Lecture 17: Long-Term Memory Flashcards
Definition of memory
• Memory: processes involved in retaining, retrieving, and using information about stimuli, images, events, ideas, and skills after the original information is no longer present
• Active any time some past experience has an impact on how you think or behave now or in the future
• When we are talking about memory, we are talking about the information that we are able to stock. Information from our environment, from the auditory (perception, tactile, olfactory).
• So everything that comes to our senses, if we want to access them we need to take the information, process it and rehearse it so that the info can go into our short term memory and then be transferred into our long term memory.
The chain of memory consolidation
1) The sensory input goes into the sensory memory (a sensory memory lasts not even a fraction of a second).
2) all the information that is not attended to is lost
3) When we have the attentional resource to process the info then this can go into our short term memory (short term memory is 30 sec). Short term memory is the window into our conscious world. The ability to process the information and compare it to the information we have in our brain again and again (rehearsal) is what allows us to take a short-term memory into a long term memory.
4) We can use multiple processes to transfer the information and the information that is not transferred is lost.
5) Some of the information that goes into our long term memory will be lost.
When you retrieve information from your long term memory, it comes back to the short term memory because the short term memory is the window to our world. Everything we do every day is associated with the interaction between short term memory and long term memory.
Dementia
A loss of ability to retrieve memory and form memory. Patients with Dementia cannot really interact with people — they forget things , they forget who they are, forget the notion of time.
Memory and consciousness is crucial for individually and uniqueness. It is all of our learning, all of our experiences, it is an archive of information.
Long-term memory
• “Archive” of information about past events and knowledge learned
• Works closely with working memory
• Storage stretches from a few moments ago to as far back as one can remember
• More recent memories/events are more detailed
What are the different types of memory?
Implicit/non-declarative: unconscious memory
1) Procedural (skill) memory:
- Skill memory: memory for actions
- No memory of where or when learned (everything that we learn without knowing that we learned it)
- Perform procedures without being consciously aware of how to do them
- People who cannot form new LTMs can still learn new skills (e.g., H.M.)
2) Priming: previous experience changes response without conscious awareness
Explicit/declarative: conscious memory
Memory we usually test in research paradigms
Memory that we know and can retrieve and tell to other people.
1) Episodic: personal events/episodes
2) Semantic: facts, knowledge
Where in the brain do the memory processes happen??
Parahippocampal gyrus - right above the fusiform gyrus and the inferior temporal gyrus. The collateral sulcus separates the two of these.
If we remove the Rhinal sulcus, we can see the parahippocampal gyrus, the hippocampus, the amygdala.
When we look at the medial view of the sulcus what do we find?
Amygdala = subcortical structure
—> it covers the perirhinal cortex (yellow) and the entorhinal cortex (red)
Hippocampus
—> also covers the perirhinal cortex (yellow) and the entorhinal cortex (red)
It is important to know what regions cover other regions because when there are some lesions made if we remove one part we might be removing another part too.
Ex: If we remove the hippocampus, we might also be removing part of the entorhinal and perirhinal cortex.
Amygdala, entorhinal cortex, perirhinal cortex, parahippocampal cortex, hippocampus are all connected to the thalamus.. These are the subcortical structures, where almost all the information is sent to.
- they receive input from all the neocortex, so all the new cortex send their information to these structures. - these structures are part of the limbic system. The limbic system is involved in the emotional processing of information and we know that memory is associated with the emotional part of information.
The pyriform Cortex is associated with the olfactory system. Also, associated to the medial temporal lobe functions and projects to the amygdala, the entorhinal cortex and the hippocampus. This means that our sense of smell is associated with memory.
Temporal lobectomy
- The most typical procedure for those with temporal lobe epilepsy not controlled with medication is a temporal lobectomy where the hippocampus and amygdala is also be removed.
- Most of the time, it results in a considerable decrease in seizure frequency or total seizure control.
- However, if this treatment is carried out on the dominant hemisphere, memory and language may be impacted. It depends if it is the right or left lesion.
- Dr. Penfield of the MNI is responsible for this surgery earning the name “Montreal procedure.”
- The majority of operations entailed removing just one side of the temporal lobe (some exceptions as patient H.M.). HM’s condition and surgery allowed a lot of researchers to understand the function of the regions that are associated with the medial temporal lobe,
- Unilateral limbic MTL damage does NOT result in global amnesia but rather in material-specific memory impairment (when specific regions are lesioned and not the entire middle temporal lobe). Depending on the projections and on the specific structures that are lesioned, then we will have specific impairment. *amnesia is the inability to retrieve or form new memories.
How do we study patients with left lesions?
Left lesions and the digit memory span task
Remember left hemisphere is dominant in language.
Procedure:
* Recurring digit sequence test by Philip Corsi (master’s with Brenda Milner) - originally from
Hebb show that we can remember 7 things at most, plus or minus 2.
–> *Memory span is how much information you can process in your short term memeory. *
* Corsi examined patients who had limbic MTL injury on one side only.
* By testing each patient with various sequence lengths and having them repeat back the
numbers, the span of each patient was determined.
* After each patient’s span was determined, all patients were tested using span+1 sequences. We add one to test the sequence that goes into the long term memory because the span is information that can be maintained in the short term memory. If we add one we are involving learning and processing which is associated with forming a long term memory.
* This task took into account the degree of difficulty across patients with various spans.
* For the first time, we were able to quantify the deficit associated with long term memory based on the number of repetitions required to learn the sequence. Also quantified using spatial tasks (see slides on right lesions).
* Test subjects with various degrees of left MTL injury:
* Just temporal pole (most interior part)
* Temporal pole, amygdala, and very anterior hippocampus (LTh)
* Temporal pole, amygdala, and much more hippocampus (LTH)
Results from left lesions and the digitt memory span task
Results:
* The LTh patients performed similarly to the controls and temporal pole patients. They were able to learn (maybe they needed seven repititions as the controls)
* The LTH patients had severe cognitive impairment (not amnesia but specific material impairment). To learn the sequence, they needed
many more repetitions.
*specific material means that it depends on what task we are using. ie digits, objects, language, visual perception
ex: the left hemisphere is more language processing, so if we damage the medial temporal lobe, we will have specific impairmentts associated with forming new memory that is specific to digits or language (anything verbal).
How do we study patients with right lesions (task 1)?
Right lesions and the spatial tapping sequence task
Procedure:
* The spatial tapping sequence was developed by Corsi for patients with right MTL lesions.
* The examiner taps out a specific sequence of blocks, which the patient must then tap back to the examiner (patient has to retain the order of the tapping).
* Each patient’s span is determined and tested on span+1 sequences - to assure that we are testing long term memory (same principle as left lesion test).
Findings:
* RTh (more anterior lesions) patients performed similarly to controls.
* RTH (more poserior and more of the hippocampus) patients had severe impairments and needed many more repetitions to learn the sequence.
We are analyzing how the information is sent to the medial temporal lobe.
How do we study patients with right lesions?(Task 2)
Right lesions and the spatial memory task
Procedure:
• Dr. Mary-Lou Smith’s (PhD with Brenda Milner) spatial memory task
• Patients were required to point out and name the objects on a table as well as estimate the true cost (to make sure that they attended to the objects).
• After a brief delay, the objects had been removed from the table, and they needed to be replaced (where it initially was).
Findings:
• If damage was in the left MTL: patients were just as good as controls
• If damage was in the right MTL: the patients with the damage extending most posteriorly were impaired (RTH > RTh).
Patients that had posterior lesions were more impaired than patients with more interior lesions - in the right hemisphere.
What were the conclusions drawn from the studies done on right and left lesioned patients in regards to language and spatial?
Conclusion 1:
• The study of unilateral limbic MTL patients concluded that the hippocampus is essential for the consolidation of declarative short-term memories to long- term memory (i.e. RTh vs RTH, LTh vs LTH). —> bigger lesions to the hippocampus were associated with greater impairment.
• Conclusion supported by the fact that even bilateral lesions of the anterior temporal lobes do not result into amnesia (Scoville patients).
Conclusion 2:
• However, we now know that each component of the limbic medial temporal lobes helps with consolidation.
Studies of patients with bilateral damage to the MTL (HM)
• Although uncommon, Dr. Scoville, a neurosurgeon in the USA, had even operated on both sides of the front temporal lobe.
• He first encountered patient H.M. in 1953, who had severe epilepsy that appeared to be caused by both limbic MTLs (quite extensive).
• Therefore, Dr. Scoville made the decision to bilaterally remove the complete limbic MTL. The different cortices that were removed were the perirhinal cortex, the entorhinal cortex, hippocampus, pyriform cortex and the amygdala. Almost a perfect model of amnesia.
• After the operation, this patient was permanently unable to create new memories.
• Damage to the limbic MTL must be bilateral to elicit global amnesia.
• From that point on, we were aware that the limbic MTLs are engaged in memory consolidation.
• HM was tested so many times (mostly by Brenda Milner)
HM’s Case
Name: Henry Gustave Molaison
Diagnostic: Severe epilepsy in both MTL and uncontrollable seizures.
Cognition: Normal in terms of language, perception, motor control, and attention, IQ score above average. He had normal cognition because he was able to retrieve the information that was already consolidated.
Memory:
• Lost ability to acquire new memories (anterograde amnesia)
• Kept clear memories acquired about before surgery (with some retrograde amnesia). He preserved his past memories, but some were lost. That is why we say that he had SOME retrograde amnesia.
*global amnesia = any time of information cannot be stored in the long term memory: spatial info, language info… anything
• No issues with short-term memory unless he was distracted.
Research and medical field: Great contribution to the medical literature and was investigated for the following 50 years, notably by pioneer Brenda Milner and her student Suzan Corkin.
The mirror drawing task
Preserved memory abilities on the mirror drawing task:
She placed HM in front of a mirror and asked him to trace based on the image on the mirror (a star). He learned and improved at this task because it is a motor task. But he will not remember doing this task.
• Results of the mirror-drawing task: Despite the fact that H.M. did not recall ever having completed the task or event Dr. Milner, he significantly improved!
• Dissociation between declarative and procedural memories: Memory exists in numerous forms—this was yet another groundbreaking finding at the time!
• Conclusions: Extensive bilateral damage to the limbic medial temporal lobes results in the inability to consolidate new declarative memories while being able to consolidate new procedural memories
It can be associated with the system reward information. Before, he had this reward of learning (implicit behaviour) and then he loses this sense of reward and learning and the fact that he implicitly knows that he doesn’t have this reward maybe can explain why he was able to say things like that.
What was the visual discrimination task used
- Animal sees a red circle or a green triangle. He knows that if he goes to red circle, he has a reward: food. But if he selects the green triangle, he will get no reward.
- Multiple trials with different objects. Learns all of these.
- This is called a visual discrimination task because you associate one form to a reward. So you have too recognize this form (so you have to compare it to what you already know).
- He was testing procedural memory by involving reward feedback learning. The animal implicitly remembered that red means food or blue means food.This is associated with unconscious consolidation.
- The striatum and the basal ganglia are also associated with unconscious memory. There is a lot of projections between the cortex and the striatal pathway.
- The cortex, the globus pallid us, the thalamus all interact to form procedural memory. They do not require activation of the frontal lobe, consciousness and they will be associated with implicit behaviour.
One lesson that we can learn from this: be sure of what you are testing when you use a task. Always understand what your experiment really measures.
The delayed non-matching to sample task
Procedure of the DNMS task (Mishkin 1978)
He had a red rectangle, the monkey would see it and see that there was food underneath. Then there is a delay of more than 30 seconds to be sure that we are testing long term memory. Then, the monkey had to chose the object that was new and in order to do this he had to recognize which was the old object. Under the new object he would find the reward.
• Never sees the same pair of objects. Always a new pair of objects.
• No reward associations.
• Delay with lowered screen
• Rely on recognition (declarative memory)
Results:
1) Bilateral amygdalectomie (A+) = bilateral lesion to amygdala
• Monkeys could perform the DNMS task
≈ 50% of entorhinal/perirhinal cortex removed (because amygdala is overlay with the cortex).
2) Bilateral amygdalectomie and hippocampectomy (A+H+)
• Monkeys failed to perform the DNMS task
Entire entorhinal/perirhinal cortex removed
Confirmed the first and second hypothesis
3) Bilateral hippocampectomy (H+)
• Monkeys could perform DNMS task !!!
≈ 50% of entorhinal/perirhinal cortex removed
Proved our hypothesis wrong.
Consolidation of memory is possible with the entorhinal and perirhinal cortex which plays a big role in recognition and memory.
The role of the entorhinal and perirhinal cortex
• Role in recognition and memory!
• How is information processed and encoded?
• Information is processed by the cortex and projections are sent to the entorhinal/perirhinal cortex (the entire neocortex projects to these structures). We can call this the hippocampal formation.
• Information is processed by the amygdala (for affective component) and the hippocampus (for e.g., spatial, linguistic, spatial context). The hippocampus is associated with the context means that when we learn something we learn that thing with its context.
• Information is then stored in the neocortex.
The part where they all work together is the most important thing in consolidating memory.
