Physicalism is the doctrine that all that exists are physical things: mass, energy, fields, and so forth. The term means basically the same thing as materialism. If you are a physicalist, you think that all human mental experiences are purely the result of the brain.
To have some type of intellectual respectability, a physicalist must have some type of semi-specific claim about where human memories are stored in the brain. It won't do for the physicalist to be totally vague about the topic. For example, physicalism would have very little respectability if physicalists talked like this:
Your memories must be stored in your brain, although I haven't a clue as to where in the brain they are stored. Maybe they are in the middle of your neurons. Or maybe your memories are in the axons that connect nerve cells. Or maybe your memories are hiding out in your DNA. Or maybe your memories are in that watery liquid splashing around in the brain. Or maybe memories are stored in synapses. Or maybe memories are stored in the network of holes in the brain, kind of like data written in the holes of punch cards. Sheesh, I have no idea, but I know your memories must be stored SOMEWHERE in your head.
Of course, talk like that sounds quite scatterbrained, so a physicalist must be a little more specific. To have any credibility, he must have an answer to the question: where in the brain are memories stored? For a few decades, a stock answer has been given by neuroscientists: they have generally claimed memories are stored in synapses. Synapses are junctions between nerve cells in the brain.
But there is a great problem with this stock answer. Human memories are known to last for more than 50 years, but synapses are not a stable platform for the storage of information. The standard tale told by neuroscientists has been that memories are stored as changes in the strengths of synapses, what are called synaptic weights. But the things that make up such synaptic weights are protein molecules that have a very short lifetime. The protein molecules in synapses have a lifetime of no longer than a few weeks (this paper finds that they turn over at a rate of about 17% per day).
Besides this very rapid molecular turnover, there are other types of structural turnover making synapses an unstable platform for long-term memory storage. What are called dendritic spines in synapses last no more than about a month in the hippocampus, and less than two years in the cortex. This study found that dendritic spines in the hippocampus last for only about 30 days. This study found that dendritic spines in the cortex of mice brains have a half-life of only 120 days. The research of Stettler suggests that synapses themselves have a half-life of only about 3 months.
In a previous post published in August, 2016, I discussed such issues, and I argued that very long-term memories (such as 50-year old memories) cannot be stored in synapses as has been so often maintained. In November 2016 there appeared a scientific paper that substantiates a good deal of my reasoning.
The paper by Patrick C. Trettenbrein is entitled “The Demise of the Synapse As the Locus of Memory: A Looming Paradigm Shift?” Below are some excerpts:
When we are looking for a mechanism that implements a read/write memory in the nervous system, looking at synaptic strength and connectivity patterns might be misleading for many reasons...Tentative evidence for the (classical) cognitive scientists' reservations toward the synapse as the locus of memory in the brain has accumulated....Changes in synaptic strength are not directly related to storage of new information in memory....The rate of synaptic turnover in absence of learning is actually so high that the newly formed connections (which supposedly encode the new memory) will have vanished in due time. It is worth noticing that these findings actually are to be expected when considering that synapses are made of proteins which are generally known to have a short lifetime...Synapses have been found to be constantly turning over in all parts of cortex that have been examined using two-photon microscopy so far...The synapse is probably an ill fit when looking for a basic memory mechanism in the nervous system.
So if memories are not stored in the synapses of your brains, where are they stored? Trettenbrein offers no answer. How can he, when there are no other places in the brain suitable for storing memories that could last for 50 years? I explained in my August 2016 post why it is that other possibilities for memory storage such as DNA in neurons are not suitable explanations. We are familiar with cellular processes that read DNA during what is known as transcription, and we know that information from DNA is read many times too slow to account for the basically instantaneous speed with which humans retrieve memory.
Imagine a Broadway producer who discovers that his star cannot perform, but who has no understudy who can fill in. Such a producer is like a neuroscientist realizing that synapses aren't up to the job of explaining very long-term memories – there is no alternate part of the brain that can be called up to fill in the theoretical hole, like some understudy filling in the gap.
Trettenbrein gives this astonishingly frank and quite correct assessment: “To sum up, it can be said that when it comes to answering the question of how information is carried forward in time in the brain we remain largely clueless.” Such intellectual candor in a scientist is a refreshing change of pace.
We get some similar candor in a new book Why Only Us? Language and Evolution by the leading linguist Noam Chomsky and Professor Robert C. Berwick. Here is an excerpt (pages 50-51):
The very first thing that any computer scientist would want to know about a computer is how it writes to memory and reads from memory....Yet we do not really know how this most foundational element of computation is implemented in the brain.
The apparent impossibility of explaining how brains could store memories for 50 years is perhaps exceeded by the apparent impossibility of explaining how humans could instantly retrieve such memories. Neuroscientists don't even have any decent theory for this, and mainly just kind of shrug their shoulders when this puzzle is mentioned.
We don't at all understand how it is that brains could create consciousness or understanding, because none of the computers we have built has the slightest bit of real consciousness or real understanding. But we do know something about the requirements for an instantaneous retrieval of information that is stored for decades, because we have built computers capable of such feats. Among the requirements for a fast computerized retrieval of information stored for decades are things like this:
- the existence of a stable place for information storage, so that stored information can last many decades, with a fast read-write capability into and out of that storage place
- the existence and use of encoding formats (for example, ASCII, .jpeg, and binary) allowing information to be written using storage protocols necessary for the physical storage of different types of symbolic information
- the creation of indexing entries allowing stored information to be very rapidly retrieved
- the existence of a positional or coordinate system allowing the system to know the very precise physical location of where some particular piece of information is stored, so that one exact piece of information (and only that piece) can be very rapidly retrieved
- the existence and use of decoding formats that are the inverse of encoding formats, allowing encoded information to be decoded
- the use of some mathematical algorithm comparable to hashing, allowing a search string or cue to be translated into an index position allowing the index entry to be very rapidly found, allowing very rapid retrieval of the stored information
A neuroscientist cannot describe in detail any brain functionality that is like any single one of these things, but somehow old men are able to instantly recall 50-year old memories. How then can we not conclude that this must involve something much more than the brain?
The synapse doctrine of memory storage is a pillar of physicalism, but that pillar is crumbling, as more and more evidence comes in that synapses are an unstable “shifting-sands” type of platform completely unsuitable to explain human memories that last for 50 years. But what happens when our scientists eventually realize that the “jig is up,” and that they can no longer get away with telling this never-proven story that memories are stored in synapses? Will they then abandon physicalism, and realize that the human mind must be at least partially the result of something more than just the brain, perhaps some soul or some mysterious consciousness reality that transcends the human body?
Don't hold your breath. I think it will be rather like this. Let us imagine a little boy who has somehow got the idea that his parents are going to give him a pony on Christmas morning. Suppose the boy woke up early on Christmas morning, and he's searched all around the house and the yard, but there's no sign of a pony anywhere: not waiting in any of the rooms, not hiding under any of the beds, not in any of the closets, and not in the garage. Rather than realistically saying, “I must change my assumptions, there's no pony around here,” the boy will probably say, “There must be a pony around here somewhere.”
Similarly, even though there seems to be absolutely no specific place in the human brain that could be storing memories that last for 50 years, our neuroscientists will probably just keep jumping around from one untenable “brain storage of memories” theory to another, telling themselves, “Those memories must be stored somewhere in your head.” They could make a leap of imagination to postulate spiritual possibilities beyond the brain – a soul or some wider consciousness infrastructure beyond the brain. But although quite plausible, and also strongly suggested by various psychic phenomena, such possibilities are taboo in a regimented academic culture that currently acts like a straightjacket on many a mind.