V=IR

where V is the applied voltage, I is the current and R is the electrical resistance of the tissue and or the stimulating electrodes. This simple equation shows that if voltage is constant, current flow will diminish if the tissue/electrode resistance goes up and will increase if the resistance decreases.  More commonly, the resistance of tissue differs from sample to sample, and the resistance of the electrodes changes with applied current over time in a process called polarization.

Types of Stimulus Devices

In the delivery of electrical energy to biological tissues, stimulus devices can hold either current or voltage constant during the stimulating process. Devices that hold voltage constant at a value set by the user and allow current to be determined by Ohm's law are known as constant voltage stimulators. Devices that hold current constant at a value set by the user during the stimulation process and allow voltage to be determined by Ohm's law are called constant current stimulators. Constant current stimulators are preferred for two reasons:

• First, current is the quantity that stimulates most excitable tissues.
• Second, stimulating electrodes tend to increase their resistance as stimulation progresses, as do some tissues. A constant current stimulator will “sense” resistance change and provide whatever voltage is needed to maintain delivery of current at the set rate.

There is obviously a limit to how much voltage a constant current stimulator can provide. If the resistance of the preparation were to become infinite, as might happen if one of the stimulating electrodes was removed from the tissue, the stimulator could not mount an infinite voltage to compensate. The maximum amount of voltage that a constant current stimulator can provide is called the compliance voltage. Once this compliance voltage has been reached, further increases in tissue resistance will cause a drop in delivered current. WPI isolators in the A360 and DLS series offer a compliance voltage of 100V with very low noise. Stimulus isolators, as the name implies, also isolate a given stimulus from ground. In an instrument design context most people think of isolation from ground as it relates to electrical safety.  From a biological recording standpoint there are other issues. Consider the circuit in Figure 1.

Fig.1

The stimulator in this case is a battery with a switch; current leaves the positive terminal of the battery, travels down the stimulating lead, passes through the tissue and returns to the negative terminal of the battery.  100% of the current delivered returns to the negative terminal of the battery.  The figure also shows a voltmeter in the preparation.  Voltages generated by the tissue as a consequence of the stimulus are recorded with respect to the voltmeter’s ground electrode.

 In Figure 2 the battery is replaced with a line powered stimulator.  Even though the stimulus source and the voltmeter have separate ground electrodes, they represent the same electrical point.  For this reason a significant portion of the stimulus current returns to ground by way of the voltmeter’s ground lead. If the currents are significant, or the voltages that you are trying to measure are very small, the I x R drop across the resistance of the voltmeter’s ground electrode will add to the recorded voltage from the tissue and will be seen as a DC artefact.

Fig.2

Capacitative coupling between the voltmeter circuit and the isolated circuit can induce current to flow in the voltmeter ground. The induced current will transiently flow across the resistance of the voltmeter ground and its I x R drop will be seen as a transient spike in front of and behind any pulse of current delivered by the current source. This is termed an AC artefact.

The primary reason why researchers use an isolated current source is to minimize artefact.  But what if you are not recording? There is no voltmeter ground to produce an observable artefact.  If you stop and consider the examples above, the artefact was minimized or eliminated by controlling the path of the stimulus current.  Knowing the current path can be critical in physiological stimulation.

Consider the current path in Figure 3.  The animal represented by the badly drawn cat is secured in a stereotaxic frame.  The frame is grounded.  The animal contacts the frame at multiple points. Figure 3 shows the battery and switch model of a stimulator. Here as before all of the current that leaves the positive side of the battery must return to the negative side.  100% of the stimulus current must pass between the stimulus electrodes. In this case the path as well as the exact amount of current delivered is known.

Fig. 3

Figure 4 shows the same experimental setup except the battery has been replaced by a line powered voltage source.  Stimulus current now returns to ground via many routes. The amount of current that flows to ground is determined by the resistance between the source and each of the ground points and is calculated as a resistive network using Kirtchoff's laws. Many unintended areas of the animal may be stimulated.

Fig. 4

In the real world we cannot use a battery and a switch, particularly if the current durations are on the order of milliseconds. Electronic devices such as pulse generators and computers are used to generate timing, and Isolators driven by these devices are used to deliver the stimulus.  By connecting an isolator (even a battery powered one) to a mains powered pulse generator, you connect the isolated ground of the isolator to the mains ground of the pulse generator. Unless the electrical connection between the two devices is accomplished without using a mechanical connection between the two devices you break the isolation.  This is what makes an isolator an isolator.  The non-mechanical contact between machines that constitutes the isolation barrier can be accomplished in one of two ways.

Originally, isolators were transformer isolated: pulse waves were applied to the primary winding of a transformer, while the actual stimulus was derived from the secondary winding.  The transference was accomplished by induction.  This approach suffered from two shortcomings.  The devices could not pass DC so no constant voltage isolated stimulations could be made. The transformer approach also has intrinsically higher capacitance.  This means that while the resistance between the primary and secondary coils is very high, the high capacitance will produce an AC artefact that is unacceptably large compared to other isolation techniques.

Optical isolation is the second popular scheme. The field has all but standardized on this.  In simple terms, the input pulse wave powers a light that shines across the barrier onto a photocell that produces the stimulus wave.  There have been countless variations on this theme and it is used for isolating recording amplifiers as well as stimulators.  All of WPI’s isolators are optical.