Semantic Knowledge in Patient HM

Semantic Memory in Psychology

As you probably know by now, memory is the cognitive process through which information is encoded, stored and retrieved. Memory is key to our survival and supports humans in everyday activities.

Semantic memory falls under long-term memory, which refers to the memory type that is semi-permanent and which can be retrieved after hours, days and even years after it has been encoded.

Fig. 1. The board game Trivial Pursuit assesses players' semantic memory.

Semantic Memory Example

All humans use semantic memory every single day. In school, for example, you are using semantic memory for all your classes. After studying this material, you would employ semantic memory when your teacher asks you to define memory, for example.

Semantic memory is knowing that the capital city of France is Paris or recalling the water cycle and its phases.

Remember, semantic memory is the knowledge of words and their meaning.

Semantic Knowledge in Patient HM and other Patients with Bilateral Medial and Lateral Temporal Lobe Lesions

Patient HM suffered from severe epileptic seizures. To relieve them, he underwent surgery in 1953 to remove both (bilateral) medial temporal lobes, including the hippocampus. Back then, it was unknown that the hippocampus was needed for memory formation.

After the surgery, HM's seizures decreased significantly, but his long-term memory was no longer present. The surgery evoked retrograde amnesia in HM.

HM's assessment revealed that:

• Specific brain regions are responsible for memory and learning processing.

  • The hippocampus is an important part of the brain responsible for memory.

• Different brain regions are responsible for the retrieval of memories and the formation of new memories.

• Different parts of the brain are responsible for short-term, long-term, and other types of memory, such as semantic memory.

The effect that damage to the medial and lateral temporal lobe has on semantic memories is slightly more complex.

Bayley, Hopkins and Squire (2006) performed a study comparing two groups of patients. One included patients with damage to the hippocampus and the other with damage to the medial temporal lobe. The results suggested that the bigger the lesion, the retrograde amnesia was stronger.

Patients with damage to the hippocampus presented amnesia of recently learned memories. Patients with more extensive lesions, including the hippocampus and the medial temporal lobe, presented amnesia for older memories.

Taken together, the evidence from HM and other patients reveals that:

• The hippocampus is crucial for the formation of new memories and the recollection of old ones.

• The temporal lobe specifically is crucial for the recollection of old memories.

Schmolck et al., (2002) Study

Schmolck et al. (2002) investigated which area of the brain is responsible for long-term semantic memory. The study aimed to examine the relationship between damage to the lateral temporal cortex and performance on semantic tests. Researchers also wanted to find out if there were any peculiarities in the test performance of HM compared to other patients.

The variables investigated were as follows:

• Independent variable: location of lesions (brain regions damaged usually due to injury or disease) found in the brain.

• Dependent variables: scores on 13 tests of semantic memory.

The sample consisted of:

• Six patients participated in the study: two patients with hippocampal damage, three with large lesions of the medial temporal lobe and different lesions of the lateral temporal cortex (MTL+), and the patient HM, who had mainly medial temporal lobe damage. The researchers used brain scans (MRI and CT) to determine which areas of the brain were damaged.

• Eight controls.

Participants were presented with nine tests. Seven tests were from the Semantic Test Battery (Hodges et al., 1992a), and the researchers developed the remaining two. The tests were based on line drawings of 24 animals and 24 objects and their names.

Schmolck et al. (2002) found that:

• The hippocampus patients performed similarly to the control group.
• The three MTL+ patients were mild to moderately impaired on almost all tests.
• HM performed better than the MTL+ group, but not other patients damaged only in the hippocampus.
• There was a correlation between test performance and the extent of damage to the lateral temporal cortex.

Schmolck et al., (2002) Study: Evaluation

The strengths of the research are:

• High reliability.

  • The researchers used 14 raters to score the transcribed data.

  • The study's procedure was standardised and, therefore, easy to replicate so other researchers could determine its reliability.

• Application: semantic memory in psychology and theories of semantic theory has shown to have high utility.

  • The research can show how the damage of specific brain regions leads to the impairment of semantic knowledge.

• The study used a matched-pairs design, so it is more confident that the differences in performance are due to the brain lesions and not to other variables such as age.

The weaknesses of the research are:

• Generalisability – the research used a small sample, so it isn't easy to generalise the results. Therefore, we can argue that the results lack external validity.

• Potential ethical issues – since the patients have brain damage, there is a question of whether they can give proper informed consent.

• Low ecological validity – as researchers did the study in a lab setting, it could have influenced the participant's behaviour, causing validity issues.

Theories of Semantic Memory

There are two main theories concerning semantic memory.

Semantic memory has been explained in terms of its hierarchical structure by Laatu and colleagues (1997). These researchers suggested that semantic memories are stored in a hierarchical manner in which there are three categories:

1. The superordinate category represents the highest level of information, for example, "food".
2. Subcategories are the mid-level category in which information is stored. According to the example, food subcategories include vegetables, milk products, fruits, meat, etc.
3. Specific memory is the last category which refers to the literal memory that is stored. In line with the example, the memory could be lettuce.

Differently, Tulving (2003) proposed a theory according to which three types of memories differ in their level of consciousness. Procedural memory, the memory used when riding a bike, for example, takes place in humans without us noticing consciously. Differently, episodic and semantic memories involve the conscious recollection of memories.