Terry Sejnowski points out the brain does not oscillate it instead has waves that travel.
He also talks about a globally integrated workspace. @Bitking
I have always thought of oscillations in this way. Is this a new way of thinking in neuroscience?
I like the analogy that a stimulus is like dropping a pebble into a pond and the ripples is its history spreading outward. The interference pattern of waves is also interesting. Depending on the order in which the different stimulus arrives, the interference patterns differ. It is like the interference pattern is the signature of what happened in that period of time.
Iāve been wondering about these wave-like patterns since I Blue Brain Project released their videos a few years back. https://www.youtube.com/watch?v=3Fmsq_Qg7Nw&t=260s
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Thanks for the link. It was very helpful. I have been hoping for more news related to traveling waves, including simply made information containing spirals described towards the end of this opening post:
With so much new information coming in so fast Iām hoping to save time by not guessing where to next go with the episodic memory and instead do my best to keep up with all the developments that are making it easier to add more detail to the model.
My best to everyone. Keep up the good work!
What I donāt understand is that it seems like there is loads of activation going on here. The level of activity in those waves goes against the idea that activity is sparse in the cortex. I did read somewhere that the lower layers are sparser than the higher layers which matches what I heard somewhere else that the frequency of activity is slower in the deeper layers.
Maybe these waves occur only in higher layers?
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Layer 6 is very sparse, but layer 5 is less sparse than L2/3. According to one study, layer 5 alone is sufficient and necessary for oscillations. I think it makes sense for oscillations to originate from a less sparse group of cells because that is more of a map level function or something like that, as opposed to processing sensory contents. So you donāt want much sparsity.
Source for layer 5 generating oscillations: Intrinsic oscillations of neocortex generated by layer 5 pyramidal neurons (Laurie R. Silva, Yael Amitai, and Barry W. Connors, 1990). Sorry, I canāt find a free version.
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What do you think the role of these oscillations are? Many believe they are used as attractor states for memory retrieval.
Interesting, Iāve just read on wikipedia that āliquidā in āliquid state machineā is used as the analogy similar to what the speaker used to describe travelling waves.
The word liquid in the name comes from the analogy drawn to dropping a stone into a still body of water or other liquid. The falling stone will generate ripples in the liquid. The input (motion of the falling stone) has been converted into a spatio-temporal pattern of liquid displacement (ripples).
Interesting coincidence, or did the speaker draw that analogy from ML to neuroscience?
I think they all started with a pebble in a pond and went from there.
I donāt know much about attractor states, so I donāt know if oscillations could be involved in memory retrieval.
I think oscillations are involved in coordinate transforms, and probably other things.
Interference between three oscillations with slightly different frequencies based on walking speed and direction has been proposed to contribute to forming grid cells. This is not proven but it makes sense, especially if walking speed is controlled by oscillation speed in the first place.
My current opinion is that L5 does not process sensory contents and is instead concerned with something like which cortical columns are receiving sensory input, the shape of that sensory input, attention, timing of sensory input (which was proposed by Jeff Hawkins), etc. Iām working on a hypothesis based on that for grid cell formation with propagating waves which solves some of my questions about layer 5.
Questions about L5
My hypothesis only suggests answers to some of these questions.
- Why does one L5 cell type have robust sensory responses with direct thalamic input (at least in some regions) and project up the hierarchy, suggesting a sensory role, but also supposedly acts as the only cortical motor output, suggesting a role in behavior?
- Why does it seem not driven by thalamus in some regions? Why does it supposedly mirror L2/3 as an output layer and why does it receive strong seemingly minicolumnar input from L2/3?
- Why does it have smaller short latency receptive fields and larger intracortically driven longer latency receptive fields?
- What is the role of burst firing, and does it even exist in normal function, or perhaps it exists in a less extreme form? Do L5 slender tufted cells really only fire much during behavior, and if so, why?
- Why are there contradictory results about the influence of the apical dendrite?
- Is there a third fundamental L5 pyramidal cell type and what is it like?
- Why are there contradictory results about the influence of input from a higher order thalamic nucleus, POm?
Generally, I think L5 processes details of how the sensory input begins, such as the sequence in which different parts of the fingertip make contact when poking a surface. Propagating waves are useful for detecting sensory onset dynamics because they occur at the level of the cortical sheet and are useful for precise timing.
Details of sensory input onset is another source of information besides the static sensory input after making contact with the object, but it is ignored by HTM.
Edit: Via another area, hippocampus can modulate walking speed. Iāve only read the abstract.
Theta oscillations regulate the speed of locomotion via a hippocampus to lateral septum pathway (Franziska Bender, Maria Gorbati, Marta Carus Cadavieco, Natalia Denisova, Xiaojie Gao, Constance Holman, Tatiana Korotkova, and Alexey Ponomarenko, 2015) Theta oscillations regulate the speed of locomotion via a hippocampus to lateral septum pathway | Nature Communications
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Scientists have proposed numerous possible roles for brain waves. A leading hypothesis holds that synchronous oscillations serve to ābindā information in different locations together as pertaining to the same āthing,ā such as different features of a visual object (shape, color, movement, etcetera). A related idea is they facilitate the transfer of information among regions. But such hypotheses require brain waves to be synchronous, producing āstandingā waves (analogous to two people swinging a jump rope up and down) rather than traveling waves (as in a crowd doing āthe waveā at a sports event).
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If the sub-cortical structures use a wave to coordinate the activities of an area of cortex is that somehow more profound than if it just blinked on and off like a Christmas tree bulb?
I could easily see the wave as an artifact of generating this alpha activity in the cortex.
Are there any neurological theories that use this wave-like pattern of activity to do anything useful?
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It seems thereās a significant link to self-organizing-maps when it comes to waves. If neurons close to each other represent similar features then waves are a product of input stimulus. The gif below shows how a bar of light gradually changing orientation causes waves in visual cortex as neurons close to each other represent slightly different orientations.
Taken from Lessons from Studies of Orientation and Direction Preference
That is, of course, if this is the same kind of waves they are referring to?
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I donāt think that would be different if the waves are subcortically generated, but as I understand it, propagating waves can be generated by sensory input intracortically. The way I picture it, a sensory input to, say, the fingertip causes a bump of activity on the corresponding point on the cortical sheet, which includes interneurons in that point on the sheet, inhibiting adjacent cells including interneurons, disinhibiting slightly further pyramidal neurons and also directly exciting them. The pattern continues from there, so the wave spreads.
What interests me is that propagating waves probably are what generate longer latency responses to parts of the receptive field further from the thalamus-driven center, at least in barrel cortex L5. Based on what I know about L5, it cannot serve a purely behavioral role, but it also isnāt well suited for sensory processing. That contradiction left me without a clue about what L5 does for a while. The longer latency responses were the clue which led to ideas about what L5 does. So propagating waves kind of underlie most of my opinions right now.
Longer latency responses to stimuli further from the center of the receptive field suggest a type of sensory processing which is not concerned with things like texture or other fine details of an objectās surface, but instead concerned with positions of things in space or on the sensor over time. Besides the need to generate a location signal, thereās a lot of information available from the shape of the objectās surface on the sensor and how it evolves over precise time scales.
At some level, the brain must treat these two types of sensory information as separate things, just as it treats sensory input with an external cause differently from sensory input caused by behavior.
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I was thinking more of the coordinated global activity like Terry Sejnowski is describing here at time index 16:00:
I think that this activation derived from sub-cortical structures.
I feel like you are describing a different activity more like what is shown at time index 8:00:
I suspect that this bias activity is due to lateral connections within the cortex. And yes - SOM does need to boost cells around the āwinningā cell so this would tend to support that interpretation.
Another idea is that at different phases of the wave, different parts of the process are occurring. Perhaps there is an āinferenceā part of the wave and a ālearningā part.
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If you check out the āthree visual streamsā paper they have temporal predictive cells based exactly on that principle.
They break the wave into PLUS and MINUS phases where the plus phase is the upper layers forming an opinion about the āground truthā of sensation and the minus phase (at the end) comparing a prediction in the lower layers to this ground truth.
The plumbing involves a pass through part of the pulvinar but that does not materially affect the basic mechanism of using timing of the wave to do temporal prediction.
BTW: This particular pulvinar based/predictive mechanism is part of the only plausible scheme that I have seen that accomplishes the long sought goal of a biologically plausible back-prop behavior. If you are interested in this topic you owe it to yourself to do the hard work of reading the paper and references. Some very good stuff going on there.
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That second video is basically what Iām talking about, although itās a bit different because what Iām talking about is partially suprathreshold, but itās roughly on the same scale.
In barrel cortex, mostly itās subthreshold, especially for cells in barrels corresponding to whiskers several whiskers away from the stimulated one, but cells still fire in other barrels than the stimulated whisker.
Source: Europe PMC
Maybe thatās a different phenomenon than found in the experiment described in the second part of the video you mentioned (8:00), or maybe itās because V1 expects to normally have some visual input everywhere and if there isnāt something next to a feature, it knows there isnāt a feature there since it is already sort of touching that part of space with the retina.
Hippocampal theta frequency waves are probably subcortically generated, but why would whatever subcortical structures cause that oscillation send a travelling wave when it could cause the same oscillation simultaneously everywhere or starting from random points? It takes a little bit of time to travel, which might indicate functional importance if the time for the wave to travel is longer latency than lateral connections. There could also be a boring reason.
Local processing does not reach everywhere; I can see that a traveling wave could be a chain of local processes.
Perhaps shaped by the feedback from the reciprocally connected cortex.
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I just ran into this nice paper on traveling waves.
Do check out the movies in the supplemental materials.
https://www.nature.com/articles/s41467-019-08999-0#article-info
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