As a “real man” I don’t ask for directions when I am lost or use a GPS. But my wife does. Sometimes we are out together with her GPS in operation and decide to ignore the advice of the device. The device will have none of that and helpfully try to reroute back to its preferred route. “Recalculating,” it tells us, often repeatedly, until we turn it off or finally go on a route we all can agree on.

This is a brief followup to my previous post Temporal Resolution of Conscious Experience.

If the lag time in pathway from the thalamus to the cortex is critically related to consciousness, there might some insight we can gain from multithreading in computer software. Multithreading in Java allows a single process (the Java Virtual Machine) to take advantage of multiple CPUs by allowing concurrent execution of different parts of a program. Typically I have used it when different parts of a program can execute independently – for example, updating multiple records (accounts, orders, etc) when the records are unrelated. However, the same technique can also be used for a complex series of computations if the overall computational task can broken into pieces.

A typical problem in multithreading is coordinating the threads. The simplest problem is knowing when all the threads are finished – all the records are updated so the program can terminate. A more complex coordination problem might involve waiting on the results of some threads before proceeding with next steps which might involve dispatching new threads. One way of doing this is to have a “monitor” thread (which could be the main thread) that tracks the other threads, accumulates the results, and takes appropriate actions.

You may see where I am going with this, but before I get there, let me clear about one thing. I am not arguing here the brain or consciousness is a computer or computer-like. It may, in fact, have computational elements but that is a different discussion. Here I am only using the concept of multithreading in software as an analogy.

The brain has millions of neural circuits that must operate in parallel. This would create problems similar to those that happen in multithread programming with perhaps similar solutions. Some of the circuits would be likely dedicated to monitoring the other circuits. With a lag time in these circuits, there would be no guarantee that all of the circuits required for any concerted action would complete at the same time. This would necessitate that there be some mechanism of accumulating and temporarily persisting the results of some circuits while other circuits completed. Could these mechanisms explain much of what consciousness and qualia are about? Is our brain constantly “recalculating” to reach agreement on its route?

Posted in Consciousness, Human Evolution, Time | 6 Comments

Temporal Resolution of Conscious Experience

An interesting article Coupling the State and Contents of Consciousness has me thinking of an idea that has been in the back of mind for a while – the temporal resolution of conscious experience. If different stimuli (different color flashing lights for example) are presented too quickly, they will either tend to fuse into one or, depending on timings, the second stimuli may mask the first. Research suggests that consciousness operates somewhat in frames with a lag time of somewhere between 50-100 milliseconds or more.

Before I get to that topic, however, let me discuss a little about the paper itself.

While most consciousness studies study state (wakefulness, sleeping, dreaming) or content of consciousness, this paper tries to unify state and contents and link both to activity of the cortical layer 5 pyramidal (L5p) neurons. “These neurons affect both cortical and thalamic processing, hence coupling the cortico-cortical and thalamo-cortical loops with each other. Functionally this coupling corresponds to the coupling between the state and the contents of consciousness.”

The most basic definition of consciousness involves wakefulness. A person in a coma or passed out from drugs or alcohol is not regarded as conscious. We could debate whether someone dreaming or in deep sleep is conscious, but the important point is that the state of consciousness is controlled by the reticular formation with its projections into the thalamus. Damage to cells in these areas will result in permanent coma. Unquestionable these areas of the brain must be intact for any consciousness to occur.

However, one cannot be awake and conscious without being conscious of something and vice versa. “One cannot be conscious of the coconut taste while being in an unconscious state. And the other way around: in typical healthy subjects, one cannot be in a conscious state while not being conscious of anything at all. In other words, contents of consciousness and states of consciousness make up an integrated whole. Studying one while disregarding the other can only provide half of an answer.”

Certainly most of the content or “action” of consciousness in mammals, however, occurs the cortex or outermost layer of the brain. The cortical layer 5 pyramidal (L5p) neurons are a type of neuron that links the thalamus to the cortex.

The paper makes two noteworthy related predictions:

  1. Cortical processing that does not include L5p neurons will be unconscious. More generally, the present perspective suggests that L5p neurons have a central role in the mechanisms underlying consciousness.
  2. Cortical processing in itself, when not integrated with the non-specific pathway thalamic nuclei via L5p neurons, is not conscious. In particular, feedforward cortical processing, where information is mainly flowing within the cortical superficial layers bypassing thalamocortical neurons, is non-conscious.

The paper cites several studies linking L5p neurons to states of consciousness and the contents of perceptions.

This paper is only one I have seen in recent years that has attempted to link with this degree of specificity a type of structure in the brain to consciousness and make what seem to be testable predictions. I think it is noteworthy in that regard. Probably someone will point out others and I can revise that statement.

The paper makes an additional observation regarding the temporal resolution of consciousness. Since “conscious perception is based on the processing within the thalamocortical loop, then it is hard for conscious perception to resolve anything that happens faster than the processing time of this loop. In other words, we claim that the temporal resolution of conscious experience stems from the propagation time between the L5p neurons, NSP [non-specific pathway] thalamus and higher cortical areas.” It also tries to explain backward masking where a second stimulus masks the first one from consciousness by this same lag.

This lag in processing has been on my mind for a while. In 2012, I wrote:

The fact is that everything we perceive really is phantasma. The red of the rose is not real. It is a particular wavelength of light. The sound of the distant thunder is not real. It is an acoustic wave moving through the air. Solid objects don’t really exist. We might kick a large rock and we might hurt our foot but physics says the rock is mostly empty space and the pain in our foot is the product of a nerve impulse. Our experiences are all in the past, delayed by a neurological time-lag and assembled into a coherent whole bearing perhaps no resemblance to what is actually “out there” in the world.

What if this time-lag isn’t a feature of consciousness but is the explanation for it?

Consciousness is what emerges to assemble experience into a whole because the pathway from the thalamus to the cortex takes time. What’s more, the bigger the brain, the longer the pathway, the longer the lag time, and… can we assume the more consciousness would be required. Does it even make sense to talk of more or less consciousness? I’m not sure. In this view, consciousness emerges as brains gets larger, not because there are more neurons or more information processing happening. It emerges as a remedy or fix for the lag time in the thalamocortical loop.

Is this idea original? Probably not.

It will be interesting to see if the predictions of this paper work out.

Posted in Consciousness, Human Evolution, Time | 5 Comments

The Mystery From 70,000 Years Ago

About 70 thousand years ago something happened that resulted in modern humans. Whatever caused this change, it resulted in what appears to have been a major leap in cognitive capabilities. It was quite possibly the change that most differentiates us from Neanderthals and other archaic humans and what has made us unique among hominid apes.

While anatomically modern human skeletons can be found dating to 200,000 or more years ago, we do not find broad evidence of sophisticated tools, artwork, and ornaments until around 70,000 years ago in Southern Africa. This period is just before a migration of humans first from Southern Africa to East Africa and the later migration of humans, which can be genetically linked to all modern humans, from East Africa to the rest of the world. We have multiple lines of evidence – linguistic, paleontological, and genetic – pointing to humans who are the ancestors of modern humans from this time and these places. In addition to better capabilities to survive and exploit any ecological niche, quite possibly these humans had superior weapons, greater organizational ability for warfare, and possibly even a greater propensity for warfare. I believe they may have lived in larger groups than other humans at that time. In the next 30-40,000 years these humans displaced other archaic humans, killed off the megafauna, and spread rapidly to Asia, Europe, Australia, and eventually the Americas. In the process, humans proved themselves able to survive in almost any environment from the cold reaches of the far north to the heat of the tropics, from deserts to rain forests, from mountains to the ocean.

The secret to this change could not have been a larger brain. By the time of appearance of the first anatomically modern humans, brain size was already the same as modern human brain size. For that matter, the brain size of Neanderthals was larger. This change occurred without a major change in anatomy so it must have been relatively subtle. We might suspect the change to involve language capabilities but that doesn’t resolve what exactly triggered the change to come about. Was it a random language gene mutation? Or was it some broader change that might affect a range of cognitive abilities?

We are now getting hints that the change that resulted in modern humans is linked to changes in the maturation rate of the prefrontal cortex (PFC). The change is a delay in the time required for the PFC to fully develop. This extended period of development meant that synaptic connections formed over a much longer time period and, we might logically assume, formed during extensive social interaction.

Evidence from a 2012 study Extension of cortical synaptic development distinguishes humans from chimpanzees and macaques suggests that a delay in the maturation of the PFC is a key feature that distinguishes humans. The contrast in development time of the prefrontal cortex between humans and other apes is quite remarkable. The time of peak expression of synaptic genes in the PFC is less than one year in chimpanzees and macaques, but around five years in humans. Although there may have been a gradual lengthening in PFC maturation during earlier human evolution, the study authors believe the major change occurred sometime after the human Neanderthal split. The study suggests, in addition, that it was likely Neanderthals had a maturation time more closely resembling apes than humans. This provides more evidence that this PFC maturation delay may be key to human cognitive abilities and behaviors.

Isolating the change in PFC maturation to a time around 70,000 years ago is a paper by Andry Vyshedskiy.

Our research into evolutionary origin of modern imagination has been driven by the observation of a temporal limit for the development of a particular component of imagination. Modern children not exposed to recursive language in early childhood never acquire the type of active constructive imagination called Prefrontal Synthesis (PFS). Unlike vocabulary and grammar acquisition, which can be learned throughout one’s lifetime, there is a strong critical period for the development of PFS and individuals not exposed to recursive language in early childhood can never acquire PFS as adults. Their language will always lack understanding of spatial prepositions and recursion that depend on the PFS ability. In a similar manner, early hominins would not have been able to learn recursive language as adults and, therefore, would not be able to teach recursive language to their children.

I think linking the delay in maturation of the PFC to modern language capabilities makes a lot of sense as well as tying recursion to the broader PFS capability. We know children not exposed to language at critical times in development do not ever acquire good language skills. Likewise, we know children exposed to multiple language from an early age learn to speak as natives and that the older we are before exposure to a new language the harder it is to pick up the language. Vyshedskiy himself points to individuals with PFS deficits associated with damage to the PFC. Since our best estimates for origins of modern languages and modern human behavior as expressed by artwork and ornaments tie to the same time, it makes sense to link all these things together.

Vyshedskiy goes on to hypothesize a somewhat complicated scenario that I won’t get into about how all of this came about around 70,000 years ago.

I think the picture how all of this came about might be straightforward.

Let me speculate that sometime early into last glacial period, perhaps about 100,000 years ago, numbers of humans migrated to Southern Africa in response to climatic changes. There they developed stable, non-nomadic settlements based extensively on shellfish. Settlements such as that would almost be prerequisite for allowing for a slower PFC development. We have evidence of widespread consumption and transport of shellfish from this location. Shellfish are rich in iodine and fatty acids which increase dopamine activity. Dopamine is heavily involved in the PFC and would be essential for reinforcing PFC reward-based social learning. At that point, seemingly the stage would be set to allow a gradual increase in PFC maturation time. The increase in maturation time could have been caused either by a single mutation or perhaps even by preexisting but initially not widely distributed genetic traits. In an environment such as this, feedback from the social learning from one generation to the next could have slowed the development of PFC to allow for additional social learning. Outgrowth of the social learning would be new cognitive skills for language, imagination, art, social control, and behaviorally modern humans.

Posted in Brain size, Human Evolution | 3 Comments