Day 2: An Unexpected Turn

In my last post, I mentioned how what I thought I would be doing isn't what I will be doing over the next few months. The difference in excitability of the motor cortex for planning position (P) versus planning the application of force along with position (P+F) was seen in the "pre-experiment," where the results showed that the MEP (motor evoked potential) values of P were higher than those of P+F. Additionally, the MEP values of both variables showed inhibition (the difference of the MEP values from the Baseline), which takes place during the planning. There are a few questions we will try to answer with this new experiment following up this "pre-experiment." Why is there a difference in the excitability between P and P+F? Is it due to different inputs or different gain from the same input? Why do we see inhibition? 

We hypothesize that two areas of the brain--the dorsal premotor cortex (PMd) and the ventral premotor cortex (PMv)--are the two areas that we should be focusing on. PMd has been linked to arbitrary force cues and associative learning. It has also been seen to be responsible for the delay in hand transport by the forearm muscle and temporal relationship between gripping an object and lifting. PMv is said to be linked to the difference between planning the position of 2 fingers (index and thumb) and the whole hand. So to see if PMd and PMv is responsible for planning force application and position, respectively, we inhibit each of the areas using the TMS!

This is where it gets really exciting!

We will use continuous theta-burst stimulation (cTBS), "where TMS bursts are applied at a frequency of 5 Hz over a period of 20 or 40 s," causing a decrease of MEP values (Gentner et al., 2007). In other words, the cTBS is a type of TMS that temporarily shuts down an area and creates a virtual lesion. When I mentioned this to Jeff today at lunch, he responded, with a worried face, "Imagine trying to grab and object and your hand gets there. 'Oh my gosh I can't squeeze the object!'" It's almost really just that.

So when we disable the PMd, ideally, we would like the results to show this.

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By disabling PMd, which we hope will remove the force application (F=0), we would expect the MEP values of P and P+F to be equal.

Likewise with PMv and position.

Click on image to enlarge

By removing the ability of planning the position of the digits, we would expect the MEP values of P to be at the Baseline, meaning that there was no excitability nor inhibition. P+F would increase since there will be no inhibition caused by the planning of digit positioning.

However, because of the sheer complexity of the brain, our expectations could be completely off. The MEP values of P+F could possibly turn out greater than those of P. Because of all these possibilities and unpredictability, our hypothesis may be wrong.

I will continue to update you more on the change of my focus of my research project. If you want to learn more, feel free to leave me a question (or questions) in the comments.

References:

1. Gentner, R., Wankerl, K., Reinsberger, C., Zeller, D. Depression of Human Corticospinal Excitability Induced by Magnetic Theta-burst Stimulation: Evidence of Rapid Polarity-Reversing Metaplasticity. Oxford Journals. [Online] 2007 2046-2053. http://cercor.oxfordjournals.org/content/18/9/2046.full (accessed Feb 7 2013).