Low-Cost Electroencephalographic Recording System Combined with a Millimeter-Sized Coil to Transcranially Stimulate the Mouse Brain In Vivo

In neuroscience, TMS and EEG recording are frequently used to advance our understanding of the brain. TMS is especially promising due to its non-invasiveness. Combining TMS with EEG recording enables researchers to explore excitability, connectivity, and spatiotemporal dynamics across various brain regions.

This protocol aims to develop a simultaneous stimulation measurement system for TMS-EEG experiments with small animals. These system have not been extensively developed, forcing researchers to create a system to trial and error according to the unique experimental requirements. This easily adaptable approach can be set up in standard neuroscience labs, making it more accessible and practical for researchers.

The findings of this study will advance research and elucidating mechanisms behind specific disorders in small animals, which are essential models for human candidates. Further, these models will improve their understanding of human conditions and help guide the creation of more effective treatments and interventions. To begin, use an intraperitoneally anesthetized mouse and expose the skull using standard surgical procedures.

Attach the stainless steel rod mounted to the 2D electrode array on the backside of the flexible substrate to a micro-manipulator. Then place the flexible substrate on the exposed skull. Adjust the location of channels 3 and 14 on the array to fit within the inferior colliculus.

Draw four small circles at the locations of channels 3, 8, 9, and 14 on the skull with a permanent marker to serve as targeting landmarks. Next, dry the skull surface to improve the adherence of the dental cement and to electrically isolate the 2D electrode array on the flexible substrate from the mouse's skull. Apply a layer of dental cement to the skull surface approximately one millimeter thick and allow the cement to cure for approximately 30 minutes.

Align the flexible substrate in line with the small circular marks on the surface of the skull. Then position the tip of a dental drill on each electrode pad hole on the flexible substrate and carefully drill into the skull. Using a screwdriver, secure the miniature screw electrodes into the drill holes of the skull.

Crimp the head of the screw electrode and the electrode pad firmly. Finally, measure the conductance between each screw electrode and the connector with testing equipment such as an LCR meter to confirm electrical conductivity. Connect the flexible 2D array to the recording system with a flat ribbon cable.

Then attach the stainless steel rod mounted on the coil to a micro manipulator. On an anesthetized and electroencephalographic electrode implanted mouse, place the coil above the bregma and adjust the position in the coddle direction to locate the focal point above the inferior colliculus. Next, prepare a stimulation system by connecting a bipolar power supply and the function generator.

Then connect the coil to the system. Connect the cable between the input terminal of the function generator and the output terminal of the data acquisition system to apply trigger signals to the function generator from the data acquisition system. Next, connect the data acquisition system to the recording system.

Prepare a suitable computer program to initiate stimuli with the trigger signals. Ensure that the stimulation timestamps are required. Start the acquisition process for the recording system, then record the response data and start the stimulation sessions.

Once each stimulation session is complete, stop the recording, which automatically saves all the recorded data. The electroencephalographic or EEG wave forms were recorded in response to sound stimulation of eight kilohertz stone burst and 80 decibels sound pressure. The responses were recorded from the channels near the auditory cortex.

The observed wave forms showed negative going responses immediately after the sound stimulation and subsequently positive going to some extent over the baseline. The EEG wave forms in response to magnetic stimulation of the area near the right inferior colliculus were also recorded. Channels near the auditory cortex showed variations in response with increased magnetic stimulation intensities leading to increased peak amplitudes of driven responses.