BDNF and Parkinson disease: Two of these factors, brain-derived neurotrophic factor (BDNF) and glial cell-line–derived neurotrophic factor, raise intriguing questions of whether US can be used to stimulate hippocampal plasticity with implications for probing learning and memory, as well as offering neuroprotective effects for neurodegenerative conditions, such as Alzheimer's and Parkinson's disease (Liu et al., 2017, Zhao et al., 2017).