___ Neurological Disorders: Amnesia ___

Amnesia - Brain Regions and their Dysfunctions

Amnesia "Three emerging strands of evidence are helping to resolve the causes of the anterograde amnesia associated with damage to the diencephalon. First, new anatomical studies have refined our understanding of the links between diencephalic and temporal brain regions associated with amnesia. These studies direct attention to the limited numbers of routes linking the two regions. Second, neuropsychological studies of patients with colloid cysts confirm the importance of at least one of these routes, the fornix, for episodic memory. By combining these anatomical and neuropsychological data strong evidence emerges for the view that damage to hippocampal-mammillary body-anterior thalamic interactions is sufficient to induce amnesia. A third development is the possibility that the retrosplenial cortex provides an integrating link in this functional system. Furthermore, recent evidence indicates that the retrosplenial cortex may suffer "covert" pathology (i.e., it is functionally lesioned) following damage to the anterior thalamic nuclei or hippocampus." S
Affected Region	Dysfunctions
Temporal lobes	"Patients with damage to the mesial and anterior portions of the temporal lobes suffer from a memory impairment involving both anterograde and retrograde amnesia." A patient (with damage predominantly in the right temporal lobe) was almost exclusively impaired in the autobiographical domain (both episodic and semantic), while another patient (with bilateral damage but more severe on the left side) showed poor performances in all domains, but much more severe in the nonautobiographical (both public events and general semantic knowledge). S "For the first time, the predictive validity of MTL (medial temporal lobe) and DGM (deep gray matter) atrophy were simultaneously compared with MS (multiple sclerosis) using reliable and validated neuropsychological measures. This study found that both compartments play significant but different roles in the amnesia of MS." S
Hippocampus	"(...) The attention and memory-related functions of these structures (hippocampus, amygdala and prefrontal cortex) are influenced by strong emotionality." S "Comparison between (a young adult with developmental amnesia due to perinatal hippocampal damage) and his controls revealed that, whereas he was unimpaired in the familiarity process, he showed abnormally low levels of recollection, supporting the view that the hippocampus mediates the latter process selectively." S "In conclusion, the whole published data question the implication of the hippocampus in every semantic learning and suggest the existence of a neocortical network, slower and that needs more exposures to semantic stimuli than the hippocampal one, which can supply a massive hippocampal impairment." S
Thalamus	"These findings stress the specific role of the left anterior thalamic region in memory and confirm that longlasting amnesia from a thalamic lesion can occur without significant structural damage to the dorsomedial nucleus." S
Mammillary body	"Mammillary body volume significantly correlated with 13 out of 14 tests of episodic memory recall, but correlated poorly with recognition memory. Furthermore, as the volumes of the left fornix and the left mammillary bodies decreased, the difference between recall and recognition scores increased. No other structure was consistently associated with memory. These findings support models of diencephalic memory mechanisms that require hippocampal inputs for recall, but not for key elements of recognition." S
Fornix	"The authors hypothesize that the lesion of the right fornix was sufficient to cause amnesia by disconnecting the hippocampal formations from the anterior thalamic nuclei and mammillary bodies and interrupting the cholinergic efferents to the hippocampus from the medial septum, according to the extended hippocampal system framework. Sparing of the left fornix may be sufficient to ensure a good recovery of memory." S

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