The overall goal of this procedure is to measure visual evoked potentials on a mouse scalp using a dry, non-invasive, and multi-channel EEG sensor. This method can help answer quick questions in pre-clinical translational research field to breach the signal gap in non-invasive EEG research between human and laboratory animals. The main advantage of this technique is it's non-invasiveness, which enhances comprehension of animal EEG.
Furthermore, it provides safe and convenient experimental environments. Begin by preparing 16 pins for one non-invasive electrode. Cut two pieces of glass fiber substrates with the size of 15 millimeters by 17 millimeters.
Next, make 16 1.2 millimeter diameter holes using a precision engraving machine. Then, drill 16 holes into the flat substrates according to the probe coordination, evenly at an interval of two millimeters. Stack two substrates, and apply one drop of fast-acting adhesive glue between the substrate layers to create a double layer of three millimeter thickness, supporting 16 stable and parallel electrodes during signal acquisition.
Then, manually assemble the 16 electrodes onto the substrate one by one. Solder and link each electrode's ending solder cap part to the touch-proof connector. Finally, cover and hide the exposed junctions with heat-shrink tubing for electrical insulation.
On the anesthetized mouse, begin by applying eye ointment with a cotton swab to keep the cornea moist. Remove the hairs around the head and shoulders with a hair clipper. Then, spread commercially available depilatory cream, and keep it on this area for three to four minutes.
Finally, remove the applied depilatory with a spatula, and then wipe up the rest with wet wipes, applying water several times. Begin by mounting the mouse's head onto the stereotaxic frame by placing ear bars into the mouse's ear canals, and tightening them in place. Mount the sensor into the custom-made electrode holder.
Then, fix the sensor holder onto the stereotaxic frame. Locate the flexible electroencephalography, or EEG sensor, taking into account the reference electrode and bregma position. Then, carefully lower the sensor in the vertical direction, so that the arrayed electrode plungers contact the mouse's scalp evenly onto the curved margin.
Check that the impedances are within the proper range, from 100 kiloohms, to two megaohms. Reposition the electrode when any impedance value is out of the range. Next, position the photostimulator 20 centimeters away from the mouse's eyes.
Set the parameters of the experimental devices such that the sampling frequency is 500 hertz, notch filtering is 60 hertz, the inter-stimulus interval at 10 seconds, flash duration at 10 milliseconds, and the number of flash stimuli at 100 trials per subject. Finally, before starting the EEG recordings, adapt the mouse for 10 minutes in the dark cage for dark visual adaptation. Average visual evoked potential responses indicate minimal signal fluctuation after stimulation, less than 300 milliseconds in duration.
The signal steadily stabilizes over time during the post-stimulation period. Further, 14 channels can be categorized into several groups based on the VEP responses, revealing similar morphologies and patterns. Once mastered, this technique can be completed in one hour if it's performed properly.
While attempting this procedure, it's important to remember to check the animal's status of anesthesia. Combining this procedure with other methods, like brain stimulation or surface-deep electrophysiological recordings can be performed in order to answer additional questions, like brain stimulation parameter optimization, or source localization. After its development, this technique paved the way for researchers in the field of translational research areas to connect basic science research to human brain studies with comparable, reliable, and efficient research without any invasive surgical preparation.
After watching this video, you should have a good understanding of how to measure visual evoked potentials on a mouse scalp using a new electrode that has the benefit of flexibility, dry type status, multichannel capabilities, non-invasiveness, and re-usability.
