Cerebral bases of consciousness: a historical view.

Attempts of modern brain research, starting at the end of the nineteenth century, to define and hierarchically order consciousness are reviewed. It is emphasized that roots from philosophy, biology, and psychology influenced brain research in its search for possible physical bases of consciousness. Among the mainstreams of research were approaches which defined consciousness by selecting numerous attributes of it and ordering these hierarchically. Similarly, there was widespread speculation on whether all kinds of animals or only some of them are conscious or manifest some of the hierarchically defined attributes of consciousness. On the brain side, the cerebral cortex and in particular its frontal aspects, the prefrontal cortex, was regarded as the principal anatomical basis of consciousness. The dependence of consciousness on memory and the ability to order processes in time were widely acknowledged and case descriptions from Korsakoff patients and epileptics were considered especially to demonstrate the interdependence between consciousness and the brain.