The common marmoset poses unique challenges for neurophysiology due to its small size and lack of gyre as anatomical landmarks. A slight shift of the electrode by just a millimeter can result in significant changes in the retinotopy map. The proposed micro drive system utilizes an XY electrode stage that allows for vertical and horizontal movement on a sub millimeter scale.
Due to the relative novelty of the marmoset monkey as a model for visual neuroscience, awake-behaving electrophysiology techniques are still evolving. Current preparations often use semi-chronic probes that do not allow access for the positioning mechanisms. This protocol demonstrates a lightweight mecha drive for useful linear array recordings, which allow flexible positioning across sessions to map out retinopathy within the chamber.
Long-term cortical damage can be avoided in the preparation when used correctly with the secure drive, slow penetration of tissue, and avoidance of major blood vessels. Additionally, the use of slastic in the craniotomies to avoid infections has been optimized in this technique. To begin, collect the parts required for the construction of electrode micro drive.
Place a nut in the slot of the base and add super glue for stability. Then insert a screw through the nut in the base. Using a rotary tool with a diamond wheel cutting attachment, cut a four by three by three millimeter section of the plastic grid with one by one millimeter hole spacing.
Place the grid section onto the top of the Y stage. Apply epoxy to fix the grid section onto the top of the Y stage. Then, fit the micro drive over the grid using a screw.
Add epoxy at the base of the micro drive for stability. Attach a plastic rectangular platform to a 28 gauge steel tube using epoxy. Thread a 23 gauge guide tube through the grid.
Then thread the steel tube attached to the plastic platform through the 23 gauge guide tube. Place the 28 gauge steel tube into one of the micro drive slots. Secure the steel tube to the micro drive with epoxy.
Using a hand-tapping tool, tap the pre-made screw holes in the X stage and base to build the base for the drive. Then insert screws through the long slots in the X stage and secure them in the corresponding screw holes of the base. Insert two screws through the long slot in the Y stage and secure them to the pre-made screw holes in the X stage.
After ensuring the micro manipulator drive is outfitted with a plastic platform, use tape to attach the 64 pin connector to the platform. Then carefully place a dab of bacitracin onto the plastic tube and attach the electrode connected to the 64 pin connector using a flexible ribbon cable. Using the micro manipulator drive, hold the silicon electrode and its connector while threading the plastic tube through the hole in the Y stage.
Detach the 64 pin connector from the micro manipulator drive and use glue gel to secure the connector to the platform on the X stage. The next day, carefully detach the electrode from the plastic tube and remove the micro manipulator drive from the assembly. Gently move the alligator clip to flip the drive over to see the unsecured electrode.
Place the electrode onto the plastic holder below the Y stage and secure it with silicone elastomer. After five minutes, using the screw control on the micro drive, retract the electrode and place the protective sleeve over the electrode. Tighten the three side located screws on the base component of the micro drive.
Connect the head stage for recordings onto the 64 pin connector. To prepare the PEDOT solution, combine EDOT and PSS in distilled water a day before the use. Open the impedance tester software, and in the upper dropdown menu at the top left, select the adapter.
In the second dropdown menu, select the electrode. Ensure that the number of channels is correct. Using a connector, connect the probe to the impedance tester system and lower the probe into grounded saline to obtain a baseline reading.
Press the Test Impedances button and verify the correct setting before pressing the Test Probe button. Record and save the baseline readings for future reference for both shanks of the electrode. Next, lower the lab jack to remove the probe from the saline.
Replace the saline in the beaker with distilled water. Raise the lab jack to submerge the electrode and rinse the remaining saline solution before lowering the lab jack again. Replace the distilled water with PEDOT solution and raise the lab jack until the electrode is submerged.
Select the DC Electroplate button and verify the correct settings. Then press the Autoplate button and save the results. After testing, first rinse the probe with distilled water.
Then fill the beaker with saline and raise the lab jack until the electrode is submerged. Press the Test Impedances button and check for the correct settings. Then press the Test Probe button.
To begin, secure the micro drive on a head cap. Secure all three screws on the micro drive system. Using the screw control on the micro drive, lower the silicon probe making one turn every one to two seconds until units are observed at the array tip.
Once the neurons are observed, retract the one turn of the micro drive to decrease the electrode's speed of entry as it moves through the slastic and dura into the cortical tissue. Slowly continue to drive the array into the cortex, advancing four to six turns over 20 to 30 minutes, until neurons are evenly distributed across the length of the probe. Then slowly retract the array by one to two turns to reduce the pressure on the tissue.
After the recording is complete, slowly retract the array from the cortex at a speed similar to the initial insertion. Once no more neurons are visible on the probe, retract the array more quickly until it returns to the fully retracted starting position. After removing the probe from the recording chamber, soak it in contact lens solution for 20 minutes to clean off any tissue or blood.
Place the probe in alcohol for one minute to remove the contact lens solution, and allow the electrode to dry. The spike-sorted array data obtained through Kilosort revealed that the principal component features of selected spikes in a cluster were distinct from background spikes, confirming the recording's stability over time. The reliability of the XY positioning demonstrated a 70.8%overlap in mean RF locations between two sessions a week apart.
Precise retina topic space movements were observed in a series of recording sessions crossing the MT and MTC borders using minor positioning changes.
