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This page was last modified on January 30, 2018

The Memory Trace: The Physiology of Memory

Types of amnesiaNeuropsychologists and neurologists have seen for years that there seem to be at least two different kinds of amnesia, that is, an inability to remember what happened in the past, depending upon when some event like a brain injury took place.
  • Retrograde amnesia: an individual cannot remember details or experiences from before the event like a brain injury. ("retro" means "behind" or "in back of")
  • Anterograde amnesia: an individual cannot remember details or experiences after an event like a brain injury.  ("antero" means "in front of" or "after")
The event that may cause amnesia could consist of a brain injury like a car crash or a concussion or some other blow to the head. It might also have been an event like a surgery or an illness such as meningitis.

Patient H.M. (Henry Gustav Mo
HM Obitlaison, 1926-2008) {W}

HM 2

Henry Molaison (Patient "H.M.") experienced severe and frequent epileptic attacks. In 1953 a surgeon removed the hippocampus on both sides of his brain in the temporal lobes in order to treat him. Unwittingly, this surgery destroyed his ability to form new personal memories. He was studied until his death in 2008 and is considered to be the most important patient in 20th century neurology and neuropsychology.
  • Short-term working memory was fine. He could remember things for 5 to 10 minutes.
  • He could not remember any experience longer than about 5 to 10 minutes, that is, anything that would be a new addition to his long-term memory. For example, he saw the same doctors and psychologist day after day, but never learned who they were.
  • He also had very significant memory loss of events in his life from before his operation.
  • His overall intelligence remained intact and he could generally care for himself, carry on conversations, and enjoy himself with puzzles and other games.
Hippocampus HM

H.M. was experiencing (1) massive anterograde amnesia and (2) significant retrograde amnesia. The destruction of most of the hippocampus and some of the surrounding tissue (e.g., entorhinal cortex, part of the temporal lobe) left him unable to form any new personal memories. As researchers came to discover, however, he could still learn other non-personal skills like how to play different kinds of games or construct different kinds of objects with blocks, etc. This was the beginning of understanding the difference between declarative and procedural memory (see below).

The Neural Circuitry of Memory
  • Richard F. Thompson: memories are stored in localized neural circuits, i.e., "unique, reusable pathways in the brain along wich signals flow" (p. 247)
  • Eric Kandel: memories result from alterations in synaptic transmissions at specific sites, i.e. there are long-lasting changes in whether synapses fire or don't fire [= "Long-Term Potentiation"]
  • Neurogenesis: while we know that new neurons are not created in most of the brain after birth, recent research finds that some new neurons may develop in the dentate gyrus of the hippocampus (= neurogenesis). From research with Patient H.M., we know that the hippocampus is central to the storage of new memories.
  • Overall, despite many years of study, we are still not fully sure either where or how long-term memories are stored in the brain.

Different Types of Memory Systems

Systems of Memory

A. Nondeclarative ("procedural memory") = Implicit; nonverbal, shown by completing a task
•    Actions & perceptual motor skills, e.g., riding a bike, driving a car, cooking a meal, etc.
•    Implicit knowledge: how to solve a puzzle, how to fix a broken object

• Conditioned reflexes, e.g., responding to sounds or other signals
• Emotional memories: the feelings which were part of an experience

[Endel Tulving]B. Declarative • “What is” Memory = Explicit, verbal, visual, auditory
= Factual information

Endel Tulving (see photo) suggests two separate systems
1. Semantic Memory: knowledge of the world, "facts" which are independent of any specific time

2. Episodic Memory: Personal, time-bound, recollections which are linked to a particular point in time

Prospective vs. Retrospective Memory

  • Retrospective memories concern events in the past or information which is previously learned
  • "Prospective" memories involve remembering to perform tasks in the future.
    • E.g., carrying your umbrella, picking up your laundry from the cleaners, writing a "thank you" note to a friend


Endel Tulving also talks about the notion of an ability unique to humans which he calls "mental time travel" or more formally "chronesthesia. (Article from APA on this notion.)

By this, he means

  • the ability of human beings to go both backward and forward in time
  • to think about the past and to imagine the future

Tulving proposes that human beings have a strong ability to travel back in time precisely because this general ability also allows us to plan for the future, to imagine what will happen and, thus, be able to take actions in order to cope with that future. Hence, our ability to travel in time gives human beings an evolutionary advantage: mental time travel is a very adaptive ability which helps humans survive.

With this ability, humans can do things like
  • Plant seeds at the right time of the growing season
  • Harvest crops when they are at their peak
  • Keep records
  • Teach children what they can expect as they grow up


This page originally posted on 9/28/07