Lecture 2: Research methods to study memory

Before we get into some clinical issues, I need to explain further the research methods for studying memory in modern days. There are three mainstream approaches to memory research. The first approach is to collect clinical cases which show memory deficits. For example, neurologists may observe the memory loss of patients with stroke or brain injury, and try to find the link between memory and a specific brain region, where the lesion is located. Such a neuropsychological approach has been adopted for more than 100 years and is still an important way to discover the potential brain mechanisms underlying memory processing.

To further clarify the cause-effect relationship between the brain and memory, we need the second approach, i.e., to directly manipulate the brain from an animal model, and observe what happens to animal behaviour. The animal behaviour can be quantified by some standardized tests, such as the Morris water maze, a test for the ability of spatial navigation.

When the hippocampus of animal subjects is disrupted, they show worse performance in spatial navigation. Therefore, such evidence strengthens the role of the hippocampus in spatial memory. The third approach is neuroimaging, a non-invasive method suitable for human subjects. We will see some cases of neuroimaging research, primarily based on magnetic resonance imaging, MRI, in our later sessions. Here I would put more emphasis on the neuropsychological approach. I just said that neurologists try to figure out the link between memory deficits and brain lesions from patients with brain injury. Here the timing of the events would play a critical role in interpreting this link. We just mentioned that the hippocampus is critical to memory consolidation.

Therefore, if brain injury occurs in the hippocampus, the processing of memory consolidation will be disturbed. Patients may fail to learn what happens shortly after injury because the consolidation is disturbed.

Such a memory deficit after an injury is anterograde amnesia, which reflects an inability to create a new memory. Patients may fail to remember what happened shortly before injury because the experience prior to the injury, though being encoded, has not been fully consolidated. Such a memory deficit before the injury is retrograde amnesia, which reflects the inability to get access to the information learned prior to the injury.

It should be clarified that when the process of memory consolidation is disturbed, patients would show a severe disability in learning new things. They can still encode new information but the information cannot be kept long enough. This is what happened to the patient H.M., a famous case in medical history. The hippocampus of H.M. is lesioned and memory consolidation is severely compromised. Therefore, he cannot keep an experience that happened just a few hours ago. Therefore, his life experience cannot be updated.

There are more cases of memory deficits in which amnesia or memory loss is not that significant. For example, patients with confabulation may talk quite a lot about some stories. They seem to be quite normal for remembering things. However, most of these ‘stories’ are come up by fabrication. In other words, these patients tend to fabricate something they have experienced, which actually never happens. Confabulation can be seen in patients with dementia or Korsakoff syndrome, which relates to an overuse of alcohol. It should be noted that confabulation is associated with a distorted information processing of memory. It is different from telling a lie, which is known as ‘not real’ by the liars themselves.

Confabulation is also different from delusion, which is associated with not just some stories but a deeper personal belief or value. Okay, now every time I told the story of H.M. to students during a class, they feel a little bit panic. They wonder if they’ve got the same memory deficits. Because I told them my name at the beginning of the class, and at the end of the class – they just forgot my name. And the next day in the corridor, they look at me saying ‘ have we met’? Of course, this is not really something to do with amnesia. Part of their ‘poor memory’ is that they did not pay attention to my self-introduction at the beginning of the course.

Secondly, they are not motivated to learn my name because I am just a nobody. Finally, for foreign students, my Chinese name is quite strange for them to learn. What I want to say is that when we study human memory, we are studying all the brain functions of learning. For example, the functions of attention and motivation are not isolated from memory. All these functions are critical to our ability to accommodate the world.