Bipolar stimulation

Hello,

I have purchased a set of four STG4008 stimulators for driving a 32-electrode stimulating array (for use in retinal stimulation).
I am interested in bipolar stimulation, in which current is sourced from one electrode and sinked in another electrode. According to the manual I can drive a pair of electrodes (say A and B) in this push-pull configuration by connecting electrode A to the + outlet and electrode B to the - outlet of the **same** stimulating channel.

However, in this configuration the source and sink electrode pairs are fixed. I wish to change the source and sink pairs at will. My question is the following. Let's assume that I connect electrode A to the + outlet of stimulating channel #1 and electrode B to the + outlet of stimulating channel #2. Also let's assume that I connect the GND outlets of both channel #1 and channel #2 to the same point (which is remote to the stimulation sites). If I delivered identical waveforms but with opposite polarities to channels #1 and #2, would I get bipolar stimulation between electrode A and B? That is would the current flow from electrode A to electrode B, without going to the remote site where the GND outlets are connected?

Is there any other way of achieving my goal, which is bipolar stimulation between any desired pair of the 32-stimulating channels?

I would appreciate any feedback you may have.

Regards,
Nicolas P. Cottaris, Ph.D.

Dear Dr. Cottaris,
I from what I can see this should work. The grounds of the STG channels are interconnected any way, so there should be no difference between the two configurations.

Frank MCS

Dear Frank,

Thanks for your prompt response. I just want to make sure that we are on the same page here.

My concern is that sinking and sourcing current between the + and the - outlets of the same channel
may not be the same as sinking and sourcing between the + outlet of one channel and the + outlet of a second channel (assuming of course that the second channel receives opposite polarity signals).

Is there anything special about the + and the - outlets of a single channel? For example if different channels have slightly different delays, sinking and sourcing between different channels may not work as intended as sinking and sourcing between the +/- outlets of a single channel, especially for fast waveforms.

What do you think?

Thanks,
Nicolas

Dear Nicolas,
I spike again with our physics guys here. You are right, it's not 100% the same. Bipolar stimulation from one channel you could in principle do also w/o a ground connection between bath an STG. When using two channels, a connection between the two STG ground connectors to the same bath/grounding point is necessary. I was wrong in that respect, by the way, the grounds of the STG are NOT interconnected. What you should do to avoid timeshifts is to assign all channels that should be used together to the same Trigger in the Trigger Settings menu of MC_Stimulus. Channels not assigned to the same Trigger may have a jitter of 20µs, which is the time resolution of the STG.
If you happen to be at the SfN, we can discuss issues there.

Regards Frank

Thanks Thomas,

I will stop by your booth at SFN.

Nicolas

Is there any other way of achieving my goal, which is bipolar stimulation between any desired pair of the 32-stimulating channels?