Week Eight Updates

This past week, the algorithm was modified. It is now possible to pause and play the music player by forcefully blinking one eye. Only one eye can be used because so far the algorithm only has variables for one channel and can only interpret the voltage changes of one eye.

Rolling graph of raw signal, filtered signal, and energy

A second channel is currently being further researched and tested in order for both eyes to be used to send signals to MATLAB. After there are two working channels, one for each eye, MATLAB will be able to recognize multiple signals and execute more commands.

The raw EMG signals were slightly different from an ideal EMG signal. An ideal EMG signal always starts around -70 mV because this is the resting potential of the neuron when it is not contracting. In order for the muscle to contract, the neuron axon undergoes voltage changes in order to tell the muscle to contract. The voltage change includes polarization, when positive Na+ ions enter the axon and K+ ions leave the axon. This is why the graph of the EMG signal increases. The decrease in the graph of the signal is the axon depolarizing, which is when the K+ ions flood back into the axon and the Na+ ions are pushed out. The frequency of the signal depends on the intensity of the stimulus. A strong stimulus will cause a very high frequency and a weak stimulus will cause a low frequency[12]. An ideal signal does not have noise, however the raw signal almost always has noise. A filter of the frequency cuts out some of this noise and make the signal clearer. The low frequency cutoff is 15 Hz and the high frequency cutoff is 150 Hz. These frequencies are used in the filter in order to get a certain range of frequencies. Frequencies less than 15 Hz and grater than 150 Hz are not included in the filtered signal. The raw signal has added noise, and depending on the filter, not all of the voltage changes are read by MATLAB. The added noise is from motion artifact and other stimuli.

An ideal EMG signal

The motion artifact is noise caused by the motion of the electrodes against the skin as they sometimes start to peel off. The frequency of the motion artifact is 2-3 Hz. The motto artifact causes small peaks and valleys in the signal when the eye muscles are not being contracted. There is other noise caused by wall noise which is about 60 Hz in frequency. All of the electrical signals in the body can interfere with the raw signal because there are constantly signals being transmitted between the neurons and the central nervous system. These signals are caused by processes such as breathing, and outside stimuli such as changes in the environment. All of this background noise can be avoided by using a filter.

The energy signal is being read by MATLAB instead of the raw or filtered EMG signal. The energy signal is not the conventional energy unit in classical physics. The energy of signal processing is the sum of the squared values of the raw signal. It is more convenient to use the energy because it is always positive and therefore does not have to account for a negative threshold.

Once the MATLAB algorithm is working properly, the EMG signal will be used to control the music player. When the amplitude of the energy signal reaches a certain point, MATLAB will execute a command to play, pause, or skip the song. A hard blink of the right eye will skip to the next song. A hard blink of the left eye will skip backwards to the previous song. A double forceful blink of both eyes will cause the song to pause or play. The MATLAB algorithm is set up so that once the energy signal reaches a threshold of twenty energy units, the corresponding command will be executed.