bci2000.org BBS

Hello,
I am an engineer of Electronics and Communication working on BCI.
Recently we have been conducting SSVEP experiments using g.USBamp and dry electrodes system. We are acquiring the EEG signal and storing as a mat file, to
analyse using matlab. Applied fft on signals to view power spectrum of the signals. We see many different frequencies components along with interested frequency. The amplitude levels of the interested frequency is lesser than others. yest. when the subject was idle(not viewing at flickering LED) the response showed 5HZ, 13HZ,11HZ,15HZ,19HZ these frequency components, while we were expecting no frequency to appear.

Thanking You
Sahana Prasanna

Hello,

there are a number of prominent EEG rhythms present in the 9-18Hz range, which also have harmonics at higher frequencies. E.g., you will see the wide-spread alpha rhythm at around 10Hz when the light is dim, and you will see the mu rhythm over sensorimotor cortex at 9-12Hz with its first harmonic at 18-24Hz. Below 9Hz, you will see low-frequency noise which comes from impedance variations at the sensors, movements of the head, movement of electrode cables, eye movements, eye blinks, etc.

Thus, you will need to minimize recording noise and artifacts, and use spatial filtering to extract signals from the visual cortex. Also, you will need to obtain a baseline amplitude at the frequencies of interest, and compare current amplitude against that baseline in order to detect whether the subject is looking at the flickering LEDs or not.

In order to be able to interpret your observations, you will need to get some background in EEG. A very good source is the book of Lopes da Silva et al, which contains many introductory chapters on all aspects of EEG:
http://books.google.de/books?q=lopes+da+silva+eeg

Some very basic information about EEG recordings may be found here:
http://www.bci2000.org/wiki/index.php/U ... ment_Setup

Also, numerous articles have been written about how to design an SSVEP-based BCI:
http://scholar.google.com/scholar?q=ssvep

Regards,
Juergen