Mammillothalamic disconnection alters hippocampo-cortical oscillatory activity and microstructure: Implications for diencephalic amnesia

Diencephalic amnesia can be as disruptive as the more commonly known temporal lobe amnesia, yet the precise contribution of diencephalic structures to memory processes remains elusive. We used discrete lesions of the mammillothalamic tract to model aspects of diencephalic amnesia and assessed the impact of these lesions on multiple measures of activity and plasticity within the hippocampus and retrosplenial cortex. Lesions of the mammillothalamic tract had widespread indirect effects on hippocampo-cortical oscillatory activity within both theta and gamma bands. Both within-region oscillatory activity and cross-regional synchrony were altered. The network changes were state-dependent, displaying different profiles during locomotion and paradoxical sleep. Consistent with the associations between oscillatory activity and plasticity, complementary analyses using several convergent approaches revealed microstructural changes, which appeared to reflect a suppression of learning-induced plasticity in lesioned animals. Together, these combined findings suggest a mechanism by which damage to the medial diencephalon can impact upon learning and memory processes, highlighting important role for the mammillary bodies in the co-ordination of hippocampo-cortical activity.

Fractionation of Memory in Patient Populations: A Memory Systems Perspective

Memory complaints and impairments characterize a number of different neurological and neurodegenerative disorders. Exactly how these impairments manifest (e.g., the type of memory that is affected, the severity of the deficit, whether the impairment is temporally-limited or extensive) can vary considerably across patient populations and depends upon which memory systems or brain structures have been compromised. Following some historical context and a brief overview of the multiple memory systems perspective, several conditions with memory deficit as a primary or secondary symptom are described (i.e., MTL amnesia, diencephalic amnesia, Alzheimer's disease, semantic dementia, and Parkinson's disease). Patterns of spared and impaired performances across conditions are compared, and the pathological profiles of each disorder are summarized. It is concluded that while neuropsychological studies support the multiple memory systems perspective, they have also been instrumental in shaping our ever-evolving views of how brain systems support memory and how they interact.