[Eeglablist] Average reference questions.

[Robert Oostenveld]

On 26 Jul 2004, at 21:27, Arnaud Delorme wrote:

>> 2. Which advantages and disadvantages average references method has?
>>
> I am not an expert on this topic. I would say that the advantage of 
> using average reference is that you recover the signal from the 
> reference channel (thus you have one extra channel even if it is 
> linearly dependent on the activity of the other channels). Also, 
> average reference is necessary for dipole fitting.

In principle average referencing is not required for dipole fitting, 
but the DIPFIT implementation in EEGLAB does require average reference 
data. The advantage for average referencing in dipole fitting, as 
compared to using a common reference, is that it does not put excessive 
weight on an arbitrary single channel (i.e. the reference channel). Let 
me try to explain:

During dipole fitting, the sum-squared error over all channels is 
minimized. If the forward model is inaccurate on the reference channel, 
the model error in the reference channel spreads over all other 
channels (since the potential on the other channels is computed with 
the model as being the difference between the potential on that channel 
and the specified reference channel). Using a common reference results 
in an strong tendancy to minimize the (model) error on the reference 
channel, since the potential on the reference channel is reflected in 
all other channels.

Furthermore, since the reference channel is commonly on mastoid or ear 
electrodes, the volume conduction model error in them is relatively 
large (the area around the ears is poorly modelled using a spherical 
model), hence the model error in the reference channel tends to be 
arelatively large if using the ear or mastoid. By taking the average 
over all channels as reference, the error in the model is assumed to 
average out in the reference, thereby minimizing its influence in the 
sum-squared error that is used as the goal function to be minimized 
during dipole fitting.

best regards,
Robert

----------------------------------------------------------------------
Robert Oostenveld, PhD
Center for Sensory-Motor Interaction (SMI)
Aalborg University, Denmark

and

F.C. Donders Centre for Cognitive Neuroimaging
University Nijmegen
P.O. Box 9101
NL-6500 AH Nijmegen
The Netherlands

Tel: +31 (0)24 3619695
Fax: +31 (0)24 3610989


----------------------------------------------------------------------
Robert Oostenveld, PhD
Center for Sensory-Motor Interaction (SMI)
Aalborg University, Denmark

and

F.C. Donders Centre for Cognitive Neuroimaging
University Nijmegen
P.O. Box 9101
NL-6500 AH Nijmegen
The Netherlands

Tel: +31 (0)24 3619695
Fax: +31 (0)24 3610989