that means... yes... there's not a whole lot protecting their brains. As you could imagine most of them wear helmets during this time. Also, some of them actually have the piece of their own skull surgically placed inside their abdomen so that the skull tissues can be kept alive before the get their skull surgically put back in!
Working with these patients gave us a unique opportunity as cognitive neuroscientists. Most of my research uses EEG to examine attention and memory processes. One of the things about EEG is that you can't accurately locate where in the brain something is happening, but you can know when it happens with excellent accuracy. However, because these patients literally have a window onto the brain we can get a much better idea of where the signal we're recording is coming from. And, for a variety of reasons, the signal quality is better over this window.
Our lab does a lot of work with humans who have had electrodes surgically implanted directly onto their brains (I'll write a more in-depth post about this topic in the near future when one of my papers on this topic is published). Because of this I see a lot of really clean data from the intracranial recordings that looks much better than the data we see in normal scalp EEG. So we decided to try and quantify these differences to a certain extent, and thus ran this study.
So in this paper we set out to quantify how the brain signals we record in EEG are different between the side of the head with the skull and the side of the head without. And because the signal quality is better, we can do a few cool things with it... like predicting when a person squeezes their hand just by looking at the brain signal.
It was a fun project, but a bit tricky to run.