With a variety of programmable stimulators and isolators available, we often receive requests for stimulator and isolator recommendations. This brief post describes how to choose an A-M Systems stimulator or isolator for your application.

Stimulator or Isolator

Before launching into a discussion on which instrument to buy, it is necessary to understand a bit about stimulation technology and terminology.

Stimulation instrumentation is used to deliver electrical signals to experimental subjects, or to control intricate timing sequences of other instruments used in the experimental protocol (for example, opening a camera shutter at a given time after the initiation of a trial).  Important parameters of the stimulation device that need to be considered before making a purchase decision include:

1. Mode: constant current or constant voltage
2. Timing aspect: pulse width, pulse frequency, etc
3. Triggering options
4. Isolation
5. Compliance Voltage

Neuroscientists have explored the impact of stimulating tissue or subjects with both current-defined stimulation signals (expressed in terms like “microamps” or “milliamps”) and voltage-defined stimulation signals (expressed in terms like “microvolts” or “millivolts”).  While some of the original Grass stimulators could only produce voltage outputs, modern stimulators can often produce either voltage or current pulses.  Instruments that supply voltage -only outputs often have accessory units called Stimulus Isolators that can convert that voltage into a current if needed.

One built-in feature of modern stimulators is to be able to provide “constant” current or voltage levels, so that the requested output level, say 10 mA, doesn’t vary even though the electrode/subject preparation might change over time (for example, if gliosis occurs in the vicinity of the electrode). This enables the researcher to be confident that they were treating their subject to a defined level of stimulation over the course of the experiment.

Over the years, particular attention has also been paid to the timing characteristics of the stimulation signal itself.  Most stimulation signals are variations of square pulses (monophasic, biphasic, asymmetric), but others studies do use more biologically driven waveforms. Square pulse stimulators, like A-M Systems Model 4100, can produce a variety of waveforms (An added feature of the 4100 is that it can also produce ramp stimuli):

Event Types

Ramps

To deliver such stimuli, the user defines in the assorted parameters using either the instrument front panel or software, and enters values such as pulse duration, pulse polarity, pulse amplitude, pulse interval, pulse quantity, and so on.

The user also specifies the rules that determine when the stimulation begins. Triggering options include: “single” or “manual” where the timing sequence begins upon the press of a button or a foot switch; “external” where the stimulation begins upon receiving a TTL signal from a computer or other device; or “free run” where the stimulator begins stimulating immediately and continues indefinitely until the unit is switched out of the free-run mode.

Isolation refers to the electrical property of the stimulator to separate the electrical circuit pathway to the subject from electrical ground; to isolate the subject. This is important for safety reasons so that stray currents do not electrocute the subject, and that noise does not prevent ground loops contaminating any simultaneous recordings.

The final specification that is critical to understand before selecting a stimulator or isolator is the concept of compliance voltage.  Everything in stimulation instrumentation is dictated by Ohm’s Law (V=IR).  The ability of a device to deliver the requested current or voltage is only true if Ohm’s law is followed.  So, while a stimulator may advertise it can deliver 10 mA, it can only do so if its compliance voltage is larger than the amount of voltage required to push 10 mA across your electrode/tissue load, or total resistance.  In the case of 10 mA of current, you would need a stimulator with at least 10 V of compliance voltage if your electrode was 1,000 Ω or less:

V=IR

Volts = 0.010 A x 1,000 Ω

Volts = 10

But if your electrode/tissue load was 10,000 Ω instead, now you need a stimulator capable of passing 100 V:

V=IR

Volts = 0.010 A x 10,000 Ω

Volts = 100

If your stimulator’s compliance voltage is only 50 V, then it will not be able to deliver 10 mA across 10,000 Ω.  Once it hits its limit of 50 V, no increase in current will be possible, so it will be stimulating at 5 mA instead of 10 mA.

The last bit of background is to determine if you need a programmable stimulator or a stimulus isolator.  A programmable stimulator controls all of the logic required to generate a series of pulses.  You define the stimulation parameters using the stimulators control panel or software, and the device generates the stimulation.  However, if you have a fast data acquisition system, it might be possible to use the data acquisition systems’ own analog output circuitry to generate the stimulation waveforms.  If you can do that, then you can purchase just a stimulus isolator or isolation unit that would be responsible for turning the data acquisition systems output waveforms into a constant current or constant voltage isolated output at the amplitude you want. Limitations to this approach are typically related to the speed of the data acquisition system processing, so fast short duration pulses are often a challenge.

Now that we have a common understanding of stimulators, stimulus isolators, and the assorted features, let’s choose an A-M Systems instrument. The answers to these questions will guide the recommendation.

1. Will your stimulation device be controlling the timing and amplitude aspects of the stimulation signal, or do you have a data acquisition system you can program to generate output stimulation waveforms?
2. Is your desired output less than or greater than 10 mA?
3. Do you require software control of your stimulator, or will front panel controls suffice?
4. Do you plan on stimulating more than a single channel (electrode) simultaneously?
5. Do you want to do complicated stimulation patterns like paired pulse?  Stepped function? Ramps?
6. Do you experiments run longer than 8 hours?

1) Will your stimulation device be controlling the timing and amplitude aspects of the stimulation signal, or do you have a data acquisition system you can program to generate output stimulation waveforms?

If you have a fast data acquisition system that you can use to generate stimulation waveforms on its analog output channel, meaning you can generate a squarewave or any waveform, vary its timing parameters and amplitude as needed, then you can simply use the A-M Systems Model 2200 to convert your D/A cards analog output to constant voltage or current (up to ±50 V biphasic; 5 mA). The 2200 uses a scaling\gain section to amplify your control signal in order to generate the actual output stimulus.  So, if you program your D/A to generate a 1.5 V pulse, and you set the 2200 RANGE switch to 0.1 mA/V, the 2200 will generate a 0.15mA isolated constant current output pulse

If, however, you require a device to generate the control stimulation signals itself, then the Model 2200 would not work for you.

2) Is your desired output less than or greater than 10mA?

If you require 10 mA or less, you can use our Model 2100 Isolated Pulse Stimulator:

If you require more than 10 mA, you can either use our Model 4100 High Power Stimulator with a 100mA output limit, a CRISPR version of our 4100 that can produce up to 350 mA (at 35 V), or in some situations, you can stack together multiple stimulus isolators in parallel to increase the output current levels (or in series to increase the voltage output).

3) Do you require software control of your stimulator, or will front panel controls suffice?

Unlike amplifiers, the settings and parameters of a stimulator are often changed frequently during an experiment.  In that case, having software control can be advantageous.  Our Model 4100 offers complete control by free Windows or Apple Computer software, or users can write their own control programs in LABView or MATLAB. Our MultiStim 8-Channel Model 3800 Stimlulus Generator can also be controlled via Windows software, LabView, or Matlab.

4) Do you plan on stimulating more than a single channel (electrode) simultaneously?

A-M Systems Model 3800 8-Channel Programmable Stimulator is essentially eight independent stimulators in a single box.  This instrument is excellent for use as a timing device to control other instruments (cameras, shutters, microscopes) or as a stimulation source when paired with its optional Model 3820 Stimulus Isolation Unit.  The 3820 SIU converts the output of the 3800 into constant current or voltage.  The 3820 is battery operated providing perfect isolation, and can generate signals up to 60V and 10mA.  All 8 channels of the 3800 can be programmed independently, or their outputs can be summed together to generate more complicated stimulation patterns.

5) Do you want to do complicated stimulation patterns like paired pulse? Stepped function? Ramps?

Traditional experimental protocols like paired pulse stimulation or automated amplitude steps are built into the Model 3800 control software or can be developed in the Model 4100 custom program feature. In addition, the 4100 can generate ramps in either voltage or current. For user designed waveforms, researchers can utilize the external signal in feature of the Model 4100 that bypasses the internal timing circuitry while enabling access to the high power high speed output section, essentially turning the 4100 into a stimulus isolator. Or, users can opt to use the Model 2200 in order to scale, convert, and isolate any waveform they can generate using their D/A system.

A-M Systems family of stimulation instrumentation can likely provide the solution you require for your stimulation protocols. Naturally, this list of 5 questions is not the complete list of considerations to be made before making a purchase. Your particular experiment might generate additional requirements so we encourage you to contact us prior to making a purchase to discuss which instrument would best meet your needs.

And, it is important to note that none of A-M Systems neuroscience instrumentation is approved for use on Human subjects.