Understanding stimulation setup and protocol with TDT's Subject Interface (SI) through template Synapse experiments.
The goal of this document is to explain the various modes of stimulation that are offered by the Subject Interface. These modes can be broadly categorized by either stimulation type (voltage-controlled or current-controlled) or by hardware method (individual banks, serial, parallel, repeated). We will briefly go into what each of these categories implies and highlight which are being used in the various template experiments that are expounded later. Lastly, the results of any stimulation paradigm are restricted by inherit hardware limitations -- we will outline what these limitations are before the template experiments to serve as a frame of reference for the user.
The goal of this section is to explain different stimulation types (voltage mode vs current mode) and special hardware configurations (serial and parallel outputs). The SIM gizmo in Synapse has several modes (normal, repeated, serial, parallel) that will also be discussed where appropriate. These concepts apply to the Template Experiments. Please refer to the IZV Stimulator section of the Synapse manual for further information regarding these elements.
Voltage mode is used to stimulate across a load at a specified voltage. The current output of the Voice (the signal) being used for stimulation will vary, depending on the impedance of the targeted load, in order to meet the requested voltage output. The compliance voltage of any Voice of the SIM is ± 15 V.
Special methods such as bipolar stimulation (see Bipolar diagram below) or Serial Outputs can be used to increase the effective voltage across a load.
Below is a diagram that shows an expected measured output across a typical load resistance (> 3 kΩ) for monopolar ("single-ended") and bipolar configurations.
Current mode is used to stimulate across a load at a specified current. The current output is limited by the voltage compliance of each Voice (± 15 V), thus you may not be able to achieve maximum current outputs (> 5mA) if the load impedance is high (> 3 kΩ) due to Ohm's Law: V = I*R.
Below is a diagram that shows an expected measured output across a 20 kΩ load resistance for monopolar ("single-ended") and bipolar configurations.
The measured voltage across the load does not change between monopolar and bipolar configurations because the positive and negative currents flow in the same direction. This is as opposed to voltage mode where the measured voltage doubles in bipolar mode. No current flows across the Voltmeter ~V due to its large impedance.
Outputs can be serialized in either current mode or voltage mode to increase the effective voltage compliance across a load. The stim configuration must be bipolar and requires at least two banks. Voltage sources in series add, thus serialization can increase the voltage compliance across a load. Each stimulation Voice never exceeds the compliance limits of the SIM (± 15 V) because the output waveform is generated with respect to an effective ground in the middle of the load.
When serializing, make note of how many voltage sources are being added
together. The measured output voltage will be
2 * requested voltage * Nbanks.
This math is explained in the diagrams below.
The diagrams below pertain to a manual serial configuration (not using Serial Mode). The manual configuration, which is setup in several template experiments, makes use of IZV sub stimulators. The sub stimulators receive duplicated bipolar Voice inputs and do not require using channels 1 and 16 only (the stim channels and shorted channels can be any two numbers 1 - 16). It can also scale beyond two boards/banks. Serial Mode, which is a special option in Current Mode, uses a single bipolar Voice, is limited to two boards, and assumes channel 16 on any sub stimulator is shorted together.
Repeated Mode is very useful for duplicating the same stimuli across
several banks with only having to configure one eStim input. Repeated
Mode is only available in Current Mode, but you can easily setup several
sub stimulators in Voltage Mode with the same eStim signal as an input
to get the same effect.
Outputs can be shorted in parallel to increase the amount of current delivered across a load. Current sources in parallel add together, so each voice that is targeted to a common node (electrode) will contribute as a current source, even in Voltage Mode.
There are other efficient ways in the IZV gizmo to increase current
compliance, namely Current Doubling (By Two; By Four). Beyond this,
channel outputs must be shorted together.
There is an important distinction between using Current
Doubling and physically shorting output channels together. The requested
current output in Current Doubling is not multiplied by two or four.
Instead, this increases the max current compliance for any Voice from 5
mA to 10 mA or 20 mA. Physically shorting output channels does multiply
requested current * Nsources.
If you are physically shorting output channels, consider using Repeated Mode to easily output the same stimulus on each output channel in use.
Parallel Mode can be used to increase the number of stimulus voices on
any given output set. The typical limit is four Voices per card/ bank.
This is useful for merging many stimuli into a 16 channel headstage or
electrode array. Parallel Mode in the IZV gizmo allows the user to
address channels on each bank in a range of 1 - 16 instead of the
typical 1 - 16, 17 - 32, etc.