Surgical Preparation

Animals were premedicated with atropine sulfate (0.05 mg/kg) and diazepam (Valium, 1.5 mg/kg) 30 minutes prior to inducing anesthesia with ketamine HCl (10.0 mg/kg). We continued anesthesia on 3% isoflurane in a 98% O$_2$ / 2% CO$_2$ mixture during the initial surgery. We inserted catheters into the saphenous veins of the hindlimbs and performed a tracheotomy. We mounted the animal in a stereotaxic apparatus then discontinued gas anesthesia. A small durotomy was made within a craniotomy of approximately 10 mm in diameter over the relevant cortical region. A plastic chamber was placed over the craniotomy and filled with agar to stabilize the cortical surface and prevent dessication, and the top of the chamber was covered with Parafilm and/or petroleum jelly.

For V1 recordings, the craniotomy was typically centered 8 mm posterior to the lunate sulcus and 10 mm lateral to the midline and the penetration was aligned vertically. For V2 recordings, the craniotomy was centered 4 mm posterior to the lunate sulcus and 10 mm lateral to the midline, and the electrode was advanced 60$^\circ$ anterior and down in the parasagittal plane. For MT recordings, the craniotomy was centered 15 mm lateral to the midline, 4 mm posterior to the lunate sulcus, and the electrode was advanced 20$^\circ$ anterior and down in the parasagittal plane. V1 neurons were recorded both on the operculum and in the calcarine sulcus, where the receptive field eccentricities are typically 2-5$^\circ$ and 8-25$^\circ$ of visual angle, respectively. All V1 receptive fields were within 25$^\circ$ of the fovea, and most were within 10$^\circ$. V2 neurons were recorded on the posterior bank of the lunate sulcus, and all V2 receptive fields were within 6$^\circ$ of the fovea. MT cells were recorded at eccentricities ranging from 2$^\circ$ to 33$^\circ$, but typically between 3$^\circ$ to 12$^\circ$.

Anesthesia was maintained throughout the experiment by a continuous infusion of sufentanil citrate (typically 4 $\mu$g/kg/hr, established for each animal) mixed with a lactated ringer's solution (Normosol). Infusion solutions were mixed to 2.5% dextrose concentration to provide adequate nutrition and infusion rate was adjusted to maintain fluid balance (approximately 4-8 ml/kg/hr). Vital signs (EEG, ECG, end-tidal ${\rm P}_{\rm CO_2}$, temperature and lung pressure) were monitored continuously. Expired ${\rm P}_{\rm CO_2}$ was maintained between 3.8-4.0%. Rectal temperature was maintained near 37$^\circ$ C through the use of a heating pad. To minimize eye movements, the animal was paralyzed with a continuous intravenous infusion of vecuronium bromide (Norcuron, 0.1 mg/kg/hr). The pupils were dilated with topical atropine and the corneas protected with gas-permeable hard contact lenses. We used supplementary lenses to bring the retinal image into focus by direct ophthalmoscopy. We later adjusted the refraction further to optimize the response of recorded units. We gave daily injections of a broad-spectrum antibiotic (Bicillin) and an anti-inflammatory agent (dexamethasone). Experiments typically lasted 4-5 days. All procedures complied with guidelines approved by the New York University Animal Welfare Committee.