Week 5 - Charlie

At the end of week 4 I determined that Dr. Marei's TMS sessions were appearing to stimulate in the mesial premotor cortex (MPMC) region. It turns out that the target region is the M1 motor cortex, which is about 10mm posterior to the MPMC. However, I obtained this result in one session dataset only. After analyzing 9 other sessions, I found that about 50% of the time, the stimuli were centered in the M1 cortex, and 50% of the time they were too far up in the MPMC. Whether the stimuli were accurate or not appeared to heavily depend on the first stimulus, which sets the visual target on the display. All subsequent stimuli in the session are aimed at this same target. My new goal for the summer is to give some sort of feedback to the TMS administrator to let them know whether they are in the correct area when they set the stimulation target.

But first, I need to perform more technical validation. As it turns out, there is not one single coordinate system used to define brain regions. There are much more, around 5 or 6 main types at least. The two most commonly used are Talairach coordinates and MNI coordinates. Talairach is based on an individual's brain, whereas MNI coordinates are obtained from a large set of averaged brain MRIs. MNI is more common nowadays, but the paper describing brain region coordinates that I have relied on is a meta analysis that uses Talairach coordinates. All sources that give coordinates in MNI were converted to Talairach. To make matters worse, there are different types of MNI coordinates and multiple ways to convert between MNI and Talairach. I will likely be spending a few days next week on this dilemma!