[Memory diseases]

MEMORY DISEASES. There are many diseases that permanently affect longterm memory and all of them have in common that they permanently and usually bilaterally disrupt specific neural circuits that underlie it. In the forefront is the Papez circuit, or hippocampo-mamillo-thalamo-cingular circuit, which is also connected to the fronto-basal regions. Its impairment leads to disorders of episodic memory, with relative preservation of semantic memory and implicit learning. The anterior temporal pole is a hub allowing access to general knowledge distributed in the cortex. Its damage results in an amnesic picture in which the loss of semantic memory dominates. The richness of memory disorders is largely deduced, in its nuances, from the lesion topographies. The most frequent aetiology of memory diseases is represented by neurodegenerative diseases, dominated by Alzheimer's disease, but the semiology of these is by far not limited to a memory disorder, because of the diffusion of lesions. Dysimmune, infectious or toxic encephalitis affecting the hippocampi, Korsakoff's syndrome affecting the thalamus and mamillary bodies, « semantic dementia » affecting the temporal pole, give pictures where memory disorders are in the foreground with remarkable semiological nuances. Post-traumatic amnesia, due to the heterogeneity of the lesions, offers a more complex picture, where memory disorders are complemented by executive disorders, sometimes major.