Scientists have no coherent story to tell of how a brain could store a memory or instantly retrieve a memory or maintain a memory in a brain that replaces its proteins at a rate of about 3% per day (an effect that should prevent you from remembering anything for more than two months, if your brain stored your memories). The phrases they mutter about such things make no sense. Asked to explain a mechanism of neural memory storage, a neuroscientist may mutter some phrase such as "synapse strengthening," which makes no sense as an explanation of memory formation. Strengthening is not information storage. And the idea that you stored memories in synapses because they were strengthened makes no more sense than the idea that you store memories in your arm biceps when they strengthen. There is no robust evidence that synapses strengthen more when you learn something.
Having no real evidence on a cellular level to support their claims of neural memory storage, neuroscientists sometimes resort to claims about parts of the brain, claiming that some-such part of the brain is needed for memory. Such claims usually involve claims that the hippocampus is crucial for memory. The experimental evidence has never supported such claims. People with no hippocampus or a damaged hippocampus usually perform fairly well on tests of memory.
In my long post here (entitled "Studies Debunk Hippocampus Memory Myths") -- a post with thousands of views so far -- I discuss very many scientific papers that discredit the claim that the hippocampus has some big relation to memory. That post is very thorough in reviewing the relevant literature, but I recently noticed that is has one omission: it fails to discuss what relation (if any) there is between the volume of the hippocampus and a person's tendency to have the severe cognitive disorder known as Alzheimer's disease. Let us look at that very issue, using a 2023 paper entitled "MRI measurements of brain hippocampus volume in relation to mild cognitive impairment and Alzheimer disease A systematic review and meta-analysis" which you can read here.
The paper does a meta-analysis of different studies comparing hippocampus volume, Alzheimer's disease and mild cognitive impairment or MCI. The paper comes to different conclusions, based on the different methods used.
I can explain the different methods:
(1) One method involves a simple method that is only checking the "raw volume" of the hippocampus. This is the easiest and simplest method.
(2) Another more sophisticated method factors in the "total intercranial volume" of subjects in addition to the hippocampus volume. This is called the "hippocampus volume measured by MRI TIV Correction" method. An AI Overview states, "Total Intracranial Volume (TIV) correction in MRI is a critical preprocessing step in neuroimaging that normalizes brain structural volumes (such as gray matter, white matter, or hippocampus) to account for variations in individual head size."
(3) Another method is called the "hippocampus measured by MRI ICA Correction" method. An AI Overview states, "Independent Component Analysis (ICA) in MRI is a data-driven, blind source separation technique used to isolate and remove noise sources (artifacts) from imaging data to enhance signal quality. It is highly effective at separating artifacts like motion, respiration, and large vessel signals from functional (fMRI) or dynamic susceptibility contrast (DSC-MRI) data, improving diagnostic accuracy." People are supposed to remain totally motionless when they undergoing an MRI scan, but they often fail to be totally motionless; and this can affect the quality of the MRI. This "MRI ICA Correction" helps fix that data quality problem.
Here are the results for the left hippocampus when these three methods were used.
Method 1: "The left hippocampus volume measured by MRI Raw volume was negatively correlated with MCI and AD (OR = 0.58, 95%CI: 0.42, 0.75)." So when the simplest and least sophisticated method was used, a correlation was reported between hippocampus volume and mild cognitive impairment (MCI) and Alzheimer's Disease (AD).
Method 2: "Results of meta-analysis showed no correlation between left hippocampus volume measured by MRI TIV Correction and MCI and AD (OR = 0.90, 95%CI: 0.62, 1.19), as shown in Figure 4." So when the first of the two more sophisticated methods was used, there was no correlation found between left hippocampus volume and mild cognitive impairment (MCI); and there was no correlation found between left hippocampus volume and Alzheimer's Disease (AD).
Method 3: "Results of meta-analysis showed that the volume of the left hippocampus measured by MRI ICA Correction was not correlated with MCI and AD (OR = 0.92, 95th CI: 0.75, 1.09), as shown in Figure 5." So when the second of the two more sophisticated methods was used, there was no correlation found between left hippocampus volume and mild cognitive impairment (MCI); and there was no correlation found between left hippocampus volume and Alzheimer's Disease (AD).
The bottom line here is that two out of the three methods did not find any correlation between left hippocampus volume and mild cognitive impairment (MCI), and did not find any correlation between left hippocampus volume and Alzheimer's Disease.
Here are the results for the right hippocampus when these three methods were used.
Method 1: "Results of meta-analysis showed that the right hippocampus volume measured by MRI Raw volume method was not correlated with MCI and AD (OR = 0.87, 95%CI: 0.56, 1.18), as shown in Figure 7." So when the simplest and least sophisticated method was used, no correlation was reported between right hippocampus volume and mild cognitive impairment (MCI); and there was no correlation found between right hippocampus volume and Alzheimer's Disease (AD).
Method 2: "Results of meta-analysis showed no correlation between the right hippocampus volume measured by MRI TIV Correction and MCI and AD (OR = 0.81, 95%CI: 0.49, 1.12), as shown in Figure 8." So when the first of the two more sophisticated methods was used, there was no correlation found between right hippocampus volume and mild cognitive impairment (MCI); and there was no correlation found between right hippocampus volume and Alzheimer's Disease (AD).
Method 3: "Results of meta-analysis showed that the volume of the right hippocampus measured by MRI ICA Correction was negatively correlated with MCI and AD (OR = 0.49, 95%CI: 0.35, 0.62), as shown in Figure 9." So when the second of the two more sophisticated methods was used, there was a correlation found between right hippocampus volume and mild cognitive impairment (MCI) and Alzheimer's Disease (AD).
The bottom line here is that two out of the three methods did not find any correlation between right hippocampus volume and mild cognitive impairment (MCI), and did not find any correlation between right hippocampus volume and Alzheimer's Disease.
Overall, the results here are quite consistent with my claims that memory is not a brain function, and that memories are not stored in brains.
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