The human brain, composed of over 100 billion cells, is a natural work of art. Groups of brain cells, called neurons, and their synapses—the gaps in between them—are the functional units of the brain that allow us to store memories. While these cells are responsible for what we remember, what enables us to recall our sixth birthday party, but forget what the professor talked about 15 minutes ago in class? Welcome to the mysterious world of the human memory.

The difference between remembering that birthday party over the lecture material revolves around the differences in how we organize and process short and long-term memories. Psychologists view the memory system as analogous to a computer in terms of consisting of three phases: encoding, the process of transforming information into a neural code that the brain can understand; storage, the retention of that code; and retrieval, the act of recalling that information.

The modal memory model proposed by Richard Atkinson and Richard Shriffin in 1968 gives a deeper understanding of how memory works, by breaking it down into three types of memory. Sensory memories are the most brief form of memory, lasting for a fraction of a second. An example of sensory memory is when you look at something and quickly glance away. Due to your sensory memories, you can recall only some of the object’s details. It allows us to take in the world as a continuous stream, rather than discrete chunks of information.

By contrast, short-term memory, also known as working memory, holds data in awareness for a brief period of time, during which our brain can use it to carry out several necessary processes. The retention period for short-term memory is around 20 to 30 seconds. For instance, when looking up a telephone number and walking across the room to dial it, your working memory is at work. Unless you repeat this task over and over again, some digits begin to fade by the time the actual number is dialed.

Another aspect of short-term memory is memory span, which refers to the number of distinct items that can be stored in the working memory at a time. Initially, this number was suggested to be seven (plus or minus two) by George Miller. However, a more recent study conducted in 2005 claims that the span may be as little as four items. Next time you try to remember a phone number for a friend, don’t feel bad when you forget a digit—your short-term memory really only retains four at a time.

On the other hand, long-term memory, the more permanent component of the memory system, is a much less transitory system. Essentially, long-term memory logs new information by associating it with other related topics already stored in your memory.

This is why it is harder to learn something completely new, as opposed to learning about something you’re already familiar with. It’s also why you shouldn’t wait to take Calculus 3 long after you have completed Calculus 2.

Furthermore, the precision with which your neurons store information depends on the strength of the neural associations formed in your brain. It is much easier to remember facts related to information already stored in your memory than to learn a wholly novel concept.

But not all content of the long-term memory is reliable. McGill professor Karim Nader proposed the process of reconsolidation, which explains why some of our long-term memories are distorted. According to Nader, once memories are activated or retrieved, they must be consolidated to be stored in the brain as memory. An improper restoration could ultimately lead to distortions in your memory. This concept could be used to treat trauma patients by helping them retrieve painful memories, and then disrupting the reconsolidation process to alleviate the pain associated with those recollections.