EEP - avrinterpol

avrinterpol - Spatial ERP Interpolation

Synopsis
Description
- Repair Single Channels
- Complete Redesign
Example

Synopsis


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EEP 3.1  Max-Planck-Institute of Cognitive Neuroscience 1996-99
avrinterpol 3.3   (OSF1 V4.0 alpha)           Wed Sep 15 13:30:28 1999
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  avrinterpol [-t] <avr> <dat> -x <excl regex> -i <interp regex> [<avr out>]
  avrinterpol [-t] <avr> <dat> -x <excl regex> <interp dat>      [<avr out>]

options:
  -x <excl regex>   to exclude channels from interpolation database
  -i <interp regex> to select the channels which are to be interpolated
  -t                test mode, list channel usage only

files:
  foo.avr  ->  fooi.avr

avrinterpol uses case insensitive extended regular expressions(POSIX.2) for channel selections. See the grep manpage for details. Use the -t option to check which channel is really used for what.

For example, the command

avrinterpol foo.avr foo.dat -x "eog|^x" -i "^F7$"

would produce a file "fooi.avr" where the channel "F7" is interpolated from all channels of foo.avr except of the channels which contain the substring "eog" or which start with the letter "x".

Description

avrinterpol computes "virtual" ERP signal data at requested positions at the unit sphere surface. Basically, a model of the amplitude topography at the unit sphere surface is generated from "good channel" signal data in an ".avr" file and the corresponding sensor positions in a ".dat" file. Using this model a interpolated value at any point of the sphere surface can be computed.

avrinterpol supports two modes corresponding to the main purposes:

Repair Single Channels

Defect ERP channels (drift, sensor failure ...) are replaced using interpolation from good channels. This way you can avoid to remove this channel or subject completetly from your study. The "repair mode" is selected by the presence of the -i switch in the command line.

To be used for interpolation input a channel must fullfill the condition:

channel data are present in the input .avr file and
channel position is present in the .dat file and
label doesn't match the exclude pattern (-x) and
label doesn't match the interpolate pattern (-i)

The generated output .avr file consists of copied and interpolated channels. A input channel is copied to the output .avr file if:

channel is present in the input .avr file and
label doesn't match the interpolate pattern

A channel is replaced by its interpolated version in the output .avr file if:

channel is present in the input .avr file and
channel has a valid position in the .dat file and
label matches the interpolate pattern

Complete Redesign

Sometimes one needs to compare ERP data obtained under different acquisition setups. avrinterpol allows the transformation from the recorded ERP (described by an .avr and the corresponding .dat file) to a new virtual .avr file which contains channels according to the positions in another .dat file. The "redesign mode" is selected by the absence of the -i switch in the command line.

To be used for interpolation input a channel must fullfill the condition:

channel data are present in the input .avr file and
channel position is present in the input .dat file and
label doesn't match the exclude pattern (-x)

The generated output .avr file consists of copied and interpolated channels. A input channel is copied to the output .avr file if:

channel is present in the input .avr file and
label does match the exclude pattern

A interpolated channel is created in the .avr file if:

channel has a valid position in the .dat file and
label does not match the exclude pattern

Example("redesign" mode)

Below you see a ERP, as mesured in a 64 channel setup and the same dataset "redesigned" to a 128 channel setup. Both versions are used to produce a xavr contour plot in a high resolution.

As expected, both versions produce the same topography - except that the 64 and 128 channel designs cover slightly different parts of the head surface. No noticeable data loss or falsification is caused by the avrinterpol run.

map, 128 virt. sensors
Surface field distributions, based on 64 real sensors(left) and 128 interpolated sensors