We have difficulty adding collars to existing lamp leads because the leads are too thick to pass into the collar bore. We must stretch the leads by their steel wires, which compromises the elasticity of the insulation and causes the end of the lead to curl up. We now start with new lamp leads, with tapering insulation at one end, and thread these into dual-bore collars. We cut off the existing lamp leads and solder the new leads in place.
The ISL's EEG amplifier has a single-ended input. It measures the potential difference between the X+ and X− electrodes, but the X− electrode is a direct connection to the 1.2-V power supply in the ISL circuit. When the lamp lead collar seal breaks, fluid around the leads connects the surface of the skull to 5 V. The point connected to X− remains at 1.2 V. Current flows from 5 V to 1.2 V. The point connected to X+ is somewhere in the potential field generated by this current, and so we have the lamp artifact. If our ISL amplfier were truly differential, and presented a 10-MΩ impedance at both X+ and X−, the current flow to the X inputs would be greatly reduced. We would, however, still need a reference connection somewhere in the body to anchor our differential amplifier's input range, and this reference connection would re-create the problem we started with. But if we use a floating power supply for our differential amplifier, such as is provided by an indepenent SCT device, we get the near-perfect differential input without any need for a reference connection. In this way, we may be able to reduce the artifact arising from broken seals dramatically.
Our plan is to test the dual-bore collar and the floating differential EEG input in a single experiment. We have A3030Ds D7.9 and D7.11 modified as shown below, and ready to ship with (A3028E) numbers E116.7 and E116.12. We attached new lamp leads and insulated the joints with silicone. The lamp leads pass through a dual-bore collar, into which they are glued with Ultragel. We have a blue LED soldered to the ends of the lamp leads. We cut off the EEG leads and covered with silicone. The A3028Es have a pin and a bare wire for electrodes, but the choise of electrode is unimportant.
Figure: ISL and SCT Lamp Artifact Devices.
The circular A3028E has 10-MΩ input impedance and will measure lamp artifact with a power supply independent of the ISL. We have not joined the EEG leads along their length, thus reducing their sensitivity to magnetic fields, because at this point we know the >1 mV lamp artifact is generated by electrical conduction through water around the lamp leads, not by the lamp lead's magnetic field. The ISL itself measures only the internal electronic lamp artifact of the ISL circuit. Its EEG leads are cut off and sealed.
We put our two ISLs in water and start them producing 10-ms pulses at 10 Hz. We record their EEG input with the LEDs out of the water and with the LED in the water.
Figure: Two ISLs Flashing, Insulated EEG Leads. Left: lamp out of water. Right: lamp in water.
We put the two SCTs in the water and measure artifact with the LEDs out of the water and in the water.
Figure: One ISLs Flashing, Two SCTs in Water. Left: lamp out of water. Right: lamp in water.
With the lamp in the water, we have the No7 (salmon) EEG pick-up electrodes 10 mm from the exposed terminals of the LED, an the No6 (brown) electrodes are 40 mm away. In the 40-mm case, if we were to use the ISL's own amplifier, we would see artifact of >27 mV. With our floating EEG input we see roughly 3 mV. We ship these components to ION. If the collar seals are effective, we should see less than 100 μV artifact on the SCT amplifiers. If the collar seals break, we should see something between 100 μV and 3 mV artifact, depending upon the resistance of the fluid that penetrates the collar.
UPDATE: [09-AUG-16] We implant ISL D7.9 and SCT E116.7 at ION/UCL. The ISL's EEG leads are cut short and sealed. The SCT EEG electrodes are on the skull. The ISL's LED is embedded in a dental cement head fixture, with the dual-bore collar within the cement. Six days after implantation we see the following artifact during lamp stimulation.
Figure: Lamp Stimulation Artifact with Brightness, Simultaneous in SCT and ISL. Six days after surgery. Pulse length 20 ms, interval 50 ms, 200 pulses. Left: from SCT E116.7, right: from ISL D7.9.
The artifact in the ISL reaches a maximum at 40% brightness, which is a characteristic fo the electronic artifact generated within the ISL circuit. Its amplitude is of order 100 μV. There is no sign of artifact in the SCT recording. We look at the spectrum of the SCT signal over eight seconds during 100% brightness and see no sign of a peak at 20 Hz (50 ms period), which means the lamp artifact in the SCT recording is <10 μV.
We implant D7.11 and E116.12 in another rat. This time, the EEG leads of the ISL are cut short but not sealed, so that the wire tips are several millimeters apart in the abdomen and exposed to body fluid. Two hours after implantation we observe the following lamp artifact.
Figure: Lamp Stimulation Artifact, Simultaneous in SCT and ISL. Two hours after surgery. Ten pulses of 10 ms, interval 100 ms. Red: from SCT E116.12, Gray: from ISL D7.11.
The artifact in the SCT signal remains <10 μV. But the artifact in the ISL signal is of order 20 mV. If the collar seals were to break, current would flow from the head fixture to the ISL's X− input, which is connected to the ISL's 1.2-V power supply. A potential field will develop through the animal's body. The SCT electrodes on the skull, being of order 10 mm apart, will pick up this pulsing potential field, as will the ISL's X+ electrode, which is a few millimeters from X−. But we see no artifact in the SCT EEG, so the artifact cannot be caused by a break in the collar seal. Instead, we suspect that the seal around our lamp lead solder joints has failed after minimal fatigue or handling, and the result is a pulsing current between an exposed lamp power lead and the ISL's X−. The ISL's X+, being nearby, picks up this field, but the SCT's EEG electrodes, being far away on the skull, do not pick it up.
