Lecture 1: Memory systems

Welcome back to the course Brain, Behaviour, and Dentistry. Now we begin the second session of the course, which will focus on the issues of memory of dental patients. After this class, you will learn the basic framework of the memory system and learn how to differentiate declarative and non-declarative memory and identify their roles in dental treatment.

Now we start to look into the different ‘forms’ of memory. We used to categorize memory into different forms based on a different time scale. For example, we keep some memory for a very short time and forget it forever. Some memory is kept for a longer time, even our whole life. From the point of cognitive neuroscience, there are also different memory systems for long-term memory. For example, some memory is about what we have done in our daily life. This is the episodic memory, like a diary that records the events in our life. Some memory is about the knowledge we gain, such as the professional knowledge of the dental instrument.

This is the semantic memory, which consists of our knowledge about the world. Furthermore, the skills that we learn are also part of our long-term memory. For example, we know how to perform a class preparation, and all the procedures are learned by heart.

While the long-term memories, such as our life history and professional knowledge, need to be accumulated by many years. Other ‘memory’ just happens transiently. Short-term memory is sometimes used for controlling our current work, and we do not tend to keep it forever. Broadly speaking, short-term memory may be kept for a couple of seconds to minutes, and without further learning, we may lose them gradually. Notably, our mental capacity to hold this short-term memory is limited. For example, when you are asked to keep a telephone number in mind, you will feel it difficult when the telephone number consisted of 13 digits! You need to effortfully keep these digits in mind.

And such a short-term memory will be easily disrupted if there’s something distracting you. In fact, researchers have found that the number of digits to be kept in a short time is about seven plus or minus two. If we try to keep a 20-digit password – we’d better write it down as soon as possible! And it’s very interesting that some memory is just ‘disposable’ – I mean we don’t really want to keep it forever. Some researchers call it ‘working memory’’ because it is not really the ‘memory’ that will be stored and recalled. Working memory is a mechanism related to attention and executive functions.

For example, in a restaurant, the server will take the order from each customer quickly and write it down. When they take the orders, they need to focus on what the customers said and learn it by heart. However, after writing down the order from the first customer, the information can be lost, and the server need to re-focus on the order from the next customer. Therefore, working memory is more like a scratchpad in our mind. It’s a buffer area where we can handle information just temporarily.

Now let’s turn to long-term memory. We mentioned that long-term memory is usually about our life experience and knowledge gained from the world. The memory can be further sub-categorized into declarative memory and non-declarative memory. Declarative memory, literally, is the memory that we can explicit tell people and write down. This includes episodic memory; it is about our daily experience, such as what I have done yesterday, what happened to me when I was a kid. The semantic memory is about the knowledge, maybe ‘common sense’ or professional knowledge we learned. These memories can be expressed verbally, i.e., declarative. In contrast, non-declarative memory is about the skills or habits that we acquired, which is more implicit, hard to be expressed verbally.

Procedural memory, i.e., we know how to drive, is one of the non-declarative memory. It should be noted that declarative and non-declarative memory is not totally irrelevant. For example, I know how to use a dental handpiece and I can finish a Class I cavity preparation smoothly. My proficiency in performing this skill is part of procedural memory. In contrast, to clearly recite the steps of Class I cavity preparation documented in a textbook, That would be part of the declarative memory. Critically, the different memory forms are mandated by different brain networks. Now we know the cortex and the hippocampus is critical to declarative memory and the basal ganglia are critical to non-declarative memory.

Therefore, a lesion in the hippocampus may lead to deficits in one’s episodic memory, but perhaps no effect on procedural memory. Likewise, a lesion in the basal ganglia may influence our skills, but not the integrity of episodic memory. Here I would like to explain further the concept of working memory. Just keep in mind that the working memory is not really the memory we want forever. In contrast, it is just a buffer for us to keep the current task in our mind. When the task is completed, the buffer will be reset to take a new task. The performance of working memory can be assessed using the 2-back memory test.

Here a sequence of letters will be displayed, and subjects are asked to respond to the letter that is shown two steps before. For example, in this sequence of displays, you need to respond to the ‘K’ letter in the fourth display because the same ‘K’ is shown two steps before. You can do this because the letters in the previous displays, such as A, K, D, are still kept in your mind. However, as the displays move on, now when you see the letter ‘O’, you need to change the task from ‘identifying K’ to identifying O’. In fact, you should discard all your focus on K. Because now it is ‘O’ that occupies your buffer.