Recording of Visual Evoked Potentials via the Skull Electrodes in a Rat Model

All procedures involving animal models have been reviewed by the local institutional animal care committee and the JoVE veterinary review board.

1. VEP Electrode Implantation

1. Anesthetize the animal with an intraperitoneal injection of ketamine (75 mg/kg) and medetomidine (0.5 mg/kg).
   NOTE: Following induction of anesthesia, observe the withdrawal reflexes (pinch test, corneal and palpebral reflex, etc.) and their absence as an indication to begin surgery. Continually monitor animals throughout the surgery and administer additional anesthetic drugs (10% of the initial ketamine dose per top-up) if the reflexes are present. Adult rats (>12 weeks) are used in the experiments.
2. Shave the skin of the surgical area. Place the animal on a warming pad (37 °C) to maintain body temperature during the surgery. Prepare skin through topical application of povidone-iodine. Apply surgical draping. Apply topical ophthalmic ointment to prevent corneal dryness under general anesthesia. Maintain asepsis by using sterile instruments.
3. Make a longitudinal skin incision on the midline of the head skin. Clear the connective tissue to achieve good exposure of the skull.
4. Carefully drill small burr holes manually using a micro hand drill at 7 mm behind the bregma and 3 mm lateral to the midline.
5. Implant screw electrodes through the skull into the cortex (area 17), penetrating the cortex to a depth of approximately 0.5 mm. Implant a reference screw electrode on the midline 3 mm rostral to the bregma. Apply dental cement to encase and fix the screws (not always required).
6. Suture the skin of the head, administer antibiotic ointment onto the skin, and allow the animal to recover from anesthesia on a warming pad.
   Note: Alternatively, electrodes can be left exposed so that skin doesn't need to be reopened in each recording. Immediately upon cessation of surgery but prior to anesthetic recovery, administer a non-steroidal anti-inflammatory drug (NSAID) (if not administered pre-operatively) or an opioid analgesic. Monitor the animals constantly until they fully recover from the anesthetic and are ambulatory.
7. Allow at least 1 week for the animals to recover from the surgery prior to VEP recording.

2. VEP Recording

1. Anesthetize the animal and prepare the skin as 1.1 and 1.2.
   Note: A lower dose of anesthetics can be used for electrophysiological recording (ketamine 40 mg/kg and medetomidine 0.25 mg/kg).
2. Place the rat in a dark room and allow it to adapt to darkness for 5 – 30 min. In some cases, rats can be dark-adapted O/N for scotopic or light-adapted for photopic VEP recordings.
3. Maintain the body temperature at 37 ± 0.5 °C by the homoeothermic blanket system with a rectal thermometer probe.
4. Dilate the pupils with 1.0% tropicamide eye drops. Open the skin over the skull to access the pre-placed in situ screw electrodes.
5. Connect the screw over the contralateral visual cortex of the stimulated eye and the reference screw to the amplifier. Insert a needle electrode into the tail as the ground. Measure and maintain the electrode impedance below 5 kΩ.
6. Place a mini-Ganzfeld stimulator directly on the skin around the eyelids to provide superior eye isolation. The illumination of the stimulator needs to be calibrated beforehand by a photometer.
7. Deliver photic stimulation through light flashes 100 times at a frequency of 1 Hz, with low and high band-pass filter settings of 1 and 100 Hz, respectively. The signal sampling rate is at 5 kHz.
   Note: Signals should be sampled at least at about 250 – 300 Hz to ensure that more than two samples are collected during each cycle.
8. Suture the skin back and keep the animals on a warming pad to recover from anesthesia. Recording can be repeatedly recorded on individual animals to monitor functional changes over a period of time.