Those circular symbols are meant to represent shielded connectors. The outer part connects to the shield of the electrode wire, the signal is the centre part. They are not electrically connected.
"DRL" is short for "Driven Right Leg". It is intended to provide a reference voltage for the body. The right leg is commonly used as it is a a distance from where the electrodes are placed. It is more commonly used for ECG (ie measuring heart voltages) than EEG.
Note that the "DRL" signal at the right must not be connected to the "DRL" signal at the left or the isolation will be compromised.
The resistor network at the right is to provide a suitable output voltage for driving the EEG signal amplifier. The output voltage will be centered on the reference voltage supplied by the DRL input on the right. The series 2k resistors are presumably to provide a suitable source resistance to imitate that of real electrodes.
The color of the pins is a stylistic choice meant to indicate the negative or signal common. It is not at all standard.
It is unusual to use that sort of photocoupler in photovoltaic mode, I'm surprised there is much signal output.
It would be useful if you put reference designators on the components so that responders can easily indicate which parts of the diagram are bing referred to.