Complex sentence: Neuroscience as an interdisciplinary field, and its branch of cognitive neuroscience particularly, operates under the ontological assumption of physicalism, according to which only the fundamental phenomena studied by physics exist.
Split into simpler sentences: Neuroscience as an interdisciplinary field, and its branch of cognitive neuroscience particularly, operates under the ontological assumption of physicalism. In other words, it assumes--in order to perform its science--that only the fundamental phenomena studied by physics exist.