In the latter half of the 1970s, neurochemical studies of post-mortem tissue specimens reported damage to the cholinergic system, resulting in decreased acetylcholine-producing choline acetyltransferase activity, decreased choline absorption, and decreased acetylcholinesterase activity. Cholinergic basal forebrain nuclei are the major neural pathways over which cholinergic neurons enter the hippocampus and cerebral cortex, and these nuclei are crucial for memory, concentration, and other cognitive procedures. In many experiments, the removal of cholinergic neurons of treatment with cholinergic antagonists, such as scopolamine or hyoscine, has been shown to elicit impairments of memory and other cognitive functions.