Most Neuralynx tethers, headstages and electrode interface boards (EIBs) include a unique “Differential Stimulation Channels” feature. The origin of these stimulation channels dates back to the NASA/NIH NeuroLab Space Shuttle E100 experiment, developed in collaboration with Dr. Bruce McNaughton for the Columbia flight in 1998. Because the “Rats in Space” E100 experiment studied hippocampal place cells in microgravity, normal food or liquid rewards would not have been practical. Therefore, electric current stimulation to the MFB – pleasure center - was perfected and proved to be a very strong behavior motivation reward!
From this point forward, Neuralynx has included the differential stimulation channel connections for the added flexibility they provide, and avoiding the need to run external wires down to the animal for stimulation.
Each stimulation channel is a differential – a two-wire pair that runs from the acquisition system through the tether (tether extension cables, commutator) and headstage to the EIB. The EIBs have accessible signal connections (vias) for connection to the animal implant or device. Two differential stimulation channels are usually provided: four wires that run from the acquisition system down to the subject. You can use these lines for many purposes: self reward stimulation, lighting LEDs, joint shock delivery, drug delivery control, eye-blink detection sensors, microphone signals, conduction of unbuffered signals from the subject back to the system, or for stimulation sources and complex digital stimulus control (Neuralynx’s HS-36-Stim & HS-18-Stim18) which contain a very small microprocessor for timing control.
Differential stimulation channel lines require four extra connector pins on the tethers, headstage and EIB connectors, and is the reason Neuralynx Omnetics connections are 44 pins instead of 36 pins for standard NeuroNexus probe connectors.
The four lines (wires) are labeled: Stim1_Src, Stim1_Ret, Stim2_Src and Stim2_Ret. The 10-Pin Stimulus Cable is used for convenient Banana Jack connections to the stimulus at the system connection. The MDR-50 Breakout Board provides convenient access to these signals. Normally, these lines do not have any electrical/electronic connection to any part of the system or headstage; they are provided and dedicated for your use. (The exception is that early HS-27 headstages had connections for video tracker LED power.)
Up to 250 milliamps can be carried by these lines. The nominal resistance is about 3 ohms per meter. The thinner, lighter Litz Wire tethers will have more resistance, due to the thinner conductors, and will not carry as much current. The older HS-27 and HS-36 Medical Grade Tethers use 36 gauge wire and have been pulsed with as much as 1 amp of current for short durations! Differential stimulation channels are not twisted pairs, so fast rising signals may cause some cross talk to the electrode signal lines, seen as very sharp spikes. (See TechTips “Noise Debug 101” series for more on conducted noise.)