I have always wondered how it is possible to store all the experiences we have in our heads. Have you ever thought about it? We have memories of the most different types, and some we keep with such richness of detail that, when we recall them, it feels like we are reliving them. The other day, I found a photo of myself when I was still a baby (in a failed attempt to eat a tangerine) and started to remember the house I lived in, the games I played, and what my days were like back then. Luckily, our memories are not stored like objects; otherwise, they would take up so much space that in no time we would have no way to keep them, which would be very sad. We know that our memories are present in our lives at all times, but after all, where are they stored? Brenda Milner's dedication to studying the case of a patient known only by the initials of his name, H. M., contributed decisively to the understanding of the processes involved in memory storage. Let's take a closer look.
At the age of nine, H. M. was hit by a bicycle, and the accident triggered epilepsy. Over time, the disease became debilitating, requiring, as a last resort, surgery to remove the medial temporal lobes and the hippocampus of the patient. The operation successfully reduced the epileptic seizures but caused devastating memory loss from which H. M. never recovered. He was able to retain new information for a short period due to the integrity of his short-term memory and had long-term memory for events that occurred before the surgery; however, the patient had completely lost the ability to convert any new memories into permanent memory. With this, Milner concluded that it is only in the hippocampus that the various bundles of sensory information necessary for the formation of long-term memory come together, and that this structure is not responsible for storing memories that have already been preserved for some time.
For many years, Milner thought that H. M. was incapable of converting any short-term memory into long-term memory; however, she observed that he was able to learn to draw the outline of a star in a mirror, and his motor skills improved each day, just as they would in the absence of brain injury. Thus, in 1962, the researcher demonstrated the existence of more than one type of memory: explicit (conscious) memory, which requires the participation of the hippocampus, and implicit (unconscious) memory, which does not depend on this structure.
Today we know that conscious memory is initially stored in the prefrontal cortex, then converted into long-term memory in the hippocampus, and finally stored in the same areas of the cortex that originally processed the information; for example, memories of visual images are stored in the visual cortex. Implicit memory, on the other hand, involves various brain systems: the association of feelings with events involves a structure called the amygdala; motor habits, such as walking or running, involve the striatum; and motor skills and coordinated activities, such as those needed to make a basketball shot, involve the cerebellum. For this reason, despite H. M. progressively improving on the mirror test due to the integrity of his implicit memory, he never remembered having performed that task the day before, due to having lost the ability to store explicit memory.
It is impressive how much the study of the pathology of a single individual has contributed, and one of the things that most piques my interest about this case is the fact that, although H. M. believed he had never performed that task before, he improved his skill after each test. This example illustrates the influence that unconscious processes can exert over our actions and how we can be guided by them. If we recall our previous discussion about how repetition can allow learning to become unconscious, what actually happens is the transformation of explicit memory into implicit memory; for example, when I was learning to drive, I had to visualize the pedals before starting the car to know where to place my feet and think about when to shift gears; today I simply get in and drive; it has become automatic! In this way, we can imagine the importance of selecting what we will or will not store, which leads me to another question: why are some memories retained for long periods while others are quickly lost? We will talk about this in our next meeting!
References
Kandel, ER. For different types of memory, different regions of the brain. In: In Search of Memory – The Birth of a New Science of Mind (Kandel, ER). 2009; p. 211-231, Companhia das Letras.
Milner B, Squire LR, Kandel ER. Cognitive neuroscience and the study of memory. Neuron. 1998; 20:445-68;
Figure 1: http://commons.wikimedia.org/wiki/File:Hippocampus.png
Figure 2 adapted from: Kandel, ER. For different types of memory, different regions of the brain. In: In Search of Memory – The Birth of a New Science of Mind (Kandel, ER). 2009; p. 211-231, Companhia das Letras.
