How Thought Propagates Part 2

I believe that consciousness and working memory are driven by the reciprocating cross-talk between fleeting bottom-up imagery in early sensory cortex and lasting top-down priming in late association cortices. The images below attempt to explore how this reciprocal interaction works.

Diagram 1: Each concept held tonically active in association cortex is represented by a letter. The order these letters are pulled into consciousness arbitrarily follows alphabetical order. 1) Shows that concepts A,B,C,D and E all diverge onto posterior visual sensory cortex. 2) Shows that neurons involved in the retinotopic imagery from figure 1 converges on the association neurons responsible for holding concept F. It also shows that concept B drops out of activation, and that A, C, D, E and F diverge back onto visual cortex. 3) Shows that this same process leads to G being activated and A being deactivated in association cortex. 4) Shows that H is activated and that E is deactivated.

Diagram 2: Sensory areas can only create one sensory image at a time, whereas association areas are capable of holding the salient or goal-relevant features of several sequential images at the same time. In other words, neurons in association cortices can remain tonic for a number of seconds, making activity there progressive because it can reflect the most important aspects of several successive instances of sensory activity. Dopamine is key to this progressive process because it helps to determine which features are relevant/motivating. This process can be described by the following list:


1)  Sensory imagery is made in early sensory cortex. This is either a product of feedforward sensory information from sense receptors or from downstream retroactivation.


2)  Salient features are extracted and communicated with higher association areas


3)  Salient features that cohere with already active features in association areas (that are consistent with the present set of concerns) are added to the active features there, whereas the least relevant features in association areas are dropped from activation.


4)  The important features of the last few images are maintained active in association areas.


5)  The new set of features is handed down to early sensory areas.


6)  This new set of features is used by early sensory areas to construct topographic imagery. In reality, sensory cortex from each sense modality feeds their inputs to association cortex (and to each other).


7)  The process starts over.


A few months ago I thought to myself that if the association cortex could be protagonized what it has to say to earlier sensory cortex would be along these lines:

“Ok, I see the imagery that you are holding now and have used it to update my own list of relevant concerns. Now, I believe that this updated list of concerns will change our perspective on the last few thoughts. Take this new concern, along with these other concerns that we have recently been handing back-and-forth and do your best to portray them all simultaneously in a single image. Then the image that you build will pass up our hierarchically-structured interface updating me of the goal-relevant concerns that your imagery was able to automatically identify”

In other words, the association cortex is always saying the following to sensory cortex: “Ok gotcha, well then what about this, can you rapidly create an informative topographic image with these specifications?” And the early sensory cortices play along by cooking up the best image they can to corepresent the constructs handed down to them. For more information see: "How Thought Propagates Part 1."

The way memories are recorded and reused is analogous to the way tracks of water form on a window pane

New droplets fall into the paths of old droplets

On a rainy bus ride in 2004 I was staring out of the window, hoping to push myself into thinking up a novel conceptualization of the neurological basis of memory. Looking up from an article on axonal migration, my attention turned to the tracks of water that had formed on the window. You know how they look. Water droplets leave trails of water behind them as gravity pulls them down the pane. Early drops form trails that subsequent drops fall into and travel within. Of course these are trails of water on glass, whereas memories are recorded in tracts made of neurons and their projections. But the analogy felt substantial to me, and I think that it can be carried at least a short way before it fails.

The qualities of the surface of the glass and the placement of the water droplets on the glass determine how the drops will travel. This is similar to how the current physical state of the brain and the locations of the activations therein determine how neural pathways are selected. During every perception, cognition and experience old neural pathways are being retraced (memories, familiar perceptions) and new pathways are being created.

Some water droplet's trails extend long distances and subsequent drops fall directly within their borders. Depending on dirt and smudges on the glass other trails dry up before they extend very far and when a subsequent drop reaches this point, it is free to delimit its own path. Similarly our memories and experiences involve the activation of neural trails some of which are only small snippets of the original encodings.

Altering synaptic weights among groups of neurons creates patterns in the nervous system that are stable and that serve as preexisting pathways that constrain the future propagation of nervous energy. In this way, previous experiences imprint stable conduits that act as barriers for subsequent stimulation, thus the activation patterns elicited by new stimuli will not be random, they will be constrained by past activity. This accounts for how humans respond to disparate situations in the same inflexible ways, it accounts for how stubborn and resilient our personalities are, and accounts for how most of our actions are simply pastiches of previously used techniques. Consciousness and higher-order thought really are composed of multiple stable states organized in different configurations. In this way, consciousness is akin to the entire window and its current streaming activity.

This perspective feels limiting and deterministic, but it is important to point out that our window is huge and that we should stimulate and push ourselves to increase the number and complexity of stable states in our repertoire ensuring that our pane of glass has many possible tracks. Also, the pane of glass on the bus sits passively as water streams down its side and creates the pathways. Humans, on the other hand do not have to have sit passively and be written upon. With the right environmental provocations we can intentionally seek out activities that will influence the creation of new pathways.

I had always wondered how memories, thoughts and behaviors were made possible by something physical. I wondered how alterations in the connections between brain cells allowed mental representations that could remain stable over days or years. Like the water droplets, neural energy finds the path of least resistance, and information in the form of action potentials is pushed down those avenues that are best facilitated by the synaptic weights in the network. The analogy here is very simple. Today it seems so obvious as to be trivial, but I remember it being powerful and instructive for me at first consideration.

How The PFC Leads to Behavioral Inhibition

After hitting snooze this morning I had a dream that I was reading a Wolverine comic written by Chris Claremont and drawn by John Byrne. In the comic, Wolverine was confronted with a villain he was in an argument with and he was being provoked. The dream’s narrator said something along the lines of: “Wolverine had an inclination to unsheathe his claws and cleave his foe in two that moment but another impulse, to remain composed and noncombative outlasted it. This second impulse subdued the first. It won out merely because it remained firing in his brain for just a moment longer than the first.”

The PFC has cells with higher-order receptive fields that can help the brain to conceptualize abstract things like long-term goals, cost-benefit analyses and the repercussions of risky acts. Often its instructions are the opposite of what our lower-order brain systems try to impel us to do. The PFC also has cells that fire for longer. Could it be that simple? In Wolverine’s case above, could that instinctive impulse to attack have been impelling and strong but temporally more fleeting than the one that stayed his hand? If that hypothetical inhibitory ensemble in his PFC would have stopped firing earlier would he have engaged?  In humanity’s case could our culture and history be due to the fact that the brain areas that hold the more complex representations fire for just a few milliseconds longer, enabling the more complex behavioral instructions to narrowly outlast the more base ones?

How Are PFC Neurons Tuned to High Complexity?

Why are the receptive fields for neurons in the PFC so “complex?” Is it simply because they normally fire for longer and thus have been able to become more broadly tuned to temporally discontinuous occurrences? If this is so, PFC neurons then would be like listeners who were able to overhear an entire lecture, instead of just snippets from it. Surely the longer a neuron fires the more time it has to “fire together and wire together” with other neurons. The high tonicity found in PFC neurons, combined with their high connectivity throughout the brain, must lead them to become maximally excited by events that do not always occur together immediately in time. In other words, their prolonged activation allows them to capture time-delayed pairings unlike other more transiently active neurons that can only capture simultaneous pairings.

Identifying the Link Between Creativity and Madness

I feel like recently I have started to put my finger on that elusive ‘link’ between creativity and madness. I think that madness may boost some forms of creativity by freeing up associations that a sane person’s subconscious would not allow them to entertain. The main point is that hyperfrontality, saneness, or the opposite of madness, is restrictive and actually inhibits some forms of creativity and the dark horse associations that can underlie it.

For a long time I have been reading about the associations between creativity and madness. I agree with the notion that madness, schizophrenia, some forms of psychosis and some drug induced states may be conducive to a certain brand of creative energy, but I have not been quite able to understand exactly why. Coming home on the bus today, I related the concept of lateral inhibition to the problem, and in doing so, contrived a plausible solution that has begun to satisfy my curiosity about the subject.

The mind of someone with schizophrenia is altered in a way that makes them process things differently. For them there is less continuity over time. When we think, we hold a number of different concepts in our association areas at any one time. Neurons in association areas and especially in the PFC remain active for a few seconds at a time, allowing their content to persist. This makes it so that some concepts are sustained for extended periods allowing them to affect subsequent thoughts. This process keeps human thought on a tight and narrow track. What it really does is it allows people to have precise thoughts, where each thought is very closely related to the preceding thought. Clearly this is the only way that planning can happen, if you continue juggling the same concepts without dropping the ones that have just come into play. Just think, if more nodes of activation are conserved during the transition between the present thought and the next thought, the two will be very closely related conceptually. If a sequence of thoughts are very closely related, there is little room for wild additions. This keeps sane people from injecting new conceptualizations and non sequiturs.  In schizophrenia these PFC and association area neurons cannot persist for as long (cannot maintain tonic firing) and thus the thought process is continually derailed by tangential (or more emotional and less cerebral) associations. Importantly, the driving element(s) may fall out of the equation and leave room for unexpected, unpredictable substitutions.

When you have a group of concepts that coactivate together and that keep each other mutually active, they inhibit “lateral” concepts and this puts specific limits on where the train of thought can go next. When you think about walking down the street and ducking to grab a dollar bill certain neurons responsible for representing the verb ducking laterally inhibit the noun duck corresponding to the bird. This makes it so that conceptualizing the word dollar bill in this context does not automatically evoke a picture of a duck bill. Lateral inhibition puts constraints on the associations that the unconscious can draw from. The brain does a fantastic job of inhibiting lateral concepts, which is good because it keeps us from making faulty associations. The drawback; however, is it limits the capacity to creatively insert a new concept in to that mix.

When you are constantly dropping association assemblies, when you continually cannot remember what you were just thinking, a new association may have the “relative momentum” to carry the train of thought off on a novel tangent. Because you may, now all at once, be missing the main theme, peripherally associated concepts can end up entering the mix - specifically because they may cohere well with what is left after the earlier ones have dropped out. So what you end up with, if you are very lucky, is something novel. They are based on true associations that are good, but are usually kept from entering the mix. Going from a dollar bill to a duck bill is absurd in some contexts but potentially innovatory in a poetic, artistic or comedic context.

Of course, creativity does not only come from madness, it can also come from its opposite. Higher-order thinking patterns can memorize creative algorithms and apply them when they deem appropriate. In these instances creativity is less spontaneous, more contrived, but also more “volitional.” Sometimes I catch myself consciously applying certain rules to my thought, attempting to infuse creativity into my thinking or attempting to think outside the box. As I see it there are two paths to creativity, more conscious ones and  less conscious ones.

Both 1 and 2 represent a train of thoughts. In example 1 a normal person thinks about A, then B, then C. Even though A was the first thought, some of the neurons responsible for it are still firing and so it influences what comes after C. In other words, the thought that comes after C is influenced by the coactivation of A,B and C. Coactivity is indicated by the box encompassing A,B and C. Example 2 is supposed to represent someone who, because of schizophrenia, dementia or PFC damage cannot keep the third-to-last thought (A) active. Here the fourth thought, E, is not the same as D because it was not influenced by thought A at all, it is only influenced by B and C. Thus, if previous thoughts drop out of the equation early, attention is not as narrowly focused, possibly leading to the creation of new, novel thoughts.