## Movement direction and linear equations

This forum deals with BCI2000 configuration issues.
Elizabeth
Posts: 19
Joined: 20 Jan 2005, 16:29

### Movement direction and linear equations

Hello,

I have a question about the linear equations that are used to determine cursor movement in the standard mu/beta rhythm task. Two strategies for training a subject to control a cursor in 1-dimension can be:

1 - Think about some type of imagery to move the cursor right, and relax to move left.
2 - Think about separate types of imagery for moving the cursor right and left (so use 2 channels, each which responds to different types of imagery, for example, C3 and C4 can be used for right and left hand imagery.)

In case 1, I'm pretty sure I understand how the linear equation and adaptation works - it finds the running average of the signal (Ymean) and above that the cursor moves right and below that the cursor moves left (or vice versa depending on the weights).

In case 2, it does something similar, but both signals are added together (after being multipled by their weights) and the average of this combined signal is used for Ymean.

If I turn off adaptation (for case 2), both signals still contribute to each direction because they are being added together in the linear equation.

Finally my questions:

1 – Is it correct that in case 2, Ymean will be the average signal after all contributing signals (for two different directions of movement imagery) have been added together?

2 – If so, couldn’t this lead to problems with non-independence of the signals? For example, right hand imagery may lead to a large response on C3 at a certain freq band, but may also have a response in C4. However, if you have C4 setup for left hand imagery (and cursor movement in the opposite direction), it seems there is potential for a problem due to correlation of the signals.

2 – Is there some way to make each direction independent (so each type of imagery is solely responsible for ‘right’ or ‘left’, and not both rolled into the overall linear equation)? If not, is there a reason why the current method is better?

Thanks for all of your help.

Elizabeth Felton

gschalk
Posts: 615
Joined: 28 Jan 2003, 12:37

Elisabeth,

As always, you are asking excellent questions. Your questions contain two components. First, one can train people to use different mental strategies (such as using one or more different mental strategies). Second, one can use either one or more brain signal features.

In regards to the first component, it is not clear which method is better. With EEG, and also ECoG, we have typically used only one mental strategy for each dimension.