Application idea and Objectives
The project’s main idea is the development of a sensor system, based on electrodes able to measure psychic patterns. The main patterns under study include variations in the occipital area where the primary visual cortex (V1) is. This area is responsible of the ionic fluctuations derived from the human visual system.
These alterations can be performed by the user at will by simply blinking the eyes. By identifying different patterns, we will be able to assign different functionalities to each of them, and hence create a human-computer interface.
The EEG (Electroencephalogram) is normally described in terms of rhythmic activity divided into different frequency bands, ranging from 0 up to 100 Hz. Scientific research has demonstrated that the mayor differences when the eyes are open in contrast when the eyes are closed, are found in the “alpha” band (8 to 12Hz). Please notice the difference in amplitude in the following graphical representation of the output spectral analysis around the alpha band (around 20dB).
As the key denotes, the red curve represents the spectral activity when the eyes are open in contrast to the blue curve, that represents when the eyes are closed.
Positioning of electrodes is a question to be answered empirically in this project. Electrode locations are specified by the international “10-20 system” map shown on the second diagram below. Although neural activity is channeled over the occipital area (O1, O2) as seen from the first figure, different locations obtain different activity fluctuations, as seen on the figure below. It is a temporal representation of sensor output regarding different spatial positioning of the electrodes, when a single blink is performed.
As it can be seen, both areas show important fluctuations. Two electrodes should be enough to be able to distinguish between both states (eyes open, eyes closed) in O1, O2 locations or Fp1, Fp2 locations. Alternatively, four electrodes will clearly reinforce the information sampled.
Hardware Software – Block Diagram
From an electrical point of view, the measurement of brain activity is quite a challenge since the order of magnitude of these signals is around 10 to 100 microvolts (for measurements on the scalp surface for a typical adult human). The following block diagram describes different parts of the system.
It is basically formed by a chain of components: a pair of active dry electrodes “pick” the signal input that will be pre-amplified and introduced to a differential instrumentation amplifier; from here the signal is cleaned from common noise and amplified to be then introduced to an ADC; posteriorly digital data will be sent via a serial port communication to finally be treated by a mathematical software as Matlab.
A LPF may be added to reject noise frequencies above 100Hz, although this option is still under study. Note that active dry electrodes do not need conductive gel to work.