The basal forebrain is anatomically diffuse rather than nuclear, with its component cell groups dispersed along the ventral surface of the hemisphere from the medial septum back to the substantia innominata beneath the lentiform nucleus. The classical work that organised this diffuse system into a coherent functional unit was Mesulam’s nomenclature of Ch1-Ch4 cholinergic cell groups, with Ch4 corresponding to the nucleus basalis of Meynert and being the principal source of cortical cholinergic innervation.

The cholinergic hypothesis of Alzheimer’s disease, proposed by Bartus in 1982, framed the early cognitive impairment of Alzheimer’s as a consequence of cortical cholinergic deficit downstream of basal-forebrain cell loss. This hypothesis predicted that cholinergic replacement therapy would benefit Alzheimer’s symptoms, which proved correct in modest measure when cholinesterase inhibitors became available in the 1990s. The therapy slows but does not prevent symptom progression, consistent with cholinergic deficit being a contributor to rather than the sole cause of Alzheimer’s cognitive decline.

Anterior communicating artery aneurysm rupture and the subsequent subarachnoid haemorrhage can damage the basal forebrain selectively because the artery sits immediately above this territory. Survivors often emerge with a striking dissociation between severe amnesia and relatively preserved general cognition, sometimes with confabulation of an unusually elaborate and consistent kind.