Amnesia

Amnesia is a memory disorder characterized by a profound impairment in establishing new memories of people, places, and events and a deficit in retrieving information acquired prior to the onset of illness. It is important to note that amnesia is not an all-or-none condition—it is functionally diverse and its severity varies based on etiology and lesion extent. However, despite the heterogeneity observed across patients, some fundamental characteristics can be established. This entry presents an overview of the syndrome, including the different subtypes of amnesias and their behavioral and neurological characteristics.

Broadly speaking, amnesic patients are unable to consciously bring to mind information encountered after the onset of injury (anterograde amnesia), despite preserved general intelligence and other cognitive functions, such as language. In this sense, their cognitive deficit is selective (i.e., limited to memory). The memory impairment is often described as “global” because patients' inability to form new memories is ubiquitous across different types of material (e.g., verbal vs. visuospatial) and different modes of presentation (e.g., visual or auditory). Amnesic patients typically also have some difficulty recalling information acquired prior to the onset of injury (retrograde amnesia), but the temporal extent of retrograde memory loss is quite variable.

One of the most striking case reports of amnesia is that of patient H. M. As an experimental treatment for intractable epilepsy, H. M. underwent surgery that removed his medial temporal lobes (MTLs) bilaterally at the age of 27. Although the surgery was largely successful in relieving H. M.'s epileptic seizures, it left him with a profound amnesia, which H. M. described as “like waking from a dream … every day is alone in itself.” His anterograde memory loss was apparent both in his daily life and in laboratory testing. For instance, H. M. could not remember where he lived or what he ate for breakfast earlier that day. In laboratory testing, he failed to recall anything about a drawing that he copied merely minutes earlier. This pattern of forgetting was observed across different types of materials and presentation formats (e.g., spoken words, pictures, abstract patterns, spatial locations). H. M. also experienced retrograde memory loss that extended years prior to his surgery.

It is important to distinguish the memory loss experienced by amnesic individuals and that experienced by patients with more global cognitive decline (e.g., dementia). Whereas amnesics' cognitive impairments are memory-specific and fairly stable, dementia patients' cognitive decline tends to be global and progressive.