Noise is any unwanted signal that contaminates your desired signal. It may originate internally from other bioelectric signals or externally from any voltage or magnetic energy source. There are many external noise sources from our high-tech world, many of which are generated by necessary technology such as computers and AC wall power. The challenge for neuroscience researchers is to find solutions that reduce or eliminate the noise source or reduce the coupling of the noise into the recording setup.
The challenge of noise in the electrophysiology environment is the magnitude of the desired signals from the brain (microvolts to millivolts) to the external noise sources (hundreds of volts) in the laboratory environment. This represents a “100 Million to One” ratio of desired signals to noise sources.
The good news is that noise signals can be greatly reduced or entirely eliminated through proper identification and coupling reduction techniques! Part 5 of this series will cover recording techniques used to eliminate noise signals on our electrophysiology data.
When you observe an external noise signal in your recordings, you must first identify the noise source. This is done by observing the signal and answering such questions as:
Remember that current ALWAYS flows in a loop/circle; the electrons MUST return back to the source.
Conducted noise coupling occurs when the expected signal path (usually in the return portion of the loop) is in common/contact with the noise source’s signal path.
Because the desired signal and the noise signal share a common section of each one’s path, the voltage drop on the common section will be the summation of voltage drop of the two signals’ paths.
Ohms Law yields 10V/110ohms ~= 0.1A flowing in the bottom noise loop. The tether has a resistance of 10 ohms, 0.1A*10ohms = 1V.
The diagram above demonstrates a conducted loop situation that has occurred in several laboratories. The subject is placed on a metallic track that has been “well grounded.” AC power line noise of a few millivolts is observed on the neural signals from the subject’s headstage as recorded on the Digital Lynx SX system.
Note: In most large buildings, currents will flow through the building structure (even through concrete) because electrical power circuits are connected to the building’s metal structure.
When we examine and model the possible signal flow “loops,” we identify two that can flow through the subject’s tether. On first observation, there should not be any current flowing from the connections to ground because there “should not” be any voltage difference in the grounds. But 1) because any current flowing through the conductors to ground will cause a voltage drop on the conductors; 2) because of the current flowing through the building; and 3) because there may be a long distance between the power outlet for the system and the main power distribution panel where the system’s “power ground” would be connected to the building, several volts may be present between the grounds in this diagram. Therefore the voltage between the metal track ground and the Digital Lynx SX ground may be significant. The signals between the subject track and the system will add to the buffered neural signals from the headstage.
Solution: Insulate the subject track from the floor with a good non-static, non-conductive material, such as wood or synthetic rubber mat material. This will break the conduction path of the current in the AC Power Ground loop. Also, all equipment, computers, amplifiers, behavior control apparatus, etc. connected for the experiment should be plugged into the same power strip.
This is one instance where the term “Ground Loop” is applicable.