February 06, 2014

Basic Concepts and some 'Assumptions' in SpikerBox Biopotential Experiments

Exploring signals of very low amplitude, as is usual in most bioelectrical signaling, is not an easy job. In many instances the power line hum often drowns these weak electromagnetic dialects. In fact, our body acts as an antenna in receiving these wayward disturbances. We can curb these interference by the following means:

  • Opting for battery operated power for the sensing device (the bio-instrumentation amplifier) as well as the data acquisition inputs, for example, a laptop that runs on battery. The brightness of the laptop display maybe set to maximum, as some laptops may emit significant stray noise when brightness is not saturated.
  • Switching off any fluorescent lights
  • By proper Earthing of the equipment
  • Using differential op-amp configuration for adequate noise suppression by eliminating common mode signals
  • Using a preamplifier; and coupler gels that reduce electrical impedance (resistance) between the electrode (lead) and the body surface
  • Choosing an operational amplifier having a high CMRR (common mode rejection ratio)
  • By using a unity gain amplifier (voltage follower) configuration that has a very high input impedance
  • Employing a noise cancelling mechanism like the 'right leg driver'
  • Using a Farady Cage for effective 'shielding' from electromagnetic interference (static)
  • Employing a hardware filter (band stop or Notch filter) or a software program to eliminate/reject a particular frequency band (e.g. 50 Hertz or 60 Hz cycle frequencies)
There's many more options to tackle the undesirable gatecrasher, the 'hum'. Yet it is so difficult to achieve. Easier said than done!

To probe a physiological or pathophysiological response, a stimulus requires that it be precise/ accurate in timing, amplitude, reproducible, relatively less damaging and that it leaves a 'desirable' stimulus artifact so that we can calculate the delay in response in the tracing. Of the various types of stimuli available we generally prefer electrical stimulation over others; such as, mechanical, thermal and chemical, for the above reasons. 

Yet, we sometimes need to take recourse to other forms; as I was kinda forced to use mechanical stimulation to elicit a 'knee jerk' or that I had to abandon an interesting project (Hoffmann's reflex) as the TENS (Transcutaneous Electrical Nerve Stimulation) unit would invariably inject more than enough electromagnetic static into the waveform to render it useless. Marking the exact instant of an applied mechanical stimulus is bothering. I tried to circumvent this by making an "induction circuit", the makeshift switch of which
would be placed on the patellar tendon and 'close' on tapping by a hammer, thus sending an intended static. Also tried to sandwich a piece of piezoelectric material between the percussion hammer and the quadriceps tendon. But none worked so far. I had to remain contented by recording with a camera simultaneously, so that it picked up the sound of patellar impact!

The experiments I have done so far were mostly on obtaining 'surface Electromyography (EMG)' and nerve conduction velocity studies (NCV or NCS). The signals were fed into the SpikerBox input through coaxial cables into its RCA jack inputs, where it was amplified by an integrated circuit (IC) INA 2126P, an Instrumentation Amplifier. There was also a provision for rendering the electrical impulses 'audible' following amplification of the signal by LM 386, which was set at its default gain of 20. [For a more complete detail, please visit "Backyard Brains Wiki"] Thus, we could hear the action potentials and also interface the output with a laptop or a smartphone by softwares such as Audacity, Backyard brains neuron recorder (my own personal favorite) etc for real-time viewing, as well as, for recording for later storage, retrieval and analysis. 

The EMG recordings was done when the muscles were at rest, and at exercise: isometric or isotonic. Since I do not have a 'force transducer', a 'dynamometer' or a Mosso's ergograph; I had to invent ways to measure force/'work done' by other ingenious (or ingenuous? only time will tell !) means. Here's some:
  • spring constant (Hooke's Law) of the device maybe obtained by calibrating with a known device and by measuring how far the cylinder moves into the outer barrel (there's a spring inside that snaps hard onto a piezo crystal when force is applied).
    Piezo based cigarette lighter producing high enough voltage causing a spark thus igniting the gas 
  • by coupling it to a linear potentiometer which have also been previously calibrated (see figure), and then displaying the resistance in a multimeter or through a linear dot/bar display using LM 3914 IC.
  • Work done = Force*Distance=m(mass)*g(acceleration due to Gravity)*displacement (see schematic diagram)
Although we would love a stereo output from a 2 channel SpikerBox, the one I have, I do not have a laptop that accepts stereo microphone input.  Stereo signals are more preferable to human interface devices than plain old mono input. A stereo signal 'may' still possibly be 'recorded' by a 
  • Desktop line-in (via Avance AC97 soundcard) input but that would entail 50 Hz hum to pass through as well. 
  • A stereo (tape/CD) recorder running on battery power is a better option. The data can later be played/digitized at a later time. 
  • A stereo FM transmitter or a 3 pin to USB adapter is yet another option. 

Meanwhile, here's some relevant issues regarding Two channel spikerbox:
  • The internal connection pattern of (4 ring TRRS vs 3 ring stereo) 'smartphone'-'laptop' cable foxed me  for some time to realize that the Left & Right channels were only capacitively coupled
  • That my low-cost Penta T-Pad tablet could NOT render waveforms in BYB app while my Samsung GT-S5360 gleefully obliged. However, the screen was way too small to allow any reasonable analysis & that data longer than >=2 seconds could not be saved in the mobile
  • There was significant 'clipping'/distortion of the waveform in my mobile phone [maybe I could have attached a resistor to the ground/common pin to attenuate the signal. But I opted for not modifying the device in any way.]
Hence, I used the other cable coming from Y splitter (audio splitter) for the measurement of output signal amplitude, since I was outsmarted by my smartphone!

My tryst with such experiments is not new. I have previously done amphibian nerve-muscle preparation experiments using Kymograph and Dubois-Reymond induction coils, in medical curriculum. This included studying the effects of external influences like temperature, load, repeated stimuli causing beneficial effect & fatigue etc. on the tissue. I also studied peristalsis & effects of external influences including drugs on Guinea-pig smooth muscle in Dale's tissue organ bath. Also being an electronics enthusiast, I tried to decode the language the heart speaks, in the honest pursuit of making biomedical instrumentation. It's tough! (fun intended)

Lastly, all that has been said above may be modified as we gain more insight and experience. Please feel free to use/modify this work, do your own research, give your feedback or point something wrong in this article. 

N.B.-Remember, electricity kills! Try to have a close friend, preferably a doc, by your side. Check that the 'Earth' outlet is really what it outta be!

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This work by Amiya Sarkar is licensed under a Creative Commons Attribution 4.0 International License.

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