Some of the terms most often used by biologists are misleading terms. Perhaps the biggest example is the term "natural selection." Selection is a term meaning a choice by a conscious agent. The so-called "natural selection" imagined by those who use such a term does not actually involve any selection or choice. The "natural selection" imagined by biologists merely involves a survival-of-the-fittest effect, in which fitter organisms survive longer or reproduce more. The duplicity of using the term "natural selection" for some imagined effect that is not actually selection is a word trick that was started by Charles Darwin, who coined the term "natural selection."
Then there is the term "body plan." To the average person this sounds like a plan for building the body of an organism. But biologists routinely use the term "body plan" to mean something much, much less: merely the features common to all the organisms that make up a phylum. According to such a definition, all species in the Chordata phylum (including humans, bears, dogs and fish) have the same body plan, which consists of little more than a backbone and a tendency towards bilateral symmetry (having the same features on the left and right side). With such a definition of "body plan," biologists can make very misleading statements that fool us into thinking they know far more than they do. Biologists may say that they know how humans got their body plan, and by "body plan" mean little more than a backbone-based body structure. 90% of the people hearing such a boast about a body plan will misunderstand, and think that such biologists are claiming that they know how the incredibly organized human structure arises from a vastly less organized speck-sized egg (something biologists do not actually know, largely because DNA does not specify anatomy).
Then there is the term "long-term potentiation." What is misleadingly called “long-term potentiation” or LTP is a not-very-long-lasting effect by which certain types of high-frequency stimulation (such as stimulation by electrodes) produces an increase in synaptic strength. The problem is that so-called long-term potentiation is actually a very short-term phenomenon. A 2013 paper states that so-called long-term potentiation is really very short-lived:
"LTP always decays and usually does so rapidly. Its rate of decay is measured in hours or days (for review, see Abraham 2003). Even with extended 'training,' a decay to baseline levels is observed within days to a week."
So-called long-term potentiation is no more long-term than a suntan. The use of the term "long-term potentiation" for such an effect is deceptive, particularly when it is suggested that so-called "long-term potentiation" might have something to do with explaining memories that can last for 50 years or longer.
Another very misleading term used by biologists is the term "synaptic plasticity." To explain why the term is misleading, let me look at what has been observed regarding synapses and dendritic spines: something that is merely instability and high variability.
As a general rule, individual synapses are too small to be well-observed in large numbers by scientific equipment. By using equipment such as electron microscopes, scientists can zoom in on one or a few synapses. But with so many billions of synapses in the brain it is effectively impossible to reliably determine whether synapses are responding to some sensory input or learning experience. Easier than observing individual synapses is the task of observing what are called dendritic spines. Dendritic spines are little bumps on dendrites. In the visual below, the bottom part shows a closeup of the tiny red circle in the top part.
The dendritic spines have a close relation to synapses, because synapses are typically found clustered around such dendritic spines.
What do scientists see when observing such dendritic spines? They see them very slowly appearing and disappearing, and very slowly randomly changing in size. Dendritic spines are rather like pimples on the face of a teenager with acne, pimples that slowly come and go, increasing or decreasing in size. The correct word to describe such constant changes in all dendritic spines is variability, not plasticity. There is no evidence of such dendritic spines changing in some systematic way, some kind of way suggesting information storage. There is no robust evidence that any dendritic spines have ever changed in some way that correlates with learning or memory formation.
There are two terms in the English language that correctly describe what we observe in dendritic spines and synapses. The words are "instability" and "variability." But neuroscientists don't like to use those words when talking about synapses. Instead, they prefer to use the term "synaptic plasticity." Such a term is very misleading.
When I do a Google search for "plasticity definition," the first result I get gives me a definition of "the quality of being easily shaped or modified." The Merriam-Webster online dictionary gives two definitions of "plasticity":
1. The quality or state of being plastic especially: capacity for being molded or altered.
2. The ability to retain a shape attained by pressure deformation.
It is rather clear what the intention was when scientists first started using the term "synaptic plasticity." The intention was to bring to mind the idea of synapses being like clay in which memories can be written. Used by the Babylonians who used cuneiform, writing in clay was one of the oldest methods used by humans to record information. Clay had two great advantages: (1) a person using a metal stylus could instantly write letters on clay; (2) clay could permanently store letters written on it.
There are two reasons why it is very misleading to be using the term "synaptic plasticity." The first is that no one has ever observed any effect in which synapses quickly take on some particular shape or pattern in response to some causal factor. Nothing like any molding or shaping effect has ever been observed.
The second reason is that term "plasticity" implies the retention of some pattern that was produced by a shaping or molding effect. The second Merriam-Webster definition of plasticity is "the ability to retain a shape attained by pressure deformation." What we observe in dendritic spines and synapses is such a high level of variability and instability that there is every reason to doubt that they could be capable of retaining any pattern if such a pattern were ever to be impressed on them.
Dendritic spines last no more than a few months 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 hippocampus have a turnover of about 40% each 4 days. This study found that dendritic spines in the cortex of mice brains have a half-life of only 120 days. The wikipedia article on dendritic spines says, "Spine number is very variable and spines come and go; in a matter of hours, 10-20% of spines can spontaneously appear or disappear on the pyramidal cells of the cerebral cortex." Referring to in vivo observations of dendritic spines in the mouse hippocampus, the paper here says the authors "measured a spine turnover of ~40% within 4 days." The 2017 paper here ("Long-term in vivo imaging of experience-dependent synaptic plasticity in adult cortex") found the following regarding dendritic spines in the cortex of rodents:
"About 80% of synapses were detectable for a day or longer; about 60% belonged to the stable pool imaged for at least 8 days. Even this stable pool was found to turn over, with only, 50% of spines surviving for 30 days or longer. Assuming stochastic behaviour, we estimate that the mean lifetime of the stable pool would be on the order of 120 days."
The paper here states, "Experiments indicate in absence of activity average life times ranging from minutes for immature synapses to two months for mature ones with large weights."
We have no good evidence that any dendritic spines survive for more than a few years. There is an often-cited paper from the year 2000 with the title "Stably maintained dendritic spines are associated with lifelong memories." The title is misleading, like the title of so many scientific papers. The paper actually found that "a tiny fraction of daily formed new spines (~0.2% of the total spines) could persist for 3–5 months." So the paper found that only 1 in 500 dendritic spines persist for as long as 5 months. The paper resorts to some dubious math to try to hypothesize that some dendritic spines may last for years.
More recent papers have made even more clear the high turnover rate of dendritic spines, and have made it seem less likely that any dendritic spines survive for more than a few years. The 2015 paper
"Impermanence of dendritic spines in live adult CA1 hippocampus" states the following, describing a 100% turnover of dendritic spines within six weeks:
"Mathematical modeling revealed that the data best matched kinetic models with a single population of spines of mean lifetime ~1–2 weeks. This implies ~100% turnover in ~2–3 times this interval, a near full erasure of the synaptic connectivity pattern."
The paper here states, "It has been shown that in the hippocampus in vivo, within a month the rate of spine turnover approaches 100% (Attardo et al., 2015; Pfeiffer et al., 2018)." The 2020 paper here states, "Only a tiny fraction of new spines (0.04% of total spines) survive the first few weeks in synaptic circuits and are stably maintained later in life." The author here is telling us that only 1 in 2500 dendritic spines survive more than a few weeks. Given such an assertion, we should be very skeptical about the author's insinuation that some very tiny fraction of such spines "are stably maintained." No one has ever observed a dendritic spine lasting for years, and the observations that have been made of dendritic spines give us every reason to assume that dendritic spines do not ever last for more than a few years.
The same studies that show such short lifetimes for dendritic spines show that while they exist, dendritic spines very rarely maintain the same size and shape. During their short lifetimes, dendritic spines tend to change very much in size and shape.
So dendritic spines and synapses are unstable and highly variable things, and there is no evidence that they can retain some pattern that might be impressed on them. There is no evidence that dendritic spines or synapses quickly change in respond to something an organism has learned or experienced. There is zero robust evidence of any kind of code used by which information is imprinted on dendritic spines or synapses. We know that the proteins in such dendritic spines and synapses are very short-lived, having average lifetimes of less than two weeks. While we can honestly refer to synaptic instability and synaptic variability, we have no observational warrant for using the phrase "synaptic plasticity."
This confusion in which mere variability is incorrectly described as plasticity is shown in the Wikipedia.org article on dendritic spines, where we read this: "Dendritic spines are very 'plastic', that is, spines change significantly in shape, volume, and number in small time courses." Such random changes will be seen in any group of dendritic spines observed, and they are correctly described as "variability" rather than "plasticity." Rather than stating that dendritic spines or synapses are "plastic" (a claim for which there is no robust evidence), we should merely be saying that dendritic spines and synapses are variable and unstable. We have good evidence that dendritic spines are constantly undergoing random changes. We have no good evidence that such changes are any type of "plasticity" shaping or molding effect produced by sensory experience or learning.
What often goes on in neuroscience literature is a very careless confusion between variability and plasticity. Variability refers to something that undergoes random changes. Plasticity refers to some effect in which something molds or shapes in response to the action of something acting like a molder or shaper. We have lots of evidence for the constant variability of synapses and dendritic spines. We have no good evidence for plasticity occurring in such things. Similarly, we have very good evidence for variability in the sky above our heads, which constantly undergoes changes as different clouds drift by. We have no evidence for plasticity in the sky above our heads.
There have been studies that have claimed to provide evidence for synaptic plasticity in the sense of synapses changing in response to some experience, but such studies have provided no actual robust evidence backing up such claims. In a typical study of this type some animal will be given some sensory experience or learning experience, and then some dendritic spines or synapses will be watched. The paper may claim that some increases in dendritic spines or synapses were observed, and that this is evidence that such things were responding to the sensory experience or learning experience. The flaw in such reasoning is obvious. Since a mouse has something like a trillion synapses and very many billions of dendritic spines, which tend to undergo random changes, there is no reason to think that some small group of dendritic spines or synapses chosen for study would be exactly the right dendritic spines or synapses that might be responding to some sensory input or learning experience. It would be far more likely that some dendritic spines or synapses chosen for study would have no connection at all to some sensory experience or learning experience, and that any change observed would be mere random variation.
Part of the problem is the enormous number of synapses. Humans have something like 100 trillion synapses, and even mice have a trillion synapses. So it is impossible to do some experiment that observes something like a molding or shaping effect in which synapses take some particular shape or configuration in response to some sensory input or learning experience. Even if you were to do some in vivo experiment in which you saw some synapses change just after a learning experience or sensory experience, you would have no way of knowing whether such a change was just a random change that would have occurred even if the learning experience or sensory experience had not occurred.
Given a brain in which there are something like a trillion synapses and dendritic spines which are undergoing random changes, like pimples on the face of a teenager with acne, you absolutely do not show an effect of plasticity (synapses or dendritic spines changing in response to a learning or sensory experience) by showing that some small number of synapses or dendritic spines increased in size or strength after something was learned. We would expect that perhaps 25% of any randomly selected dendritic spines or synapses would increase after some learning occurred, even if this was in no way produced by learning or sensory experience. Similarly, if I claimed that stocks sometimes rise in response to what I write, I would provide no robust evidence for such a claim by showing that five or ten stocks had risen in value on some day I wrote something. At least a quarter of all stocks will increase in value on a random day.
A 2021 scientific paper gives us a sentence of unproven dogma, followed by another sentence confessing the lack of observations to support such a dogma:
"A defining feature of the brain is the ability of its synaptic contacts to adapt structurally and functionally in an experience-dependent manner. In the human cortex, however, direct experimental evidence for coordinated structural and functional synaptic adaptation is currently lacking."
Or to put it more concisely, there's no good evidence for synaptic plasticity, in the sense of synapses molding in response to something learned or experienced. Scientists looking for evidence of memories forming in the brain are still empty-handed, although their misleading words often suggest otherwise. Another recent paper kind of gives us a hint that "there's no there there" by saying at its beginning, "After decades of research on memory formation and retention, we are still searching for the definite concept and process behind neuroplasticity," which has a "still grasping for moonbeams" sound to it.
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