Tag: attention

Essay 19: Nucleus Isthmi

On October 5, 2023

Essay 18 was trying to solve the problem of maintaining behavioral state. When a fast neuron synapse takes only 5ms, behavior that lasts seconds or minutes needs some circuit to sustain attention on the task. Essay 18 explored the striatum as a possible model to maintain behavior. In zebrafish, this problem is partial solved with a paired system consisting of the optic tectum (OT) and the nucleus isthmi (NI) [Gruberg et al. 2006].

Optic tectum

The optic tectum (OT – superior colliculus in mammals) is a midbrain action and sensor system that organizes vision, touch, sound, and action into retinotopic map like an air controller radar screen that activates only for important triggers. So, it’s not like the movie screen of primate vision, but is an action-oriented, sparse map that focuses on a few important items. In the larva zebrafish, the OT activates for hunting prey (paramecia) and avoiding obstacles and predators.

The OT itself has no persistence, When it detects potential prey and moves toward the prey, the OT doesn’t remember that it’s hunting or recall the previous location of the prey. Without enhancement, it forgets the pretend fails the hunt. The nucleus isthmi (NI – parabigeminal in mammals) provides that attention and persistence function [Henriques et al. 2019].

Nucleus isthmi circuit

The NI has a simple organization that is topologically, bidirectionally mapped to OT. The return signal from NI to OT is acetylcholine (ACh), which amplifies the sense input, biasing the next action to follow the previous action. Essentially this is a simple attention circuit that maintains consistent behavior.

In the diagram above, a left action sends an efference copy to the matching nucleus isthmi area, which can remember the activation for longer than the 5ms fast activation in the OT. In turn it sends an ACh modulator to amplify the left touch sensor, biasing the direction toward the same action.

For the essay simulation, the original problem was hitting an obstacle head-on, which triggered both left and right touch sensors, which then caused jitter as the animal randomly chose left and right without maintaining consistency. By adding an NI system, an initial left action would bias the left input sense to choose a next left action.

Acetylcholine attention system

As a speculation, or perhaps a mnemonic, this NI system where ACh enhances senses based on action might be a model for some attention mechanisms else were in the brain. NI is a sister nucleus to other ACh nuclei, specifically the parabrachial nucleus (B.pb) and the pedunculopontine nucleus (V.ppn), all developing from the same stem region near the isthmus. V.ppn is one of the major ACh attention nuclei and is part of the midbrain locomotive region (MLR). It seems plausible that V.ppn might share some organization with NI where its upstream ACh might support sense attention like the NI does for OT.

Engineering note

After implementing the nucleus isthmi support, both the proto-striatum and NI solve the jittering problem equally. The algorithms are slightly different — NI is a straight enhancement, while proto-striatum is a disinhibition with selection — but for the current complexity of the animal and environment, there’s no behavioral difference. Both proto-striatum and NI can be enabled simultaneously without interference problems.

References

Cui H, Malpeli JG. Activity in the parabigeminal nucleus during eye movements directed at moving and stationary targets. J Neurophysiol. 2003 Jun;89(6):3128-42. doi: 10.1152/jn.01067.2002. Epub 2003 Feb 26

Gruberg E., Dudkin E., Wang Y., Marín G., Salas C., Sentis E., Letelier J., Mpodozis J., Malpeli J., Cui H. Influencing and interpreting visual input: the role of a visual feedback system. J. Neurosci. 2006

Henriques PM, Rahman N, Jackson SE, Bianco IH. Nucleus Isthmi Is Required to Sustain Target Pursuit during Visually Guided Prey-Catching. Curr Biol. 2019 Jun 3

Marín G, Salas C, Sentis E, Rojas X, Letelier JC, Mpodozis J. A cholinergic gating mechanism controlled by competitive interactions in the optic tectum of the pigeon. J Neurosci. 2007 Jul 25

Motts SD, Slusarczyk AS, Sowick CS, Schofield BR. Distribution of cholinergic cells in guinea pig brainstem. Neuroscience. 2008 Jun 12;154(1):186-95. doi: 10.1016/j.neuroscience.2007.12.017. Epub 2008 Jan 28

18: Neuroscience issues with proto-striatum

By ferg

On October 3, 2023

In Uncategorized

The previous proto-striatum model is flawed because it focused too much on sensory input and not enough on action efferent copies. To fix this focus, the model can use midbrain locomotive region (MLR) actions as a bias selector.

Recall that the simulation needed the striatum to solve an action jitter problem by introducing a win-stay bias. Once the animal turns left, it should bias toward continued left turns. Before the fix, the animal randomly chose a direction every 50ms, reversing itself, causing problems in avoiding corners and obstacles. The simulation problem was an action-selection problem not a sensor problem.

In the vertebrate striatum, action feedback comes from the MLR via the parafascicular thalamus (T.pf). The T.pf connection to the striatum is unique, both in its targeting of striatal interneurons (S.cin and S.pv), but also for its connection to the medium spiny projection neurons (S.spn), the main striatal neurons [Ragu et al. 2006]. T.pf connects directly to S.spn dendrites, not merely the spines as with other inputs. This direct connection potentially gives a stronger stimulus, and its uniqueness suggests it may be an older, more primitive connection.

Action-focused striatum model

So, I’m changing the striatum model to follow an action focus. After an action fires the motor command neurons (B.rs reticulospinal), the MRL sends an efferent copy of the motor command to the striatum via T.pf.

Action feedback model for proto-striatum. B.rs reticulospinal motor command, MLR midbrain locomotive region, Ob olfactory bulb, Snc substantia nigra pars compacta, S.pv striatal parvalbumen interneuron, S.spn spiny projection neuron.

In the above diagram, the main sensor path is still from the olfactory bulb (Ob) to the substantia nigra pars compacta (Snc / posterior tuberculum) and then to MLR, basically a stimulus-response path. A previous action biases the sensory path for the next action by activating a corresponding S.spn, which disinhibits Snc, making the next sensory input more powerful.

Comparison with the previous model

As a comparison, the following diagram shows the previous striatal model. Unlike the new model, the final selected action didn’t bias the next action because there was no feedback connection. (The reset signal to S.pv is a different circuit, and doesn’t bias the decision because it applies to all choices equally.)

sense-focused proto-striatum model. — *Previous photo-striatum, where a prior selected sense biased the next sense. B.ss somatosensory touch.*

In addition, the sensory input must coordinate striatal disinhibition via S.spn with its excitation of the Snc action. Although not impossible evolutionarily, the double coordination required makes it less likely. The new model not only incorporates the action but simplifies the sensor circuit.

Parafascicular thalamus

For personal reference, here’s a summary of the T.pf connections [Smith et al. 2022].

Essentially all the T.pf inputs are motor efference copies and all the T.pf outputs are to the basal ganglia. Inputs include the following areas: vision/optic motor (OT and pretectum), midbrain locomotive region (MLR, M.pag, V.ppt, V.ldt), diencephalon locomotive region (H.zi), consummatory action (B.bp), forebrain attention (P.bf) and cortical action (C.fef, C.moss, C.gu). The cingulate cortex might be unusual (C.cc), although it also has motor areas.

Striatum as attention

Attention is a difficult topic, in part because it’s used in so many diverse ways that the word is often more confusing than helpful [Hommel et al. 2019], [Krauzlis et al. 2014]. However, I think it’s interesting that the action-based striatum model looks like selective attention.

*Simplification of proto-striatum showing resemblance to selective attention.*

When a left action biases the next action to stay the same, its mechanism is to enhance the sensory path, as if it’s paying attention more to one side than another.

Engineering feedback: dopamine mistake

When implementing this idea, the simulation doesn’t need dopamine feedback. Instead of forcing the dopamine just because the basal ganglia has dopamine feedback I’m taking it out from the model. Since I’ve only implemented a prototype portion of the basal ganglia, this may be okay instead of a fatal flaw. When the full model arises, we’ll see if this is a mistake.

*Actual simulation implementation, removing dopamine and reset feedback.*

Notice that the only dopamine in this model is descending, with no ascending dopamine [Ryczko and Dubuc 2017].

References

Hommel B, Chapman CS, Cisek P, Neyedli HF, Song JH, Welsh TN. No one knows what attention is. Atten Percept Psychophys. 2019 Oct

Krauzlis RJ, Bollimunta A, Arcizet F, Wang L. Attention as an effect not a cause. Trends Cogn Sci. 2014 Sep;18(9):457-64

Raju DV, Shah DJ, Wright TM, Hall RA, Smith Y. Differential synaptology of vGluT2-containing thalamostriatal afferents between the patch and matrix compartments in rats. J Comp Neurol. 2006 Nov 10

Ryczko D, Dubuc R. Dopamine and the Brainstem Locomotor Networks: From Lamprey to Human. Front Neurosci. 2017 May 26

Smith JB, Smith Y, Venance L, Watson GDR. Thalamic Interactions With the Basal Ganglia: Thalamostriatal System and Beyo nd. Front Syst Neurosci. 2022 Mar 25