“All the crazy things I did to ni-i-i-ight, those will be the best memories for meeee!” Come on now don’t act like you have never heard of it, I know that you can now hear the song playing in your mind. Even though David Guetta’s Memories was released almost four years ago you probably can still remember its lyrics and can sing along the entire song from beginning to end, when played. Did you ever think how?
Among all of the complex functions of the brain, the one that I find the most fascinating is the ability to store information that can be retrieve later on, or memory. This ability is surely the basis of most of the cognitive functions that give us the life quality that we usually take for granted. However very rarely do we question how this process occurs.
Learning and memory are often used side by side. Learning refers to acquiring of new information by the nervous system and the resulting changes in behavior. Memory refers to the encoding, storage and retrieval of the learned information. This means that when forming a memory, the information form the outside world as reaches our senses, where it is processed to be recorded as visual, elaborative, organizational, acoustic, semantic encodings. A lasting record of the encoded information is stored and maintained over long periods of time and is made available to recall in response to different kinds of cues. Even though this is a crude generalization, it is roughly how you are able to remember the lyrics of the song in response to a auditory cue, which is the song playing in your mind or in the club that you are in.
There are many different ways to categorize the different types of memories that we may have. Sensory memory stores sensory information such as visual, auditory, olfactory, for a very short period of time (few seconds). An example could be when you hear your doorbell and remember that you heard it few seconds after the auditory cue (e.g. to open the door to your guest). Working memory, sometimes used synonymously with short-term memory, actively holds multiple transitory information in mind where they can be manipulated and is thought to underlie some of the very important daily tasks such as thought processes. Long-term memory, on the other hand can store information potentially for a very long time, potentially or an unlimited duration.
Memory can be classified into two different groups depending on the type of information encoded: declarative and non-declarative. Declarative memory, or explicit memory, is all about the facts and actual events and require conscious recall. Your ability to know the words of a language (semantic memory) or remember what you did last week Sunday (episodic memory) are types of declarative memory. Non-declarative memory, implicit memory, depends on implicit learning and does not require conscious recall. Riding a bike, playing the piano or other similar actions that require motor learning are examples of non-declarative learning, where remembered memories are usually automatically transferred into actions.
However things don’t always go in the forward direction; just as we make new memories, we also loose some. So, another very intriguing concept is the ability to forget. Pathological forgetfulness, or amnesia, is the inability to learn new information or to retrieve acquired information. Comparisons with the normal memory function have made amnesia very instructive for scientists to understand the properties of memory.
One very interesting example is the case of a patient called H.M.. The case of H.M. is one of the most fascinating cases taught in every introductory Neuroscience class due to its importance for understanding some of the characteristics of memory. H.M. (1926-2008) had suffered from minor seizures since the age of seven and more serious seizures after turning sixteen. By the time that he was 27, his seizures had become so severe that he would go through generalized convulsions with tongue biting, incontinence, and loss of consciousness, which made him unable to work as an electrician and quit his job.
H.M.’s neurosurgeon had found that the seizures were localized to the right and left medial temporal lobe (MTL). So H.M. went through a bilateral medial temporal lobe resection, where the MTL of both the right and left hemisphere of H.M.’s brain was removed. The regions that were removed included the right and left hippocampi and amygdala, both of which are now known to be very important for memory, which is an information that was not available at the time.
Not surprisingly H.M. had serious memory deficits after the surgery, even though the surgery was successful in its goal of controlling his epilepsy. H.M. suffered from a severe anterograde amnesia, which means that even though his working memory and procedural memory were working fine, he could not store new information in his explicit memory. His IQ before and after the surgery was similar, he was able to recall earlier memories from his childhood and he was able to learn new skills (eg. puzzle solving), which is a process that requires implicit memory. However he would not recall having undergone a surgery, he would not be able to recall events in his daily life, and when asked the date he would say “March 1953.”
In one of his regular controls, H.M.’s neuropsychiatrist Brenda Milner made him take a mirror-drawing task. He would be instructed to draw a star by looking at it in a mirror for three successive days, which would be a new perceptual and motor task for him. Dr. Milner observed that H.M.’s performance improved over time. However he had no memory of the events that took place during those three days, he did not know that he would be repeating the tasks he had previously done and he would not be able to recognize Dr. Milner as someone that he had met before. The case of H.M. is still, to this day, one of the most important ones in the field because it lead to the understanding that there are actually different types of learning and memory, where each depend on distinct systems and circuits in the brain.
If you are interested to learn more here are some links you might want to check out:
– Charlie Rose interview with Brenda Milner (starting at 12:00): http://www.youtube.com/watch?v=GsLa3nJvXW4
– Mirror Drawing test by Brenda Milner: http://www.youtube.com/watch?v=8MS-cnmczws
Image source: Flickr