The neurotransmitter acetylcholine has been linked to hemispheric activation in northern fur seals. Researchers studied seals in controlled environments by observing behavior as well as through surgically implanted EEG electrodes.[1] Acetylcholine is released in nearly the same amounts per hemisphere in bilateral slow-wave sleep. However, in USWS, the maximal release of the cortical acetylcholine neurotransmitter is lateralized to the hemisphere exhibiting an EEG trace resembling wakefulness.