Today, I am reminded of this experience not only by a photo of my grandparents standing under the pear tree with me, but by all the beautiful memories I have stored in my mind and that come back from time to time. Then I start a kind of time travel back to my childhood, a process in which some factual knowledge has to be activated, i.e. when, where and how did I experience the story of the pear tree? And at the same time, all of this evokes feelings from long ago – to the point where the smell of warmth, security and the taste of ripe and precocious pears come to mind.
Everyone has childhood experiences like mine that come back to haunt them, whether they want to or not. Childhood memories can be nostalgic and beautiful, or oppressive and traumatic. When and why they resurface at certain moments is what I want to explain here. It is precisely these splinters from the large memory structure of growing up, of becoming an I, that make up our being and our psychostructure. Our memory determines who we are. Our memory is our life. It is essential for us individually, but it manifests itself only in the social context. For it is both a prerequisite and a means of communication with ourselves, with others, and across time as a culture.
256 storage systems and no single hard drive
In the following, I would like to walk with you through the rooms of human memory and introduce you to a powerful part of the mechanisms of our memory. I will focus primarily on autobiographical memory. The Canadian Endel Tulving, one of the founding fathers of modern memory research, came up with 256 different memory terms in the course of his career. This seems exaggerated, but it underscores the point that human memory is not a unitary store, but is made up of multiple subsystems distributed across different areas of the brain with different capabilities and functions.
An abstract and systematic division of memory into tangible subsystems becomes clearest when one considers what happens when a person’s memory fails. As in the case of Clive Wearing, a British musician and BBC reporter, who in 1985, at the height of his career (he had just performed for the Pope with his London choir), contracted a herpes simplex infection, which in rare cases can cross the blood-brain barrier and cause massive, life-threatening inflammation in the brain. The well-known journalist fell into a coma, and when he awoke, he was no longer able to store autobiographical memories and recall past experiences. His hippocampus, a central switching station of explicit memory in each hemisphere of the brain, had been highly selectively but just completely destroyed bilaterally, resulting in a total loss of autobiographical memory. He believed, at intervals of minutes and whenever someone visited him, that he had just awakened from the coma and had just regained consciousness. He knew who he was, however, and could recall that he had studied at Cambridge and had performed Handel’s Messiah. The rest of his life was more or less buried – caused by a tiny virus particle. Whenever his wife visited him in the hospital, he greeted her with great joy – not remembering that he had married her several years earlier. He could no longer read a book or watch a movie, because after a few moments he would forget the plot. He was trapped in the present, a state he himself described as “hell on earth. Only his musical memory was still enormously good: when his former choir was with him and sang a song for him, he could easily tune in and sing along, including all the verses and the correct intonation. He also had an enormous amount of facts about the world at his fingertips.
It is neurological cases such as these that provide scientific evidence that brain injury can destroy certain aspects of human memory without affecting other categories of memory. And they show that memory exists neither as a place nor as a sign.
Let’s begin our journey into the epicenter of our ego-consciousness by looking at the different memory systems in our brain. One is divided into working memory, implicit memory, and explicit memory. Or, if you divide them according to temporal aspects, into short-term and long-term memory. A certain part of the short-term memory is now usually called working memory. We use it, for example, to store subtotals in complicated calculations or to remember how a sentence began at the end. It can hold no more than six to eight items at a time and is located primarily in the front part of the frontal lobe (in the prefrontal cortex). In many ways, working memory acts as the crucial pinhole of our memory performance. It determines how long we can concentrate on a task, i.e. how many steps of thought we plan in advance and how long we can pursue a goal with all our might. Its performance affects all memory functions. The following applies: The better we can concentrate, the more facts we juggle in our head, the better our memory capacity.
While working memory has a small storage capacity, long-term memory has an almost inexhaustible capacity. It permanently stores information in our brain. There are two types of long-term memory: declarative and implicit. Declarative memory stores facts, experiences, episodes, and events in a different subsystem depending on the content of the memory.
- In autobiographical memory, we store episodes from our lives in the first-person perspective, i.e., information about when something happened, with whom, and where. This domain also includes our (rather poor) source memory, i.e. the memory of the origin of a memory.
- Episodic memory can include stories from books, narratives, or movies.
- Semantic memory contains our knowledge of the world. This includes names for things, places, animals, plants, but also our classical school knowledge and our semantic-grammatical knowledge.
Implicit memory includes all unconscious memories that are difficult to articulate. This includes motor learning, habits and routines, perceptual memory, and a subsystem known as priming.
Generally, when we think about memory, we think mainly about explicit memory, that is, personal memories, factual knowledge (What is the name of the capital of Lithuania?), and singular knowledge (What is the name of our eldest son’s history teacher?). But this is only a fraction of our memory. The far greater part of what we have learned in life is generic memory, that is, memory based on habit.
Brain-organic bases of autobiographical memory
A systematic consideration of all aspects of our memory would be akin to an anatomy lesson on human brain regions. Therefore, we will focus on autobiographical memory because it is the least studied. For a long time it was counted as episodic memory because it was difficult to recognize the first-person perspective of an experience as a distinct quality. It was (and to some extent still is) thought that this first-person experience is rather an epiphenomenon without causal effect – we believe to be the doers and experiencers, but it is the neurons that dictate how we act through their circuitry. Already in the late 19th century, the psychologist William James called this assumption a big mistake. But it was not until the 21st century that neuroscience dared to take a closer look at this aspect of our memory.
Although there is no clear place where our memories are organized like books in a library, certain areas of the brain are essential to the functioning of our autobiographical memory. Brain organically, the hippocampus is a critical structure of explicit memory. Together with parts of the frontal and temporal lobes, it is responsible for storing and retrieving autobiographical events.
To integrate the hippocampus, which develops last in evolution, into the brain as a circuit, an entire information highway is needed: a thick strand of nerve fibers (the fornix) carries information from the hippocampus to the basal forebrain and to parts of the hypothalamus, the mammillary bodies. It does not begin to function at its normal rate until the third year of life, which is why we have difficulty recalling memories from our first three years. The fornix pathway is not fully functional until about six years of age. Parents, educators and teachers should keep this in mind. In addition, the hippocampus is one of the few brain regions where neurons continue to form significantly after birth. The scientific term for this process is adult neurogenesis. Fred Gage, a neuroscientist at the Salk Institute in San Diego, theorizes that the hippocampus is probably necessary to mark individual experiences in time and to take into account the constant remodeling processes in the hippocampus – whether the many events and facts we experience and store.
It would be a mistake to assume, based on what has been described above, that the individual memory systems work side by side in isolation. On the contrary, our lives are a series of learning situations in which all memory systems interlock. To store (and retrieve) autobiographical memories, we need a whole network of areas called the Papez circuit. It consists of the hippocampus and the anterior part of the cingulate (gyrus cinguli), as well as the thalamus and the mammillary bodies, located deep in the brain. This neural circuit is extremely powerful because in many ways it is the neural network that determines who we are and what we remember about ourselves, as well as what we forget.
At this point I would like to point out the lateralization according to left and right hemisphere: The left Papez circle (in right-handers) is for storing facts, episodes, and autobiographical events, while the right is for spatial information and relationships between objects. It is precisely these brain structures and pathways that are affected in Korsakow’s Syndrome, which occurs as a result of vitamin B1 deficiency, often caused by prolonged heavy alcohol consumption. Patients suffer from anterograde amnesia, i.e. they can no longer store autobiographical memories and hardly any factual knowledge from the time of the onset of the disease.
On its way to autobiographical and factual memory, information must pass through a filter called the limbic system. The name is derived from Latin and means “hem,” because its structures surround the corpus callosum, which connects the two hemispheres, like a belt or ring. The limbic system is divided into the amygdala, the hippocampus, parts of the hypothalamus, and the cingulate gyrus. Like a spoked wheel, it pushes up against the cerebral cortex from the inside (with the diencephalon as the hub), virtually lining it from the inside. It is the entity that sorts out relevant information, provides it with emotions, and bundles it before it is deposited in widely distributed areas of the cerebral cortex. Learning, memory, and emotion are thus closely related in terms of brain anatomy, which also explains why a certain emotional state corresponding to the mood you were in is stored as a memory, making it easier to retrieve that memory. But even the smell of freshly cut grass or the song we heard during our first kiss can evoke memories from long ago.