2 questions about reference

Post Reply
Bo
Posts: 7
Joined: Mon Dec 01, 2008 10:37 pm
Location: IL

2 questions about reference

Post by Bo »

Hi

Our lab recently started working with ActiveTwo system.
I have 2 questions about reference to which I couldn't find direct answers so far.

1. While I recorded signals from some subjects, I noticed that whenever I choose to display the data with the reference option set on "none" (CMS and DRL), I got a flickering purple light from the reference panel on the left side. From the reading I've made I understood that the data are saved in any case in a non referenced format and that the panel I'm talking about is for display only. However, as it looks to me like a warning signal I wonder what it means and whether I'm doing something wrong.

2. When uploading the BDF files to eeglab (version 6.01b) I am asked for a BioSemi ref channel number (with the option of non=average ref). Two issues are not clear to me here - If I use EX1 channel as a nose ref, how do I choose it? In addition, I get a message saying that if I do not choose a ref channel I loose 40dB of SNR. Why would that be the case with average ref, which is the default option when importing the data to eeglab?

Thanks for any advice!

B.S

Coen
Site Admin
Posts: 1124
Joined: Fri Mar 26, 2004 7:00 pm
Location: Amsterdam, Netherlands
Contact:

Post by Coen »

You are not doing anything wrong. The purple box is just a warning to let users know that no reference is selected for the displayed signals. You are fully correct that the reference selection does not influences the signals saved to file (indeed, signals on file are always unreferenced).

The channel number of the EX1 channel depends on the particular selection that you have chosen in file dialog in ActiView. For example, if you save the first 32 channels plus 8 EX channels, then the EX1 will be on channel number 33.

The average of all channels (or of a subgroup of channels) can certainly be used as a valid reference.

Best regards, Coen (BioSemi)

Post Reply