In Vivo Cortical Imaging in Mice

All experimental procedures used in this study were approved by the Animal Care and Use Committee of USC. Female adult mice (12–16 weeks, C57BL/6) were anesthetized with urethane (1.2 g/kg) and sedative chlorprothixene (0.05 ml of 4 mg/ml), as previously described28 (link)–30 (link),39 (link). Lactated Ringer’s solution was administrated at 3 ml/kg/hour to prevent dehydration. The animal’s body temperature was maintained at ~37.5° by a heating pad (Havard Apparatus, MA). Trachotomy was performed to maintain a clear airway, and a ventilator (Havard Apparatus, MA) was connected. Cerebrospinal fluid draining was performed to prevent the cortex from swelling. The animal was placed in a custom-built stereotaxic holder. The part of the skull and dura mater (~1×1 mm) over the V1 was removed. Artificial cerebrospinal fluid solution (ACSF, containing (in mM) 140 NaCl, 2.5 KCl, 2.5 CaCl₂, 1.3 MgSO₄, 1.0 NaH₂PO4, 20 HEPES, 11 glucose, pH 7.4) was applied onto the exposed cortical surface when necessary. Throughout the surgical procedure, the lids were sutured. After surgery, right eyelid was reopened and drops of 30k silicone oil were applied to prevent the eye from drying. The whole procedure of RF mapping was finished within 25 minutes. Previous studies showed that in nonparalyzed mice the drift of the measured RF was negligible within an hour, compared to the average RF size27 (link)–29 (link),39 (link). Our cell-attached recording also showed that the drift of the measured RF of single unit was never more than 2–3° per hour, so that the largely overlapped excitation was not due to the eye movement.