During learning, neurons deep in brain engage in a surprising level of activity -- ScienceDailyIt's the part of the brain that makes sure you cannot tickle yourself. The cerebellum, an apple-sized region near the base of the skull, senses that your own fingers are the ones trying to tickle, and cancels your usual response. Now an international team of researchers has learned something surprising about this region, which despite its small size contains roughly half of all the neurons in the brain. These neurons, which were thought to fire only rarely as they take in information from the senses, are in fact far more active than previously suspected. The finding, published March 20 in the journal Nature Neuroscience, may signal a major shift in our understanding of how the cerebellum encodes information. "People used to think that the cerebellum's input layer of neurons was only sparsely active, and encoded only information collected from the external world," said Sam Wang, professor of molecular biology and the Princeton Neuroscience Institute, and a senior co-author on the study. "It turns out that they light up like a Christmas tree, and they convey information both from outside the body and from other areas within the brain."