Mice aged P3-P9 were deeply anesthetized with isoflurane (2.5%) in oxygen and then placed on a heating pad set to 36.5°C via a homoeothermic temperature monitor (NPI TC-20, ALA Scientific). Local anesthesia was produced by subcutaneous injection (0.05 ml) of 1% Xylocaine (10 mg/ml lidocaine/0.01 mg/ml epinephrine, AstraZeneca) under the scalp. After removal of the scalp, steel head posts were fixed to the anterior and posterior portions of the exposed skull using cyanoacrylate glue. Isoflurane anesthesia was adjusted between 0.5-1.0% as necessary to maintain a stable respiratory rate. A ~2 mm oval craniotomy was created by gentle etching into interparietal skull and removing the resulting bone flap above the superior colliculus, just posterior to lambda using the tip of an 18G syringe needle. After achieving hemostasis, the dura was carefully removed using forceps and microdissection scissors. The craniotomy was filled with warm (37°C) low temperature melt agarose (A9414, Sigma, 1.5% in sterile buffered saline, 150mM NaCl, 2.5 mM KCl, 10 mM HEPES, pH 7.4).