Where early work on the neuroanatomy of amnesia focused on the hippocampus, it now appears that this structure is essential only for episodic memories. In this type of learning, it is a part of a uniﬁed system that includes the mamillary bodies and anterior thalamus. By contrast, the perirhinal cortex appears to be required for the type of learning that can be classed as semantic learning. Damage to the mediodorsal nucleus of the thalamus also appears to have an amnestic eﬀect through disruption of frontal cortical function. Despite there being this variety of memory structures in the brain, each with a diﬀerent function, recent work is indicating that the cholinergic basal forebrain acts to modulate these diﬀerent systems. In modulating these systems they allow normal memory formation, although do not aﬀect memories once they are encoded in these structures. Future work must examine the speciﬁc role of the cholinergic cells in this modulation, and also test the model proposed here that the basal forebrain is not required for the retrieval of memories that are already encoded. The interactions of memory structures in the retrieval of encoded memories is examined further in Chapter 8.