Source: Laboratories of Jonas T. Kaplan and Sarah I. Gimbel—University of Southern California

Anterograde amnesia is the loss of the ability to form new memories. This can be distinguished from retrograde amnesia, which is the loss of old memories. Anterograde amnesia can result from damage to structures in the brain that are involved in the formation of new memories. Patients who have damage to the structures of the medial temporal lobe, including the hippocampus, amygdala, and the surrounding cortices, often have severe deficits in the formation of certain kinds of memories. These cases can be informative as to how memory is organized in the brain, and how different systems support different kinds of memories.

In this video, we will test a patient with medial temporal lobe damage on a series of memory tasks designed to distinguish between different forms of memory. First, we will test short-term or working memory, which is the process we use to keep information in mind temporarily. Next, we will test two different forms of long-term memory: explicit and implicit memory. Explicit memories are conscious and easy to verbalize. For example, memories of facts or episodes from our lives are explicit memories. We can easily tell someone what we ate for breakfast, or what city is the capital of France. Implicit memory involves knowledge we gain from experience but that is not easily expressible. For example, knowing how to do things, or becoming habituated to a stimulus are forms of implicit memory.

These procedures are based in part on studies of the famous patient Henry Molaison, also known by his initials H.M., who had severe anterograde amnesia as a result of a surgery for intractable epilepsy in which parts of both temporal lobes were resected.¹ We will perform a test of digit span, which measures short-term memory, a test of paired-associate learning, which measures explicit or declarative memory, and mirror-drawing, a test of implicit skill learning.²