Summary

In brain information preservation, an important question is the extent to which one should optimize for the conservation of physical states in (a) the brain or (b) the whole body including the brain. A relevant parameter is attempting to preserve traits as opposed to states. The fundamental claim of brain preservation is that adequately conserving the brain alone would be enough to retain the information for long-term memories and core personality traits, because it is the only part of the body that is known to be irreplaceable without massive effects on this information. There is a spectrum of possible procedures that each privilege the brain to a different degree, each with their own upsides and downsides. My personal view is that preserving the brain alone should be the focus because it has clear value and is a difficult enough problem. Others may feel that the rest of the body is irreplaceable or that preserving the brain is not such a difficult task, in which case they may want to focus on preserving the rest of the body as well.

The embodied and embedded brain

Even for a materialist who thinks that the brain is sufficient to produce a mind, it does not make sense to speak of human brain function without considering the function of the rest of one’s body. They are completely intertwined.

In other words, the brain is “embodied.” Here we are using the word “embodied” in the sense of “include or contain something as a constituent part.” The brain functions in large part to make predictions about the world and then produce actions in behavior-sensation feedback loops with the rest of one’s body.

Similarly, the embodied nature of our brains goes beyond this to our “embedded” relationship with the environment. Humans are organisms that exist in a contextualized environment with other humans, other animals, objects, places, and natural phenomena. Our dependence on other people, other organisms, and objects in the external world has been called the “extended mind”.

A friend once asked me whether after the theoretical revival from brain preservation, the idea is that someone would “live in an iPod or something.” Generally speaking, the answer is definitely not. Without the rest of one’s body and an appropriate environmental context, it’s hard to conceptualize how a person’s brain could function anywhere close to optimally.

Instead, the idea is that the rest of one’s body and environmental context are replaceable. The rest of one’s body is necessary in the same sense that legs are necessary to walk but can be replaced. Whereas, in the way that many people think about it, the brain is not: their brain is who they are. Basic neuroscience teaches us that your brain controls your memories and personality traits such as thinking, planning, judgment, and emotions (Stangor et al., 2014). So while a successful revival must involve the non-brain parts of the body and an environmental context for the person, it doesn’t need to be the original non-brain parts and the original environmental context. These could be replaced, synthesized, and/or simulated.

States and traits

In order to address this question in more depth, it’s helpful to consider the concept of a psychological state vs trait (Kraemer et al., 1994).

States tend to be short-term. They are endogenous (e.g. hormonal) or exogenous (e.g. environmental) sources of variation within individuals. If one is in the midst of a major depressive episode, that’s a state. If someone just smoked cannabis, that’s also a state.

Traits, on the other hand, are long-lived and stable across contexts. They refer to long-term personality styles, such as shyness, honesty, or curiosity.

Cicero made the state vs trait distinction in 45 BCE: “It is one thing to be irascible, quite another thing to be angry, just as an anxious temper is different from feeling anxiety. Not all men who are sometimes anxious are of an anxious temperament, nor are those who have an anxious temperament always feeling anxious.” (Chaplin et al., 1988).

From the perspective of preservation metrics, our focus is on long-term traits as opposed to short-term states. States are transient and are not generally valued in the same way. States are also less robust and more difficult to preserve. Let’s examine a few examples of state vs trait reasoning to flesh it out a bit.

Tyler Cowen once noted that he thought cryonics had a low chance of working in part because of the impact of hormones from non-brain areas, such as the gut, affecting one’s brain and mental states. But generally speaking, gut-brain interactions would affect states, not traits. We likely wouldn’t say that someone’s core traits and personhood would be lost if their gut-brain interactions were altered. Otherwise, fecal microbiota transplants might be thought of as killing someone and creating a new person.

A similar discussion could be had about altered levels of thyroid hormones. If someone’s thyroid stops producing thyroid hormone, they frequently develop depression, memory problems, or other changes to their mental states. When thyroid function is replaced by medications, it can dramatically affect their mental states. And yet, people don’t tend to worry about their identity changing upon replacing thyroid function. On some deep philosophical level, given certain assumptions about personal identity, maybe they should. But most people don’t. It is in this every-day sense of “let’s just replace the thyroid function so I get back to normal and feel like myself again,” similar to the concept of ordinary survival, that states are not a priority to preserve.

Long-lasting states

There’s a wrinkle here in that some aspects of someone’s experience could be life long, and yet they might still consider this aspect of their functioning a state as opposed to a trait. To me, this is an area of uncertainty. Perhaps the key question is whether the state can be easily reversed.

An example of a long-lasting state, as discussed by Mike Darwin, is hereditary hyperinsulinemia caused by disordered pancreatic function. If someone is born with a mild hyperinsulinism, it’s possible that they could become irritable when they become hypoglycemic, especially if they have had a longer than normal period of fasting. This propensity towards irritability might even seem like a somewhat important aspect of their personality, at least from an outsider’s perspective. If the person had a new body upon theoretical revival, the degree of hyperinsulinism might not be matched, either because it was hard to ascertain or because of the person’s preference. Either way, this might lead to a change in that person’s personality, wherein one might become less irritable when hypoglycemic.

My guess is that for most people, a theoretic revival procedure involving a new pancreas would not cause them to feel that their fundamental identity had changed. In the same way that they probably wouldn’t feel their identity had changed if they started taking the medication diazoxide to treat their hyperinsulism. Instead, this would likely be seen as a welcome quality of life improvement. It is in this sense that non-brain organs can be thought of as having important – and even life long – effects on states, but still not identity-critical effects on core personality traits. Ultimately, parsing out a long-lasting state vs trait can be somewhat of a semantic distinction and it is not absolute (Chaplin et al., 1988); it is on a spectrum. But I still find it a helpful way of reasoning about this topic.

Everything else other than the brain mediates a state

Basic physiology tells us that organs other than the brain can certainly have state-like effects on one’s personality. However, they can be lost or replaced without changes in one’s personality traits or long-term memories. Hearts, lungs, livers, kidneys, and other organs can be replaced by transplantation. Sensory organs like the cochlea or retina can be at least partially replaced by artificial versions (Stanga et al., 2021). These replacement procedures don’t seem to have clear effects on long-term memories or underlying personality traits. The main function of non-brain organs could be thought of as modulating the brain’s expression of mental capacities.

It’s worth noting that not everyone who loses function in one of their senses feels that it is a loss. For some people, in the right contexts, it can lead to growth. Jorge Luis Borges discusses the ways in which his blindness was a gift in a remarkable lecture.

Some people have claimed that certain organ transplants can cause transfer of memories through some sort of epigenetic mechanism (Liester, 2020). I find the evidence for this to be anecdotal and weak. But even if this were the case, memories and personality would still be instantiated in the brain via electrochemical activity, so the non-brain organs would still not be necessary to preserve if the information in the brain were intact.

One way to think about the fundamental claim behind brain-focused preservation is that the brain is the only part of the body that uniquely mediates traits, while other parts of the body can mediates states. From this perspective, the brain is the only part of the body that is identity critical.

Non-brain nervous system

The spinal cord and other parts of the nervous system other than the brain are worthy of a more detailed discussion.

Spinal cord

Numerous, reciprocal connections exist between the spinal cord and the brain. In fact, it kind of looks like an arbitrary distinction between the brain and the spinal cord when you look at it grossly: 

Human brain and spinal cord; Specimen from the National Museum of Health and Medicine

The microanatomic level tells a different story. The spinal cord tends to be mostly involved in sensory and motor functions, including communication of these with the brain. Whereas the brain is also involved in higher-order functions such as memories, emotions, and personality traits. This is why descriptions of the anatomy of the spinal cord tend to involve motor and sensory fibers that communicate with the brain:

Spinal cord tracts (red motor, blue sensory); Sobotta’s Anatomy Atlas, 1908 edition

Where does the brain end and the spinal cord start? What we call the most inferior part of the brain is the medulla and what we call the most superior part of the spinal cord is the cervical spinal cord region. Technically, this transition is defined to occur at the level of the pyramidal decussation, which is where the motor fibers in the pyramids of the medulla cross over the midline (Beuls et al., 1993 May-Jun).

In the end, it’s likely somewhat of an arbitrary distinction between the end of the brain and the start of the spinal cord. This is most relevant to the brain preservation procedure in which the brain is removed from the skull and then conserved. But even in this type of procedure, part of the cervical spinal cord is often able to be retained. And while the boundary zone may be indistinct, there are clearly true differences in the functions of the brain and the spinal cord.

The somatic nervous system and the afferent autonomic nervous system carry information to the brain through the spinal cord. Specifically, it carries sensory information from all over the body including touch, vibration, and proprioception (the sense of where one is in space) to the brain.

These connections are lost when people have spinal cord transections that cause these connections to be completely severed. Clearly, spinal cord injuries have effects on the way that people tend to think and behave. For example, some have suggested that the diminished autonomic arousal feedback after spinal cord injury may affect emotional experience (Chwalisz et al., 1988) (Deady et al., 2010). But it does not seem that these effects would be irreversible if autonomic arousal feedback could be restored. More generally, the function of the spinal cord seems to affect one’s states, not traits.

Unless spinal cord injury coincides with a traumatic brain injury or causes secondary changes in the brain, then to the best of my knowledge it has not been associated with loss of pre-existing memories or personality traits. What this suggests is that the spinal cord itself is not the primary storage site of that information.

One detail here is that there are certain types of simple implicit memories, such as pain pathways and spinal cord reflexes, that are stored in and mediated through the spinal cord (Squire et al., 1996). In my opinion, these are not memories in the every day sense of the word. But to be more precise, I’m talking about non-reflex, non-pain memories that are present even after a complete upper cervical spinal cord transection.

Peripheral nervous system

There are also peripheral nerves that do not travel through the spinal cord and therefore are not lost in a spinal cord transection. So we cannot use the example of people who have had spinal cord transections to claim that these nerves are not essential for memories and other aspects of personal identity.

It seems that each of these individual nerves, at least on one side of the body, can be transected, lost, or manipulated without clear irreversible effects on memory or personality. For example, the facial nerve supplies sensory information from much of the face including the tongue to the brain. As far as I know, bilateral facial nerve replacement has not been shown to have long-term effects on memory or personality, as evidenced by the procedure of whole face transplantation with repair and regeneration of facial nerve connections (Siemionow et al., 2011).

The efferent (outgoing from brain to body) parasympathetic nervous system, including the vagus nerve system, is also not affected by a spinal cord transection. It is difficult to study the effects of bilateral damage to the vagus nerve nuclei in humans because it causes clinical death due to laryngeal paralysis and cardiac arrhythmia (Walker, 1990). It is clear that the vagus nerve system has a big impact on mental states, as suggested by the beneficial effect of vagus nerve stimulation on depression (Kong et al., 2018). Like other manipulations of the peripheral nervous system, one can turn vagus nerve stimulation on or off or modulate the degree of stimulation. It seems that the effects of vagus nerve stimulation affect states, such as one’s mood, not one’s underlying memories or core personality traits.

Replacement of nerves with future technology

It is reasonable to ask whether new nerves could ever be connected to the brain. In fact, it has already been shown that nerves can be replaced and the brain can adapt to the new inputs. The brain has a high degree of plasticity to accommodate new sensory inputs, including sensory map reorganization following digit amputation (Merzenich et al., 1984) and remapping of brain areas to auditory sensation following visual loss (Merabet et al., 2010). Given enough time, our brains have been shown to be highly adaptable.

Nerve transfers are a nice example of this phenomenon that are already in routine clinical use. Instead of a nerve graft, nerve transfers involve redirecting part of a healthy nerve into a nearby damaged nerve in order to restore nerve conduction to that region (Moore, 2014). You can think of it as “plugging in” your healthy nerve to the region with the injured nerve. Nerve transfers are another proof of principle that cognitive remapping to novel sensory inputs would be possible after a complicated revival procedure following brain preservation.

Some people point out that neural plasticity decreases with age and that many people choosing brain preservation are elderly. These critics are thinking in terms of early 21st century biotechnology. Our ability to manipulate neural plasticity parameters in targeted ways will expand dramatically in the future if we ever reach the point of reviving conserved people.

Topographic maps and the replaceability of sensory and motor systems

There is a line of reasoning that makes one, or both, of these arguments:

  1. While individual sensory or motor inputs to the brain can be lost or replaced, that does not mean that all of them are necessarily able to be lost or replaced at the same time.
  2. Replaceability relies on the availability of external reference points that may be necessary for sensory remapping. If these reference points were lost, as they might be after brain preservation, then the process of decoding which sensory inputs different areas of the brain correspond to might not be possible.

Ultimately, there is some degree of uncertainty on the basis of these arguments, because we cannot point to real-world examples that address them. But our best current understanding of neuroscience suggests to me that the uncertainty involved in this aspect of the procedure is very low.

One highly relevant neuroscience principle is topography. Topography refers to order in the way that neurons are positioned in the brain relative to the sensory input surface or the motor output system.

Let us consider the retina input stream as an example. In a simplified version of this, retinal ganglion cells project axons to the lateral geniculate nucleus, which then project axons to the primary visual cortex, known as V1. Neurons in the lateral geniculate nucleus and primary visual cortex are ordered in topographic ways.

One example of topography in the lateral geniculate nucleus and V1 are retinotopic maps. Retinotopy means that neurons in these areas are arranged in order of the visual field (Swindale, 2008). As a result, electrochemical activity in one part of the brain region will be preferentially stimulated by light stimuli presented to one part of the visual field. Much is already known about how these topographic maps are arranged. For example, there are known distortions of the visual field present in these maps, such as magnification of the center of the visual field.

If it were not for topography, then decoding how synthesized sensory inputs and motor outputs should be attached to the brain without the corresponding organs might be much more challenging. But because topography is a general principle of brain organization, decoding the static neural code of sensory and motor areas will be much more simplified. This will allow for reattaching synthesized sensory inputs and motor outputs to circuitry in the brain.

More generally, if one only had access to the preserved brain as a physical artifact with no external knowledge of how the brain works, then decoding it would seem highly under-determined and potentially impossible. But given that our knowledge of the brain will have to be immense before the time that revival from brain preservation is attempted, we will have a detailed knowledge of how different areas of the brain work and how they connect to sensory inputs or motor outputs. As a result, my assumption is that the two objections listed above will be surmountable by future technology. This seems like a substantially easier problem than many of the other problems involved in a theoretical revival procedure.

Limitations of subjective report

The style of reasoning I have often used above to determine whether a part of the body can be lost or replaced without effects on personal identity is people’s subjective report: asking people who have undergone a certain loss or replacement whether they feel different.

It’s important to recognize the limitations of the subjective report approach. First, you’re asking a person who has an evolutionary bias and psychosocial interest in maintaining a sense of continuous subjective identity whether they still feel like themselves. And second, it varies by culture to say whether somebody is still themselves or not after a given event.

Instead, when possible, it would be better to focus on more objective things that we can measure, such as long-term memories that can be recalled, behavior, neuroimaging metrics, or characteristics observed by third parties. Unfortunately, such data is often difficult to find. I recognize that this is a limitation of my reasoning. Perhaps the most honest thing is to acknowledge that my conclusions stem primarily from my strong prior beliefs based on the extensive basic neuroscience literature showing that the brain mediates cognitive functions. And that the available data I have evaluated merely fails to shake those priors.

Uncertainty is left to the individual

Focusing on the preservation of the brain alone, followed by replacement of the rest of the body at the time of a theoretical revival, is effectively the same as the proposed procedure of a brain transplant. Although a brain transplant has not yet been performed, it certainly seems more plausible near-term than brain preservation followed by revival, and there is an extensive philosophical and ethical literature on this topic.

It remains uncertain exactly how a brain transplant would play out in practice. But the consensus seems to be that the person whose brain was transplanted would simply have a new non-brain body, because a person effectively is their brain.

There is not only some amount of uncertainty about the underlying biology, but what would be the outcome of a brain transplant is also a personal decision. Some say that they define their identity as including parts of their body, not just their memories, personality, and other things mediated by the brain. Perhaps this would mean that they would consider themselves to have died if they required a heart transplant. Ultimately, personal identity is subjective. As (Pascalev et al., 2016) stated: “there are good reasons to leave it to the individual undergoing the [brain transplant] transformation to negotiate the parameters of his or her identity, on her own terms and at her own pace.”

It is certainly the case that someone might consider a non-brain part of their body to be an important part of their personhood. For example, an athlete, an artist, or a musician might value their peripheral nerves and/or musculoskeletal system and think that they are a crucial part of their personal identity. Perhaps this person would choose not to focus just on preserving their brain, but on preserving their whole body. This decision obviously has trade-offs in terms of cost, preservation quality, and other factors.

But my impression is that most people would not consider themselves to have died if a non-brain organ was lost and needed to be replaced. Or even, in the extreme case, if all of their non-brain organs were lost and needed to be replaced.

Some of this philosophical discussion is probably a waste of time. Ultimately, brain preservation is about trying to save lives in the same way that CPR is, so it shouldn’t necessarily require any more philosophical justification. I call this the “CPR test”: if a philosophical objection to the idea of people pursuing brain preservation would also suggest that someone should not perform CPR on someone who has just clinically died of cardiac arrest, and desired an attempt at resuscitation, then it’s probably not a very humane or practical suggestion.

Robustness of brain information

In contrast to all other organs and parts of the body, damage to the brain is liable to lead to serious changes to one’s memories and personality. Following a serious traumatic brain injury, it’s not uncommon for people and their loved ones to wonder the extent to which they have the same personality as they had prior to the event that damaged their brain (Stone et al., 2004).

However, it’s also worth pointing out that brain information can be surprisingly robust to damage as well. For example, some patients with intractable seizures can have this cured by hemispherectomy, a procedure in which half of the brain is removed or rendered nonfunctional. Amazingly, people still tend to retain their personalities and memories after this procedure and can go on to live normal lives. The authors of one case series noted: “We are awed by the apparent retention of memory after removal of half of the brain, either half, and by the retention of the child’s personality and sense of humor.” (Vining et al., 1997)

One downside of the endless debate of preserving the brain or the whole body in cryonics is that it obscures perhaps a more interesting metrics-based discussion, which is: What parts of one’s brain are identity critical? One might imagine that the cerebellum, brainstem, visual cortex, or other regions might be considered more or less identity critical for different aspects of one’s memory or personality traits.

Of course, even non-identity critical brain regions are, generally speaking, necessary for optimal brain functioning. For example, even though people have been found to be able to live without a cerebellum, they have also been found to have functional deficits (Lemon et al., 2010).

A heart is necessary for living, but it’s replaceable – can the same be said for some brain regions, or are they all identity-critical? This is a practical question because different brain preservation methods could lead to better or worse preservation of different parts of the brain. As far as I can tell, it is an area of high uncertainty.

One study has systematically evaluated the reactivation of engram ensembles in different brain regions to see which brain regions are most associated with memory recall (Roy et al., 2022). They note that while different brain regions can be associated with the same memory, each brain region might contribute a particular type of information for that memory:

The distributed nature of engram cell ensembles of a specific memory has led to the suggestion that the memory engram within an individual brain region may contribute a subset of the overall memory information. For example, hippocampal engrams are thought to primarily contribute contextual information by acting as an index for cortical memories of various sensory modalities, whereas amygdala engrams hold valence information for a given experience. In addition, cortical engrams such as those in the retrosplenial and prefrontal cortices may support spatial navigation and top-down control of memory retrieval, respectively. Further, engrams in auditory and olfactory cortices may support auditory recognition memory and odor-induced learned behaviors, respectively.

That said, even if a brain region is not identity critical, the information present there could still be useful in the case that there were damage to identity critical brain regions. For example, redundant information in potentially non-identity critical brain regions such as the cerebellum might be helpful for inference in the case of damage to regions that are almost certainly identity critical, such as the frontal cortex. Because of this possibility of inference, as well as our degree of uncertainty of whether a given brain region might not be identity critical, it seems prudent to focus on preservation of the whole brain.

Preservation types and trade-offs

Just because one focuses on the brain for preservation metrics does not mean that one must only preserve the brain. From a technical perspective, there are a variety of different ways one could imagine performing a brain preservation procedure that would also include the rest of the body to different degrees. It’s not just “neuro vs whole body.” Among other procedures, one could imagine:

  1. Extracting the brain from the skull and only preserving the brain
  2. Preserving the whole skull with the brain inside
  3. Preserving the whole cephalon (technical term for head) with the brain inside
  4. Any of the above but also separate secondary preservation of the rest of the body
  5. Preservation of the brain and the body separately through separate perfusion tracks (for example, femoral + carotids/vertebral arteries) but leaving the body intact
  6. Preserving the whole body at once, for example via perfusion through the heart

Historical perspective on cephalon and whole-body preservation

The initial cryonics procedures in the 1960s focused on preservation of the whole body. Cephalon-only preservation was started at the cryonics organization Alcor in the mid-1970s (cephalon is a technical word for the head). This shift in preservation type seems to have been independently proposed: both by one of Mike Darwin’s colleagues as well as by Linda and Fred Chamberlain. Darwin and the Chamberlains are pioneering cryonicists who co-founded Alcor. Both proposed it for the same reasons: decreasing the expense of preservation and improving the quality.

The historical context is that at the time, the Cryonics Society of California (CSC) and the Cryonics Society of New York (CSNY) had both recently run out of money and the conserved people had thawed. At the time, Mike Darwin, who is probably the most influential cryonicist, was so pessimistic about the chances for cryonics to be successful that he considered throwing in the towel on the whole idea.

It was essential for cryonics organizations to be steely-eyed about how many resources were actually available in the face of a society that was indifferent to cryonics at best and hostile at worst. Money that would be spent on preserving and storing non-essential parts of the body is money that could instead go to other critical aspects of the preservation mission, such as into a patient care trust that would pay for maintenance and eventual revival. The first cephalon-only preservation, also called a neurosuspension, was performed on Fred Chamberlain’s father in 1976.

When it comes to resources, little has changed over the last half century. People who are interested in brain preservation are still highly resource-constrained. As will be discussed in a later chapter, being realistic about our resource limitations is especially important if we want to ensure that anyone who wants brain preservation can access it.

The rest of this section will be a discussion of the key trade-offs involved in the different preservation types.

Downsides of brain-only preservation

1. Loss of structural information in the rest of the body, requiring replacement and/or inference.

This is the major downside of brain-only preservation. As a result, in brain-only preservation, inference about the structure of the rest of the body would need to be done via external and internal information.

External information is information about the person other than one’s brain itself. An example of this is a “mindfile,” which is a database of information including text, video, audio, and image files about one’s behaviors, memories, appearance, and personality traits. This external information could also be used if there is damage to the brain requiring inference about the original structures. For example, if one had access to an MRI scan of one’s body, this might be helpful.

But likely the far more useful type of information is internal information, which present in the brain. Brains contain numerous sources of information about the rest of one’s body, including memories of one’s appearance and one’s internal sensations such as proprioception. During the revival and rehabilitation process, brain information could be leveraged for the re-creation of the rest of the body, in an iterative process.

There is uncertainty about how high-fidelity this inference process would be. The peripheral nervous system may pose a particular challenge. For example, inferring the precise degree of myelination of peripheral axons would likely be impossible without access to the tissue itself. People who choose brain-only preservation should expect that the rest of their body may be different post-revival than it was prior to legal death. This might cause people to have different somatic sensations and motor abilities, requiring a process of rehabilitation and adaptation. Of course, many people might not care too much about this or would actually prefer to have a new rest of their body with their old brain.

In brain-only preservation, some of the central nervous system would be lost too. Sensory organs such as the eyes and ears would be lost. As discussed above, these organs can already be replaced today, so it is likely that they are not essential. But if there was damage to the brain during preservation, then other parts of the nervous system – if they were also preserved – could potentially be used for inference about the original brain states; of course, this would not be possible in brain-only preservation.

2. The next downside of brain-only preservation is that manipulating the brain may expose the person or team performing the procedure to a higher degree of hazardous materials than in other preservation methods. These include pathogenic prions, tuberculosis, HIV, and HCV.

There are standard operating procedures in place within brain banks to deal with the risk of hazardous agents (Ravid, 2008). In the context of brain banking, the main risk factor for prion disease is thought to be accidental parenteral (i.e. non-oral) inoculation with nervous system or lymphoid tissue (Ironside et al., 1996).

While a brain with prion disease or other infectious disease poses a serious transmission risk and needs to be handled with appropriate safeguards, it is also important to not over-exaggerate the risk to the point of causing harm. Those suffering from prion disease or another infectious illness prior to legal death frequently have their brains examined for neuropathology studies. As Ironside and Bell note in their review of brain banking in cases with known prion disease and other high-risk cases, “in light of current information on the nature of these agents, it is possible to establish safe working protocols for autopsy procedures, brain dissection, tissue handling, microtomy and staining, which should be reinforced by training and education which is updated at regular intervals” (Ironside et al., 1996).

Given the appropriate precautions, handling brain tissue is generally thought to be acceptably safe from a risk management perspective. There are also brain preservation procedures that can preserve the brain tissue in addition to deactivating and/or isolating prions and other infectious agents.

3. Another hazard of removing the brain from the skull is that it can cause significant damage to the brain itself. This process generally involves the use of an electric saw to cut through the skull, followed by cutting of the cranial nerves and arteries, and then manual removal of the brain out of the skull (Poloni et al., 2020). All of these steps pose the risk of traumatic damage to the brain.

First, in brain banking, people sometimes talk of the “Dienerian sulcus” that can result if the diener (also known as an pathology specialist) accidentally cuts into the brain when cutting through the skull.

Second, lifting the brain out of the skull is a significant mechanical trauma that is likely to cause damage. For example, this is thought to cause an artifact known as “dark neurons” (Jortner, 2006). These are contracted neurons seen on hematoxylin and eosin stained tissue when the brain is removed from the skull prior to preservation by fixation. The formation of dark neurons themselves are probably not too damaging from an information content perspective, as will be discussed in a later essay. But they are likely just one example of the type of damage that can occur as a result of this mechanical trauma.

A key factor in how much damage will occur when removing the brain from the skull is the material state of the brain. The brain generally has a jello-like integrity, but how soft that jello-like material is can vary significantly. It turns out that how soft the brain is depends on how much agonal and postmortem damage has occurred prior to removal.

Hayman’s 2013 thesis (Chapter 4, p 123) scores the amount of brain decomposition based in part on how easy it is to remove the brain from the skull intact. Hayman reports that prior to decomposition, the brain is firm, it can be removed from the skull without separation, and it can be sectioned into thin slices with distinct anatomy.

In the first stage of decomposition, the brain becomes slightly softer, and the sulci and gyri can separate when the brain is removed from the skull. In the second stage, the brain becomes softer and the lobes can easily separate when the brain is removed from the skull. In the third stage, the brain is so soft that it starts to fall apart when the dura is opened and begins to disintegrate when removal is attempted.

In the fourth stage of decomposition, the brain is held in place by the dura, but it is in a liquid state and flows out of the skull when the dura is cut into. In other words, the brain appears to melt after it is taken out of the brain. This is obviously hugely damaging to neural structures.

In advanced stages of decomposition, removing the brain from the skull may cause more harm than it does good, and the brain should likely be at least partially fixed within the skull prior to removal. This could be done via perfusion through blood vessels in the neck or via convection or immersion after creating a window in the skull.

This is why William Osler injected zinc chloride into the carotid arteries into brain donors with longer postmortem intervals, to increase the firmness of the brain and aid in its removal from the skull (Shrady et al., 1880). Performing some sort of fixation prior to removal of the brain from the skull, especially in the context of significant decomposition, largely eliminates this downside of the brain-only preservation method.

Notably, if the brain is at an advanced state of liquidation, it may be at or close to information-theoretic death regardless of the brain preservation method. But many would argue that it is still the most just course of action to attempt to minimize damage during the brain preservation procedure and leave the conclusive determination of information-theoretic death to the future.

4. There may be diminished access to blood vessels as a result of removing the brain from the skull.

Perfusion-based preservation is an effective method of postmortem preservation that takes advantage of the brain’s existing vasculature system to distribute preservative chemicals throughout the brain. The brain’s supplying blood vessels, the carotid and vertebral arteries, may be more easily accessed in the neck with the brain intact.

It is possible to attempt to access the internal carotid and vertebral arteries after the brain is removed from the skull, but it is a more difficult problem. Blood vessels need to be cut during the brain removal process, which means that one must cannulate the stump of the blood vessel. Downstream parts of the vasculature may also be damaged as a result of the trauma of removing the brain from the skull. One upside to the brain being removed from the skull, though, is that it is easier to access the vertebral/basilar arteries, which distribute blood to the posterior circulation of the brain. And it is easier to immediately visualize whether the perfusion preservation process is working or not.

In brain banking, both ex situ (brain removed from the skull) and in situ (brain still present in the skull) methods have been used for perfusion fixation (McFadden et al., 2019):

Human brain perfusion fixation methods employed over time; McFadden et al 2019

It’s unclear to what extent ex situ perfusion methods may be worse than in situ perfusion methods. This will be discussed in a later essay. But one important point here is that perfusion-based preservation prior to removal of the brain from the skull largely eliminates this downside of the brain-only preservation method.

Upsides of brain-only preservation

1. It’s easier to distribute chemicals to the brain without relying on high-quality perfusion.

Other than unprotected cryopreservation, all of the preservation methods we will consider require chemicals to reach the brain tissue. This can be done via perfusion of the chemicals through the circulatory system, but this can be difficult to do postmortem, especially in the presence of significant decomposition, and/or with chemicals that are difficult to perfuse, such as high viscosity agents. The main advantage of brain-only preservation is that it is much easier to distribute preservative chemicals.

The ability to more easily distribute preservation chemicals makes a whole universe of brain preservation procedures, such as plastic embedding, more feasible to accomplish. Theoretically, it’s also possible to perform these methods without extracting the brain from the skull, for example if a skull window is created, but that is potentially more technically challenging.

2. It’s easier to assess preservation quality. When the brain remains inside of the skull, it’s harder to assess what it looks like. This includes macroscopic appearance, although that can be mitigated by imaging techniques like a CT scan that can see through the skull.

A bigger problem with leaving the brain inside of the skull is that it makes it more difficult to evaluate the microscopic tissue quality. During and after a brain preservation procedure, one needs to know the state of the brain tissue; otherwise, organizations can end up with the “no feedback problem,” where there is no knowledge about what the actual preservation quality is like or, more importantly, an incentive to maintain and improve it. As a result, brain biopsies help to develop and maintain a high-quality brain preservation procedure.

Selfishly, you might not want a biopsy of your brain tissue, which is, after all, a part of you. But you almost certainly want to conserve your brain with an organization that routinely performs brain tissue biopsies and actually cares about preservation quality. Biopsy-based feedback is also necessary for advancing the field. Fortunately, the damage of a microliter-scale, targeted brain biopsy is quite minimal.

Even though assessing microscopic preservation quality is easier in the brain-only preservation approach, it’s important to point out that taking a biopsy can also be performed in the other approaches if a skull window, such as a burr hole, is used. However, the biopsy will be more challenging to perform. While not as dispositive about brain preservation quality, a biopsy can also be taken from a part of the spinal cord in the other approaches.

3. In addition to using biopsy samples to test preservation quality, it is also easier to extract brain biopsies for human brain research with brain-only preservation. This could help, for example, with research into neurobiological disorders such as Alzheimer’s disease or schizophrenia. Similar to genomics, my impression is that a small amount of one’s brain tissue would be enormously helpful for discovering the etiologies of neuropsychiatric disorders and developing better treatments. This is because brain tissue structure tends to be similar across the brain. People with more altruistic goals may find contributing to medical research via small biopsy samples to be a major advantage of brain-only preservation methods.

4. In many circumstances, shipping a brain is legally less problematic than shipping a whole body. This is because it is legally considered an organ, so many of the byzantine rules regarding transporting a body do not apply. It also costs much less to ship a brain. As Canatelli-Mallat point out, “A practical reason for choosing brain over full body cryopreservation in countries where there are no cryonics facilities is that shipping a head or a brain is simpler and much less costly than doing so with a whole body.” (Canatelli-Mallat et al., 2020)

5. Brain-only preservation is commonly used in brain banking and autopsies for neuropathologic assessment. Indeed, brain dissection has been performed for many centuries:

The Anatomy Lesson of Dr. Deijman, Rembrandt, 1656

This means that there is already an infrastructure set up to support it. Specifically, there are funeral directors, pathologists, pathology assistants, and others who are capable and aware of the procedure to remove a brain from a legally dead person’s skull, perform the initial preservation with fixatives, and then help with shipping it to a central location. Tapping into this existing network has the potential to dramatically reduce the costs of the procedure.

6. The volume of a typical human brain is approximately 1300 cubic centimeters, whereas the volume of a typical human body is approximately 65,000 cubic centimeters. Being 50 times smaller means a significantly lower cost of storage space, cost of liquid nitrogen, refrigerator space, or other form of cooling method (if relevant), and cost of chemicals for preservation. It also means a substantially lower environmental cost of cooling and chemicals.

Isolated brains are easier to maintain in storage and they’re easier to ship to alternative storage sites. It’s also easier to assess any changes in preservation quality over time in isolated brains. Many brain collections have stored human brains for a multitude of decades, in some cases for more than a century. Even though they are not as focused on long-term preservation as cryonics organizations, in some circumstances these brain collections have had better success.

These advantages of brain-only preservation could be mitigated by using a different preservation method for the brain as opposed to the rest of the body.

7. Another advantage of brain-only preservation is that it potentially allows people to be organ donors.

Is this possible today? It’s tricky. It depends on the organ one is trying to donate, the level of cooperation from the organ donation team, the goals of the person interested in brain preservation, and the acceptable amount of post-mortem delay.

Generally, even for people who only want neurosuspension, the cryonics organization Alcor recommends that its members not sign up as organ donors. This is because of a concern that it will significantly delay the brain preservation procedures.

It is also possible to donate one’s organs and then do brain-only preservation very shortly afterward. From a preservation quality perspective, this would cause damage because brain decomposition will occur during this time period. But some people may choose this regardless because they believe so strongly in organ donation and it’s important to honor this request. One might also advise their healthcare proxy that they want to be a conditional organ donor in the case that their brain is dramatically damaged as a result of the way that they legally die, since brain preservation is more likely to be futile in that scenario.

The only reason we need to have this discussion is because the medical field has not accepted brain preservation as a reasonable course of action – that many people want – and established the quite feasible procedures that would allow for organ donation to be done at the same time as brain preservation. If this ever occurs, it would be very much possible for people to do both organ donation and brain preservation. Indeed, this would likely help brain preservation to be higher quality, as it would involve trained surgeons and high-tech medical equipment. For example, Peter Gouras suggested that brain preservation procedures could be done by the organ procurement team at Columbia University. It might also help organ donation to be higher quality, because the next-of-kin consent process would be more streamlined.

Cephalon-only preservation

Relative to whole-body preservation, cephalon preservation shares a lot of upsides with brain-only preservation: lower preservation cost, easier to maintain over time, theoretic ability to perform organ donation, and easier to assess preservation quality (including access to spinal cord tissue to assess preservation quality). Regarding the relative ease of storing a cephalon as opposed to a whole body over time, a relevant data point that none of the early suspension failures in cryonics were cephalon-only preservations. Cephalon preservation also shares one of the downsides of brain-only preservation, which is that it requires inference of the rest of the body (although to a lesser degree).

Here are some specific advantages and disadvantages of cephalon-only preservation:

Downsides of cephalon-only preservation

1. Separated heads are horrifying to people.

This seems to be an innate reaction. Research has shown that chimpanzees are terrified when they are shown images of isolated chimpanzee heads (Marks et al., 1987). It is a danger sign, similar to the whites of an animal’s eyes. There is a reason that Medusa’s head is a classic symbol of horror.

Separated heads are a terrifying symbol; Perseus by Benvenuto Cellini

People’s strongly negative association with separated heads is widely acknowledged in the cryonics community to be very bad; for example, see Steven Harris’s comment here. It is a violation of social norms. It makes cryonics seem even weirder than it already is. The use of separated heads is a major way that cryonics is parodied and cryonicists are put into an “out-group”, for example in Futurama:

Richard Nixon’s preserved head; Futurama

However, the emotional reaction against separated heads needs to be balanced against reason. Alternatives to the separated head approach, such as whole-body, have downsides. They may be much more expensive, more environmentally taxing, and depending on the specifics of the method, may offer lower preservation quality.

Humans also have visceral reactions against plenty of other things that we have learned to ignore or suppress. Much of what happens in medicine causes disgust or fear reactions: colonoscopies, debridements of necrotic wounds, brain surgeries – the list goes on ad nauseam. But we’ve learned to suppress or ignore our immediate reactions to these procedures because they save lives, and we can do the same with separated heads, if necessary. Related to this, a common approach to deal with the problem of disgust and fear, perhaps with limited success, is to emphasize that the goal is brain preservation and that the whole cephalon is preserved mainly as a surgical detail.

2. Separating the cephalon from the rest of the body at the neck and accessing the blood vessels for perfusion-based preservation is technically difficult and can take time. As opposed to removing the brain from the skull, there is no large existing network of people who know how to accomplish this that one can tap into.

3. The use of cephalon-only preservation has often been associated with lawsuits. For example, the costly Pilgeram lawsuit was related to the fact that Laurence Pilgeram had signed up for whole-body preservation, but Alcor switched his preservation to neurosuspension as a result of lack of funds.

These lawsuits have been primarily against Alcor, which often “converts” members to neurosuspension in the case of limited funding because cephalon preservation is much cheaper than whole-body preservation. As of this writing, Alcor’s costs are are $200,000 for whole body cryopreservation or $80,000 for neuro cryopreservation (which is cephalon-only preservation). It seems that it is likely the switching that helps trigger lawsuits, rather than the neurosuspension per se. As a result, some people have argued that organizations should only offer whole body preservation or brain-focused preservation.

Upsides of cephalon-only preservation

1. The cephalon-only approach allows for better access to blood vessels in their original locations. As discussed in the brain-only section, it can be difficult to perfuse the brain’s blood vessels once the brain has been removed from the skull.

2. It avoids the trauma involved in removing an unpreserved/unfixed brain from the skull.

Caveat: For both #1 and #2, how one does the initial chemical perfusion is independent of how the brain is eventually stored. The brain could be initially perfused through the vessels in the neck and only removed from the skull once it is suitably well preserved. This would achieve the technical benefits of cephalon-only preservation in the brain-only preservation method.

3. Cephalon-only preservation retains more tissue than brain-only preservation. Psychologically, there may be an important difference for some between the prospect of brain transplantation and head transplantation, although this might more frequently work in the opposite direction due to the horror/disgust reaction.

4. In cryopreservation protocols, cephalon-only preservation allows for a faster cooling rate than whole-body preservation. Ben Best describes a cooling rate of ~ 1xC/min for the cephalon compared to ~0.2xC/min for the whole body. A faster cooling rate is likely associated with less ice damage during the preservation process. It seems that this is likely beneficial for preservation quality; for example, see Rafal Smigrodzki’s comment about this.

Ben Best has claimed that the cephalon-only cooling rate is fast enough to achieve vitrification, but the whole body cooling rate is not. I’m not sure how strong the evidence for this is and it likely depends on the quality of cryoprotectant distribution to the brain either way.

5. There is possibly less decomposition during stabilization/transport in the cephalon-only approach than in whole-body preservation. Immediately separating the cephalon from the rest of the body has been shown to lead to less postmortem decomposition of the brain (Madea et al., 2017). This may be because this method allows a faster rate of cooling during stabilization. Or because the method prevents bacterial spread from other areas of the body such as the gastrointestinal tract.

Downsides of whole-body preservation

1. On the most basic, intuitive level, attempting to preserve the whole body may lead to lower quality preservation of the brain. How this actually plays out in practice is a somewhat controversial point within the cryonics community, so let’s dive into the details.

There aren’t many direct comparison studies available. One study, (Smith et al., 2015), compared perfusion fixation via the femoral vein to perfusion fixation via the carotid arteries. They found that perfusion of the femoral vein did not yield sufficient formaldehyde perfusion to the brain parenchyma, which was poorly preserved. On the other hand, perfusion via the neck vessels led to high-quality perfusion fixation that was sufficient for downstream studies.

This isn’t quite a fair comparison for whole body vs cephalon-only preservation, because in cryonics, the ascending aorta is what is usually cannulated for whole-body preservation, not the femoral vein. Perfusion via the ascending aorta will definitely lead to better preservation of the brain than the femoral vein, although it is also much more technically difficult to access, requiring a sternotomy. One published brain banking method performed thoracic dissection and perfusion through the aortic arch, with good reported brain preservation, although they had to deal with cerebral edema (Böhm, 1983).

There is an argument that perfusing via the aorta may actually lead to better brain preservation, because there is only a need to cannulate one vessel, whereas isolated cephalon or isolated brain perfusion would require cannulating four vessels (both carotid arteries and both vertebral arteries). However, in practice other factors seem to outweigh this, for example as de Wolf and Platt describe (de Wolf et al., 2019) (p 48):

“Median sternotomy used to be the standard surgical approach for both whole body and neuro cases at Alcor prior to going to isolated head perfusion for neuro patients, and as of this writing is the default approach for all cases at the Cryonics Institute. For neuropreservation cases at Alcor’s facility, isolated cephalon perfusion is now the preferred method because it allows better venous drainage, better monitoring of brain perfusion (venous effluent from left and right jugulars can be measured separately), lower likelihood of pushing clots into the brain in cases of significant pre-perfusion ischemia, faster surgery, faster cryoprotectant equilibration, and decreased perfusate utilization.”

Ultimately, in my view, any focus on non-brain structures is liable to lead to worse preservation of brain structures. It’s important to recognize just how vexing of a problem postmortem perfusion-based preservation is. Even if one is just focused on the brain, getting adequate perfusion to the brain is hard. Trying to perfuse the whole body, which is 50 times larger in volume, has the potential to make things a lot more difficult. As one example, in Alcor case A-1151, attempting to perfuse through the aorta resulted in problems, leading to a lack of perfusate return through the jugular vein. The procedure ultimately had to be converted to a cephalon perfusion, leading to lost time during which the brain was decomposing.

It’s also important to recognize that perfusion-based preservation becomes more difficult as the amount of damage during the agonal state and post-mortem interval increases. As a result, attempting to perform whole-body perfusion-based preservation in realistic cases – as opposed to ideal experimental studies – is going to be even more difficult.

2. Whole-body preservation makes it more difficult to test the quality of preservation of the brain because it makes imaging more difficult. Max More has reported that cephalon-only preservation is more easily compatible with CT scanning without taking the conserved cephalon out of liquid nitrogen.

3. Other downsides of whole-body preservation have already been discussed above, such as the higher cost and difficulty of transport.

Upsides of whole-body preservation

1. Of course, a key upside of whole-body preservation is that, at least in theory, it doesn’t require inference of the rest of the body.

In practice, it is hard enough to preserve the brain at a high level of quality. Preserving the brain and the rest of the body to a high level of quality is even harder, and the rest of the body may not be preserved well. And if it is not preserved well, then there is a question of whether there is much of a value add over alternative approaches, especially considering the costs.

2. Perhaps the main upside of whole-body preservation is that it is more consistent with the ritual practices of the rest of society.

As (Krüger, 2010) describes:

“In my view, in explaining the comedown of cryonics we should also take into account the lack of ritual, or rather the incompatibility with the common ritual tradition of funerals in the United States, and the violation of its own ideological preconditions. The latter point refers mainly to the neuropreservation, carried out by Alcor. Beheading a dead person severely violates the cultural need for the preservation of the whole corpse.”

Some people in the cryonics community go so far as to attribute the relative lack of interest in cryonics in society to the fact that Alcor offers cephalon-only preservations. It’s hard to quantify exactly how important this is, but in my opinion, this is a fairly strong take relative to the available evidence.

Brain-privileged whole-body preservation

Related to whole-body preservation is an option where one type of preservation is used for the brain and a different type of preservation is used for the rest of the body. The idea is to do whatever is necessary to preserve the brain with high quality, while also attempting to preserve the rest of the body as a secondary goal.

In terms of practical techniques, probably the most brain-privileged approach would be to extract the brain from the skull or separate the cephalon at the neck and perform the preservation of the brain separately from the rest of the body.

Why is it necessary to privilege the preservation of the brain? For almost all methods of brain preservation, there are likely advantages for brain preservation quality if the brain is the main focus. For example, the cooling rate achievable is much faster in cryopreservation if the cephalon is separated from the rest of the body.

One way to accomplish this would be to perfuse the brain and just cryopreserve the rest of the body without cryoprotectants. But if you’re not going to actually do the cryopreservation of the rest of the body, there is a question of whether there’s much of a point to doing it at all? The other organs and body parts would likely require significant amounts of repair and/or replacement regardless. In part, this discussion relates to the quality of cryopreservation that’s achievable in non-brain tissues without cryoprotectants. This likely depends on the tissue type and is largely unknown.

The main problem with brain-privileged whole-body preservation is cost. Given unlimited resources, there is little downside to brain-privileged whole-body preservation.

What do most people prefer?

Market data – what people choose in practice – seems to suggest that most people prefer whole-body preservation. As of this writing, the cryonics organizations Cryonics Institute, TransTime, and Osiris only offer whole-body preservation.

In a survey of cryonicists (n = 316) conducted in 2017 by (Swan, 2019), the majority of people preferred whole-body preservation to neuro preservation. However, the difference was not very large:

  1. Whole-body 53%
  2. Neuro 39%
  3. Depends 16%
  4. Leave to discretion of cryopreservation provider 2%

In this same survey, these were the factors that helped people make the decision:

  1. Cost 48%
  2. Perceived technical feasibility 46%
  3. Time to revival 19%
  4. Legal considerations (personhood status of whole-body versus neuro) 16%
  5. Ability to have cryopreservation facility determine what is best at the time 15%
  6. Social acceptance 10%
  7. Recommendation of others I trust 9%
  8. Choice of other persons enrolled 3%

At Alcor, which offers both whole-body and cephalon-only (“neuro”) preservation, data from 2019 and 2020 suggest that most people who are preserved are choosing cephalon-only over whole-body. In 2019, the ratio was 10 cephalon-only preservations to 1 whole-body preservation. In 2020, the ratio was 4 cephalon-only preservations to 2 whole-body preservations. One organization, Oregon Cryonics, only offers brain-focused preservation (cephalon-only or brain-only). As of Dec 2022, Tomorrow Biostasis offers whole body or brain-only preservation.

Summary table

Brain-Only Cephalon-Only Whole Body Brain-Privileged Whole Body
Average volume 1200 cc 3900 cc (calculated from head volume in Table 2) 62000 cc 62000 cc
Volume relative to whole body 2% 6% 100% 100%
Peripheral nervous system present None Cephalon only (e.g. aspects of cranial nerves) Whole PNS Whole PNS
Inference of non-brain body required Yes, whole body Yes, non-cephalon parts Likely, depending on degree of damage Likely, depending on degree of damage
Hazardous/infectious exposure Yes Yes Yes Yes
Ability to move/store conserved brain Easiest Easier More difficult More difficult
Brain banking infrastructure in place Yes No No No
Ability to donate organs Yes Yes, slightly more difficult May still be possible, but much more difficult/lower quality May still be possible, but much more difficult/lower quality
Horror/disgust reaction Minimal Yes No Potentially yes, assuming the head is still separated
Faster cooling rate? Yes Yes No Most likely, depending on procedure

Conclusions

There are some technical differences between the four options for preservation that may favor brain-only or cephalon-only preservation. However, these differences are largely mitigated by the option of brain-privileged whole-body preservation. The main remaining trade-offs relate to cost and available resources.

One perspective is that brain preservation is not easy and choosing brain preservation at the time of one’s legal death is not for the timid. It has been likened to attempting to escape from a burning building with a rope of knotted towels and bedsheets. It has also been likened to jumping out of a crashing plane with an experimental parachute. It’s not a good option, but for some people the alternative is worse.

With this perspective, there is a need to be practical about what is essential and what is not. For some, that means that one of the brain-focused preservation options may be the best choice; for others, it will be a whole-body option or bust. My personal opinion is that the brain is all you need. This may feel like a heavy choice, but even if you don’t choose brain preservation, you or your loved ones are going to have to make some difficult choices about what to do with your body after you die. So in a sense, it’s normal for this to be a hard decision.

Endnotes

Brain parts

As a more speculative part of this essay, here I discuss a little bit about which parts of the brain might be more important to preserve. First, it’s important to discuss brain biopsies. This is a surgical procedure in which a small area of the brain, often approximately 1 cubic centimeter in volume, is removed in order to determine the etiology of a condition (Hawasli et al., 2013). Generally, brain biopsy itself has not been found to cause long-term neurological sequelae (Schott et al., 2010).

This is not surprising because the aspects of the brain that are responsible for cognitive functions tend to be widely distributed across the brain and are therefore robust to the removal of a small part. For example, for long-term memory recall, the experiments of Karl Lashley in the early-mid 1900s showed that the size, but not the location of damage, to the cortex of the brain was associated with memory loss in mammals (Josselyn et al., 2017). In the 1940s, Hebb reported that the destruction of large amounts of cerebral cortex in humans also led to little effect on memory (Josselyn et al., 2017). This has practical importance in brain preservation, insofar as I believe it is essential to take very small biopsies of brain tissue in order to query preservation quality; otherwise, the whole procedure runs the risk of the “no feedback problem.”

Based on recent neuroscience studies, we can say more about the location of stored memories than simply “the brain.” Evidence suggests that memories are initially formed via neural activity in the hippocampus and nearby regions. Over time, memory consolidation seems to spread the information for these memories into areas of the neocortex. However, hippocampal activity may still be important for detailed recollection of the memories (Josselyn et al., 2017). So, very crudely, for declarative memories, the most important areas appear to be networks and circuits found primarily in the hippocampus and the neocortex.

For non-declarative memories, networks stored in other brain regions may be important. For example, the cerebellum likely plays a role in the memory of certain learned reflexes, such as the timing of sensory-motor habits (Eichenbaum, 2016).

The corpus callosum is the largest region of white matter in the brain, which primarily contains myelinated axons that connect the two hemispheres of the brain. This can be cut during a corpus callosotomy, which is a surgical procedure designed to treat certain types of severe epilepsy. What people have found in studying the outcomes of corpus callosotomy procedure is that people do not have significant loss of their already formed memories or personality traits, although they can have deficits in forming new memories or in accessing newly-formed memories (Zaidel et al., 1974). One might imagine that the deficits in memory consolidation could be repaired, or if they are not able to be fully repaired, replaced with a species-generic or improved type of memory consolidation. This is also relevant to brain preservation because certain ways of delivering preservative chemicals may involve damage to the corpus callosum.

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