A case of frontal network amnesia.

A 63 year old man developed an amnesic syndrome coupled with an array of "frontal lobe" signs after bilateral small subcortical infarcts. His amnesia was characterised by severe difficulty in voluntary recall of recently memorised verbal and non-verbal materials, while his recognition for the same materials was less affected. The symptoms remained unimproved at a follow up evaluation eight months after onset. Magnetic resonance imaging showed two small circumscribed lesions, one in the dorsomedial nucleus of the left thalamus and the other in a region of the right globus pallidus and anterior limb of the right internal capsule. The mammillothalamic tracts and anterior nuclei of the thalami were clearly spared bilaterally. The left dorsomedial nucleus lesion disrupted the thalamofrontal circuit, while the anterior limb lesion of the right internal capsule disconnected the same circuit by damaging part of the anterior thalamic radiation. Thus the amnesia in this patient may have been caused by disruption of the bilateral thalamofrontal circuits. This type of amnesic pathology should be separated from more conventional types of amnesia that are produced by disruption of the so called Papez circuit or the Delay-Brion memory system.