Speaking at the Big Ideas in Neuroscience session at the recent CNS annual meeting, Angela Friederici of the Max Planck Institute discussed language as a unique human trait. Understanding of the words we use comes from different types of memories in different networks the brain. Neuroscientists often gain insight into these connections through individuals with neurological conditions that impair different types of recall. With data from two patients with different conditions, a new study is calling into question the idea that memory of words can develop independently of memory of events.
Past work has suggested that individuals with early damage to their hippocampus can still perform well on standard tests of “semantic memory” – memory of words – despite a significant impairment in their “episodic memory” – memory of events. “We wanted to see whether the hippocampus was important for some types of semantic knowledge, in addition to its more classical role in episodic memory,” says Anna Blumenthal of the University of Western Ontario, lead author on the new study in Neuropsychologia.
Her team turned to two patients, one with hippocampal damage and the other without: HC and NB. They specifically wanted to look at the conceptual structure surrounding word knowledge – how people come to understand so-called extrinsic features (such as how something is used or where it’s located) compared to intrinsic features of objects (such as color, shape, etc.). For example, “lives in the ocean” would be an extrinsic feature for the word “squid,” while intrinsic features might include “slimy” or “has tentacles.”
The team tested HC and NB, as well as control participants, on semantic memory tasks related to these features, such as listing as many features as they could for a word like “squid”. They also asked participants to judge how typical a certain concept is of its category. For example, participants would have to rate on a scale of 1-9 (9 being the most typical) how typical a penguin is for the category “bird.”
“This is an interesting task, because while there are no correct answers, most individuals will rate things similarly; for example, on average, most people agree a robin is a much more typical type of bird than is a penguin,” Blumenthal explains. “Deviations from the norm show us that the structure of semantic knowledge is organized differently.”
Looking at the differences between HC, NB, and the controls, the research team found that episodic and semantic memory are not entirely independent. The hippocampus plays an important role in learning some aspects of conceptual knowledge, they wrote. CNS spoke with Blumenthal about these results, the patients HC and NB, and next steps for this line of work.
CNS: How did you become personally interested in this research area?
Blumenthal: When I was an undergraduate, I loved my biological psychology course, and I started hanging around the lab helping out with some olfactory learning and memory experiments with rodents. This experience led to a summer internship exploring the role of the hippocampus in spatial learning and memory. Hearing place cells firing away while rats explored their environment got me hooked on the hippocampus! I imagined doing rodent electrophysiology, but then I developed a severe rat allergy and so decided to explore those other organisms with interesting memory capacities – humans.
During my master’s program, I worked in a language lab, which I loved, but questions about episodic memory and the hippocampus are what truly got me most excited at the end of the day, so I came back to tackle this area for a Ph.D. The current project brings together my interest in language, particularly conceptual knowledge and semantics, with episodic memory.
Additionally, I have always been fascinated by the history of neuropsychology, and I have great respect for the amount we can learn from these cases. As most of my work is fMRI based, actually being part of a neuropsychological study for the first time was a really exciting experience for me.
CNS: How did you come to work with patients HC, and NB and what can you tell us about them?
Blumenthal: Both HC and NB have been avid research participants and have contributed much to our current knowledge of memory and medial temporal lobe function. We are lucky and thankful for their enthusiastic participation.
HC came to be a part of this research study through her participation in a large research program run by our collaborator Shayna Rosenbaum. HC was 22 at the time of our testing, and at this time had finished one year of technical school and one year of vocational school (culinary). Despite her severe memory impairment, HC has normal intelligence and does well on neuropsychological tests that tap into
NB has participated in a number of studies in our lab, she was 26 at the time of testing. Five years prior to testing, NB had surgery to remove a mass in her left amygdala that was causing seizures that could not be treated with medicine. This surgery required removal of part of the surrounding tissue in her left medial temporal lobe, including the perirhinal cortex but excluding the hippocampus. Our lab is interested in the perirhinal cortex because it is involved in processing conceptual knowledge, and in recognizing whether objects in the world are familiar. Conducting experiments with NB allowed us to test the causal involvement of this area in these processes.
CNS: What was most challenging and rewarding working with the patients?
Blumenthal: Unfortunately, I cannot speak to that, as I was not the person who collected the data.
CNS: What is developmental amnesia, like what HC has?
Blumenthal: Developmental amnesia is a disorder that occurs when there is damage to the extended hippocampal system early in life. The primary symptom is amnesia – or a lack of episodic memory. Episodic memory refers to the ability to relive or re-experience events from the past – for example, a lunch you went to yesterday, your 16th birthday party, etc.
CNS: What are examples of semantic memory/knowledge previous work has seen retained in developmental
Blumenthal: Much of the previous research uses traditional neuropsychological tests or IQ tests. These show that certain aspects of language – such as naming objects or coming up with as many words as you can that refer to an animal (called semantic fluency) – are normal. They found the same for learning and reporting facts about the world, such as knowing that Paris is the capital of France, or that the Eiffel Tower is a building in Paris.
Our study suggests the development of concepts – the basic buildings blocks we use to understand our world and communicate with others, which were previously thought to be largely independent episodic memory – may actually be more complexly intertwined.
CNS: Why is the distinction between extrinsic and intrinsic features in your study important?
Blumenthal: We believe this distinction is important because learning extrinsic features may require the hippocampus, whereas learning intrinsic features may not. Specifically, learning an extrinsic feature, like the fact that a squid lives in the ocean, requires linking the feature (lives in the ocean) with the object (squid), and we believe that making this link from an experience or set of experiences (seeing a squid washed up on the beach on during a vacation at the Jersey shore, for example) depends upon the hippocampus.
CNS: What were you most excited to find?
Blumenthal: It was really interesting and exciting to see that HC’s semantic knowledge differed from those of controls in terms of extrinsic feature knowledge specifically. The idea that some aspects of semantic knowledge are abnormal in developmental amnesia, and that the hippocampus may be necessary for some aspects of semantic knowledge, is a really new idea, and goes a bit against the grain.
CNS: What do you most want people to understand about this work?
Blumenthal: I think the key big picture take-away from this project is that the hippocampus may be contributing to many aspects of cognition and behavior, outside of episodic memory. Evidence for this in the field is generally increasing, but there is still much work to be done to understand the core functions of the hippocampus and how those functions contribute to broader domains of cognition, like episodic memory, language, and navigation, to name a few. Our study suggests the development of concepts – the basic buildings blocks we use to understand our world and communicate with others, which were previously thought to be largely independent episodic memory – may actually be more complexly intertwined.
CNS: What’s next for this line of work?
Blumenthal: There is still so much to be done! Like most studies, I think this one opens more questions than it answers. I would say the first step would be to replicate these findings in other individuals with developmental amnesia. A further step would be to test individuals with amnesia that was acquired as an adult on the same experiments. Comparing adult amnesiacs will help us understand whether the hippocampus is crucial for developing this concept knowledge, for using it, or for both.
-Lisa M.P. Munoz