This page concerns the description of several common EEG archival file formats which can be imported by the Prolate1 EEG display and analysis software. This enables the software to import and parse many EEG data files which are currently available free of charge via the World-Wide-Web and other sources of distribution, significantly enhancing the software's utility and appeal to a wide audience of researchers in the fields of theoretical and clinical neuroscience, diagnostic medicine, and cognitive study.
Need for sensor coordinates:
A large and important part of the Prolate1 software is the ability to generate a continuous interpolation of the 3-d electric field potential from data values acquired at a set of arbitrary sensor positions. Therefore it is essential that the software have access to the angular coordinates (longitude and latitude) of each sensor (EEG electrode) in a given experimental setup. The actual interpolation of the electric field potential is performed via a triangulation generated by the program from the coordinates of each sensor.
A necessary part of the importation of EEG datasets is the specification of the exact coordinates of each sensor's location on the spheroid. However, many EEG file formats do not contain explicit coordinates for each sensor. Therefore, part of the importation process includes the automatic generation of sensor coordinates from information which is contained within the imported EEG dataset (if possible), or the generation of such coordinates from a separate geometry description file which is also read by the software.
After importing datasets:
After importing data, you may wish to perform the following steps to archive the data as a Prolate1 document, and to change settings for displaying the data:
Currently supported file formats:
The format of an example ASCII text file containing two sensors is as follows. Files containing additional sensors will also contain additional columns of data:
File type: ASCII text Data format: Rectangular table of numbers, delimited by commas, spaces, or tabs. Column 1: Size, format and time. Element 1 is # sensors. Element 2 is # frames (samples/sensor) and format: >0 = unsigned byte format (0-255 values), <0 = real format (any real). Elements 3-n: Time values (secs) for corresponding sensor data. Columns 2-m: Sensor position and data. Element 1: Longitude angle (radians) of sensor position. Element 2: Latitude angle (radians) of sensor position. Elements 3-n: Data values of sensors for corresponding times: Data is 0-255 for format >0, Data is any real for format <0. Note: Longitude angles must be 0 to 2*pi. Latitude angles must be 0 to pi.
Click here to see an example text file containing 15 channels and 25 frames of real-value data.
The format of an example binary file containing two frames (samples per sensor) of data is as follows. Files containing additional sensors will also contain additional interleaved data.
File type: Binary Data format: Stream of binary data. 4b int: # sensors. 4b int: # frames (samples/sensor) and format: >0 = unsigned byte format (0-255 values), <0 = real format (any real). 8b reals: Longitude angles (radians) of sensors. 8b reals: Latitude angles (radians) of sensors. 8b real: Time value for frame 1. 1b unsigneds, or 8b reals: Sensor values for frame 1. 8b real: Time value for frame 2. 1b unsigneds, or 8b reals: Sensor values for frame 2. Note: Longitude angles must be 0 to 2*pi. Latitude angles must be 0 to pi.
EGI Netstation Format
The Prolate1 software can read EGI (Electrical Geodesics Inc.) 128 channel Netstation version 2 or 3 files directly. Simply select the Netstation button from the "Read Sensor Data..." dialog in the File menu. You will then see the following dialog:
Fill out the entries for the subset of the file you wish to import. Note that only version 3 files have segments. Also note the following memory constraints:
You must also specify a sensor coordinate file, which is an external text file containing the longitude and latitude angles of each sensor in the data file. This is simply a text file containing two rows: the first row specifies the longitude angles of each sensor, the second row specifies the latitude angles of each sensor. Click here to see an example coordinate file for the standard 128 channel Netstation sensor layout. You may create your own coordinate files for datsets containing other than 128 channels, or you may modify the example coordinate file shown above.
Click here to download an example Netstation file and sensor coordinate file (2.1 Mb).
There is also a stand-alone program which will convert a Netstation file and associated sensor coordinate file to the text format which can be imported by the Prolate1 program. This program has been derived from code posted at the EGI website. This program can be used to conveniently convert Netstation data to a form which can be viewed and edited with a standard text editor. Click here to download this program and a sensor coordinate file (72 Kb).
Neuroscan CNT Format
The Prolate1 software can read Neuroscan CNT (continuous) data files directly. Simply select the Neuroscan CNT button from the "Read Sensor Data..." dialog in the File menu.
Note that Neuroscan is a PC file format. Therefore, you must port your PC data file to the Mac, probably using PC Exchange. Do not perform any translation on this file when porting. The Prolate1 program will perform all necessary data unpacking and byte reversal in order to correctly read this PC format file. Also note the following memory constraints:
Sensor coordinates in a Neuroscan file are determined by the extended 10-20 nomenclature of the electrode name for each channel. The conversion from electrode names to spheroidal coordinates is automatic. Generally, the x-axis will be the nose of the subject.
Also note that the Neuroscan file must be saved with a valid event table, even if you are not using events. If your data file does not contain an event table, you will get an error message when you attempt to read it.
Click here to download an example Neuroscan file (64 Kb).