Direct Electrical Stimulation: Insight into brain networks

I have just been going through a thought provoking new article by Hughes Duffau (2011), and thought I would provide a brief synopsis of the methods, and highlight a result or two. For those of you who are not very familiar with Direct Electrical Stimulation (DES) work, this would be one of the better places to start, as it is very well written and the reference section covers nearly the entire literature.

The first strength of the paper lies in the methods. Whereas the vast majority of human neuroimaging (MRI, EEG, MEG) attempts to measure cortical activity through the scalp, skull, and meninges, DES goes straight to the source: In patients undergoing brain surgery (awake patients to be specific), bipolar electrode tips deliver biphasic current pulses to the cortex and white matter of the human brain. While the current is being delivered, a patient’s performance on some task is being monitored. When the patient’s performance changes due to the stimulation, you have successfully mapped an “eloquent region.” In other words, that region’s neuronal activity plays a direct role in producing that behaviour.

Dr. Duffau's results using DES support a view of the brain that is very dynamic, and do not adhere to the localization hypothesis. To put it another way, complex human behaviours such as language rely on many different brain regions operating in concert. If one of the regions is damaged, it is possible to re-organize the circuit and preserve the function (as long as the white matter connections are left intact). Bear in mind that this is in relation to so-called frontal/executive functions (cognitive processing, language, emotion, etc), and basic sensory systems might operate far differently (not surprisingly, Dr. Duffau has published on this as well).

Below are a few references... I am including two other intriguing articles for those who want to know more about DES, and how the brain might be wired. As always, email me with any questions or just follow up with a comment.

Duffau H (2011) The “frontal syndrome” revisited: Lessons from electrostimulation mapping studies. Cortex (In Press).

Mandonnet E, Winler PA, Duffau H (2010) Direct electrical stimulation as an input gate into brain functional networks: principles, advantages, and limitations. Acta Neurochirurgica 152: 185-193.

Mandonnet E, Jbabdi S, Taillandier L, Galanaud D, Benali H, Capelle L, Duffau H (2006) Preoperative estimation of residual volume for WHO grade II glioma resected with intraoperative functional mapping. Neuro-Oncology 9: 63-69.

DTI preprocessing in FSL: B-vector correction

Using FSL to pre-process DTI data is easy to do, and the use of a standard FMRIB pipeline is ubiquitous in the literature. An ongoing concern with this common pre-processing pipeline is how to appropriately deal with eddy currents. FSL uses a 12 degree of freedom affine registration as a method of eddy current correction. By definition, therefore, it corrects for both eddy currents AND gross subject motion. So far this seems great... however...

It is really important to alter your b-vector file based on subject motion (Leemans and Jones 2009), but one should NEVER correct b-vectors based on eddy currents, since these vary across the brain (randomly) depending on a variety of factors. This can have profound effects on not only tractography, but also general diffusion modelling (DTIfit). Current wisdom on the FSL forum suggests that using the downloadable script “rotbvecs” will solve your b-vector correction issue. The problem is that this will correct for both subject motion AND eddy currents.

Here is a simple solution that will allow you to keep using FSL for preprocessing, and keep you up to date on appropriate b-vector corrections:

1) Create a new eddy_correct script that only uses 6-degrees of freedom, and call it eddy_correct_6dof. To do this, just add “-dof 6” to the following line of code:

${FSLDIR}/bin/flirt –in $i –ref ${output}_ref –nosearch –o $i –paddingsize 1

Then run the rotbvecs script as normal (this changes the bvecs file to reflect subject motion).

If you still want to correct for eddy_currents, simply run the regular eddy_correct script, and don’t touch the b-vector file. This might seem like a time consuming extra step, but probabilistic tractography is incredibly sensitive to the initial quality of the data. If you are looking for more detailed instructions, just send me a quick email (look to http://csl.psychol.cam.ac.uk/people/) for my contact info.

Of course ... one can also simply choose not to use FSL to pre-process the data... some great alternatives are emerging.