Digitize EEG Sensors and Head Shape

Getting Started

The Brainstorm Digitize application allows for a connection to a Polhemus device and collection of digitized head points and/or EEG sensor locations.

The connection to the device is made via a serial port connection. This can be accomplished using a direct serial connection or with a USB-to-Serial cable connection to the host computer.

Figure 1. Polhemus system with two receivers and one transmitter

Figure 2. Typical Polhemus setup

The application is designed for two receivers (i.e. the stylus and reference) one transmitter.

The stylus is used by the operator to point to the desired position for collection

The reference is fixed to the subject’s head, typically with glasses or headband. While the subject’s head can move during the collection, the reference sensor should not move from the starting position relative to the head.

The transmitter is fixed behind the subject, typically to a non-metal chair or table. The orientation is shown here, with X+ pointing up and Y+ point to the right.

Figure 3. Transmitter orientation

****Care should be taken when setting up this system to ensure accurate collection. Thorough testing should be performed using known head coordinates (phantom) to determine accuracy.

The head coordinate system used in the Digitize application is based on the CTF head coordinate system, see /CoordinateSystems.

To start the Digitize application, start Brainstorm. Then select ‘Digitize’ from the File Menu.

Figure 4. Starting Digitize

The Brainstorm GUI will be hidden during the use of the Digitize application and will reappear after exiting Digitize.

The Digitize GUI

Figure 5. Digitize GUI

This application allows you collect:

• Three anatomical landmarks for computing the head coordinate system
• EEG sensor locations
• Scalp points for head shape

The collection can performed by:

• One operator who will press the button on the stylus to collect a point
• Two operators, which one will point the stylus at the desired location and one will click the ‘Collect Point’ button on the lower panel of the GUI.
• Points are collected in order from Nasion, Left , Right, EEG (if desired), then head shape.

If an error is made during collection:

• The last point can be removed by clicking “Delete Last Point” on the lower panel of the GUI
• The entire session can be restarted by selecting ‘Start Over’ from the File menu

When complete the coordinates can be save in a variety of formats including CTF (*.pos), megDraw (*.eeg) or tab-delimited ASCII text.