Behavioral/Systems/Cognitive
The Operating Point of the Cortex: Neurons as Large Deviation Detectors

Dario L. Ringach and Brian J. Malone

Department of Psychology and Neurobiology, Jules Stein Eye Institute, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California 90095-1563

Animal preparation. Experiments were approved by the University of California, Los Angeles, Animal Research Committee and were performed by following National Institutes of Health's Guidelines for the Care and Use of Mammals in Neuroscience. Old World monkeys (Macaca fascicularis, 3–5 kg) were used. Animals were sedated with acepromazine (30–60 µg/kg) and anesthetized with ketamine (5–20 mg/kg, i.m.). Initial surgery was then performed under 1.5–2.5% isoflurane. Two intravenous lines were put in place for the continuous infusion of drugs. A urethral catheter was inserted to collect and monitor urine output. An endotracheal tube was inserted to allow for artificial respiration. Pupils were dilated with ophthalmic atropine, and the eyes were protected with ophthalmic Tobradex (Alcon Laboratories, Ft. Worth, TX) and custom-made gas permeable contact lenses.

At the completion of this initial surgery the animal was transferred to a stereotaxic frame. At this point the anesthesia was switched to a combination of sufentanil (0.15 µg · kg–1 · h–1) and propofol (2–6 mg · kg–1 · h–1). After monitoring the anesthetic plane for 10–20 min, we performed a craniotomy over the primary visual cortex. Only after the completion of all surgical procedures, including the insertion of the electrode array, was the animal paralyzed (Pavulon, 0.1 mg · kg–1 · h–1).

To ensure a proper level of anesthesia throughout the experiment, we continually monitored rectal temperature, heart rate, noninvasive blood pressure, end-tidal CO2, SpO2, and EEG by a Hewlett-Packard Company Virida 24C neonatal monitor (Palo Alto, CA). Urine output and specific gravity were measured every 4–5 h to ensure adequate hydration. Drugs were administered in balanced physiological solution at a rate to maintain a fluid volume of 5–10 ml · kg–1 · h–1. Rectal temperature was maintained by a self-regulating heating pad at 37.5°C. Expired CO2 was maintained between 4.5 and 5.5% by adjusting the stroke volume and ventilation rate. The maximal pressure developed during the respiration cycle was monitored to verify that there was no incremental blocking of the airway. A broad spectrum antibiotic (Bicillin, 50,000 IU/kg) and anti-inflammatory steroid (dexamethasone, 0.5 mg/kg) were given at the beginning of the experiment and every other day.