The earliest contributions to a neurobiology of consciousness came from basic discoveries in electrophysiology made possible by the invention of the electroencephalogram (EEG) by Hans Berger (Millett, 2001). The EEG had the then-remarkable capacity of providing a graphical distinction between conscious and nonconscious states. Next, the classical cerveau isole and other modifications performed on animal brains provided initial insights into the brainstem's role in regulating consciousness. Beginning in 1930, Bremer's surgical procedures on animal brains provided key experimental foundations for a neurobiology of consciousness. In the encephale isole, where the spine is severed from the brain, consciousness was not found to be impaired but freedom of movement was devastated. A transection at the level of the high midbrain, termed the cerveau isole, left the animal in a coma for several weeks, with eventually some partial restoration of limited EEG desynchronization but with presumably permanent impairment of consciousness. In a lower transection, known as the midpontine pretrigeminal isole, a great deal of waking EEG activity was observed, suggesting that the space between the cerveau (midbrain) and the mid-pontine isoles contributed crucially to wakefulness. Additionally, because the midpontine isole provided no possibility of additional sensory input, wakefulness could only be due to contributions from the brainstem tissue between the cerveau and the midpontine isole, and not due to some quantitative or qualitative "threshold of sensory input" as the earliest concepts of consciousness and brain function emphasized.
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