Cortical Dynamics & Neurocircuitry: Mapping Sensory Loops & Cognitive Pathways
Exploring the cerebral cortex's complex structure and dynamic neural processes, this scroll delves into sensory processing, neural circuitry interactions, and higher cognitive function integration.
Our focus this hour is on the intricate workings and changes within the cerebral cortex, a central area of interest in neuroscience. It encompasses both the static architecture and the dynamic processes of neural activity. It's a comprehensive and systematic exploration of the networked nature of neural pathways. How different neural circuits interact and function. How sensory information is processed in the brain, with discussions on feedback and feedforward mechanisms within sensory processing, and a brief mention of higher-order cognitive functions suggesting a higher order integration of sensory information with cognitive processes like attention, memory, and decision-making.
Today’s article is also note a0403z which is part of the Self Aware Networks Theory of Mind found at https://github.com/v5ma/selfawarenetworks
If you have Questions about how the brain works, need help researching artificial general intelligence or are interested in learning more about Self Aware Networks Theory of Mind you can ask Self Aware Networks GPT now available for paid users of ChatGPT (It’s the Alpha Version, meaning the Robot is still under construction) https://chat.openai.com/g/g-FA3lrTWTq-self-aware-networks-gpt
This scroll presents a simplified model of the direction of information flow in the brain: from your senses (eyes & ears) to incoming subcortical regions (like the thalamus) to the primary sensory cortices. It examines how information flows from subcortical areas (like the Thalamus) to the superficial layers (of Cortical Columns) and progresses towards the deeper layers.
Then from the Deeper layers (of Cortical Columns, like Layer 6) some of the activity goes into a subcortical loop (like the Cortical Thalamic Loop) back to the higher superficial layers (like Layer 1 which is closer to your skull, skin, and hair therefore it's a superficial layer (obviously superficial layers have great functional importance).
I think of the Cortical Column as a vertical information flow (from the Sensory Info to Thalamus to the Cortical Column from the top to bottom of the Column) and they come with vertical loops of information.
The bottom or 6th layer of the cortical columns often connect to a subcortical structure (but not always, and not exclusively). Then signal information may travel back to Layer 1 of the Cortex. The main loop you want to know about in this discussion is the cortical-thalamic loop.
I think of Cortical-Thalamic Loops as Vertical Column Loops, which is definitely true for Thalamic Core Neurons, or as Local Slanted Vertical loops (as is the case with Thalamic Matrix Neurons which may pass information between Cortical Columns in the nearby or local region, such as within a Hypercolumn.) (more about Matrix and Core Neurons later on in this scroll)
The idea of Vertical Loops I think helps contrast them with the other type of information flow in the brain which I think of as Horizontal Loops (Feedforward & Feedback Loops) which are also not strictly horizontal, but relative to the Cortical-Thalamic Loops they are much more horizontal, and they travel much greater distances across the entire brain from front to back, and back to front).
There is a section later on in this scroll that discusses Feedforward & Feedback neurons in more detail. These horizontal feedback loops to a great extent involve IT Neurons (from the Cortical Columns) in the 2/3rd layers (but actually include layers 1-4, and IT neurons are included in a lot of Feedback connections) and PT Neurons (5 Layer Pyramidal Cells or PT Neurons are usually the largest neurons, they are the major outputs of cortical columns and are involved in a lot of feedforward connections (including motor outputs))
Key points about the Neural Direction of Flow:
In general the direction of a component in a loop (which is a neuron) can be considered from the Basal Dendrite or Dendrite towards the Axon or Exit Terminal or Apical Dendrite. The same is true whether we are talking about a vertical loop in a column, or a horizontal feedforward / feedback loop.
This reflects the basic principle of signal propagation in neurons, where electrical impulses generated in the dendrites travel down the axon to connect with other neurons.
However, when considering a loop within a single neuron, the concept of a single, linear direction becomes less straightforward. A loop implies a circular flow of information, where signals not only travel from dendrites to axon but also potentially back to the dendrites again through various synaptic connections.
Different types of loops (vertical, horizontal, feedforward, feedback) can involve unique signal pathways and directions within the neuron. For example, a vertical loop in a cortical column might involve dendrites receiving inputs from both lower and higher layers, potentially creating more complex interactions than a simple dendrite-to-axon flow.
Neurons are highly complex structures with intricate dendritic trees and numerous synaptic connections. The flow of information within them can be dynamic and multidirectional, not always restricted to a single, linear path. Different dendrite types can receive and process information differently, contributing to the overall loop dynamics.
Key points about the Cortical Direction of Flow:
Vertical Information Flow in Cortical Columns: This emphasizes the sensory information's journey from subcortical regions like the thalamus to the cortical columns, moving from superficial layers to deeper ones, and often looping back via structures like the thalamus. This vertical flow is essential for the initial processing and integration of sensory data.
Cortical-Thalamic Loops: These loops play a critical role in refining sensory processing, with layer 6 projecting back to specific thalamic nuclei and the thalamus then relaying information back to superficial cortical layers. This loop acts as a mechanism for fine-tuning sensory inputs.
Horizontal Feedforward and Feedback Loops: These loops represent the broader, more integrative aspect of brain function, encompassing larger distances across the brain and involving multiple brain regions. They are crucial for higher-order processing, decision-making, and the integration of sensory information with cognitive and emotional contexts.
Layer-Specific Roles: Each cortical layer has unique connections and functions. For example, layer 6's involvement in feedback to the thalamus, layers 4 and 5's roles in processing monaural and binaural auditory information from the medial geniculate body, and the significant role of layer 5 pyramidal cells in feedforward pathways.
Influence of Non-Thalamic Connections: Layer 6 also establishes connections with other cortical layers and plays a role in local circuit modulation and integration of information within the cortical column.
Specialized Thalamic Nuclei: The distinct roles of medial geniculate nucleus divisions (MGN and MGB) in auditory processing, and the LGN in visual processing, demonstrate the specialization within the thalamus for different sensory modalities.
Interactions Across Cortical Columns and Regions: Layer 6's communication with nearby columns or regions, targeting various layers depending on the specific functional context, illustrates the interconnected nature of cortical processing.
The brain's ability to integrate feedforward and feedback information, process it across different layers, and loop it through cortical and subcortical structures is fundamental to its function.
Understanding the diverse paths from our sensory inputs to subcortical inputs to layers 1 and 2 and beyond provides a deeper appreciation for the path of the flow of sensory information through brain regions in shaping our perception, cognition, and behavior.
In the olfactory system, the olfactory bulb directly projects to layer 1 of the piriform cortex, relaying odor information that shapes odor perception and memory. The Olfactory bulb affects layers 1 and 2 (mitral cell layer) by Influencing the experiences of odor intensity and hedonic valence. It's worth noting that the olfactory system's direct cortical projections are unique compared to other sensory systems, which typically relay through the thalamus.
The Medial Geniculate Nucleus (MGN, thalamus) provides auditory input to layer 1 of the primary auditory cortex.
The Ventral Posteromedial Nucleus (VPN, thalamus) Relays taste information to layer 1 of the gustatory cortex.
Structures like the raphe nuclei (serotonin) and locus coeruleus (norepinephrine) send widespread projections to layer 1 across various cortical areas, influencing arousal, attention, and mood, impacting the processing of incoming information.
The striatum within the basal ganglia projects to layer 1 of motor and premotor cortices, modulating motor planning and execution based on reward and reinforcement signals.
The amygdala, a limbic structure, sends projections to layer 1 of sensory and association cortices, influencing emotional responses and biasing the processing of stimuli according to their emotional salience.
The hippocampus & entorhinal cortex loop projects to layer 1 of various cortical areas, especially those involved in memory and spatial navigation, influencing the context-dependent processing of information, navigation, and memory retrieval & consolidation.
The Retrosplenial cortex connects with layer 1 of diverse cortical areas, playing a role in spatial navigation, episodic memory, and self-referential processing.
The Orbitofrontal cortex sends reward and decision-making signals to layer 1 of other cortical regions, influencing emotional responses and value-based choices.
The Insula projects to layer 1 of sensory and association cortices, integrating visceral and interoceptive information with external stimuli, shaping emotional and cognitive responses.
The Hypothalamus provides hormonal and autonomic signals influencing arousal and attention, impacting information processing in various cortical areas.
Anterior cingulate cortex: This prefrontal region connects with layer 1 across diverse cortical areas, playing a role in error detection, conflict monitoring, and cognitive control, modulating information processing based on internal cognitive demands.
The Substantia nigra pars reticulata sends inhibitory projections to layer 1 of motor and premotor cortices, contributing to movement selection and suppression of unwanted motor actions.
The Subthalamic nucleus modulates motor control and basal ganglia loops through projections to layer 1 of motor and premotor areas.
The Cerebellum provides feedback signals influencing motor planning and execution through connections to layer 1 of motor and premotor cortices.
Ventral tegmental area sends dopaminergic projections to layer 1 to influence reward processing, motivation, and learning.
The Nucleus basalis of Meynert sends cholinergic projections to layer 1 of widespread cortical areas, modulating arousal, attention, and cognitive flexibility.
The Parabrachial nucleus modulates pain perception and emotional responses through projections to layer 1 of sensory and limbic cortices.
Quick Reference: The six layers of the Neocortex from top to bottom:
Layer I (Molecular Layer): The outermost layer, closest to the surface. It's primarily composed of dendrites (extensions of neurons) and axons (nerve fibers) from other layers, with very few neuronal cell bodies. This layer plays a key role in integrating neuronal signaling.
Layer II (External Granular Layer): Contains small granular neurons and some pyramidal cells. It's involved in intracortical communication.
Layer III (External Pyramidal Layer): Characterized by medium-sized pyramidal neurons, this layer is important for communication between different areas of the cortex.
Layer IV (Internal Granular Layer): Contains densely packed stellate and pyramidal cells. It's a primary recipient of sensory inputs from the thalamus, especially important in processing sensory information.
Layer V (Internal Pyramidal Layer): Contains large pyramidal neurons, including Betz cells in the motor cortex. It's crucial for sending output signals to other parts of the brain and the spinal cord.
Layer VI (Multiform Layer): The deepest layer, consisting of various cell types, including pyramidal and multiform cells. It primarily sends outputs to the thalamus, creating a feedback loop between the thalamus and cortex.
While the descriptions of the functions of different cortical layers in this scroll are broadly accurate, these can vary somewhat across different cortical areas and in different species.
The Visual System in Focus
In general a lot of phase wave differential signals come in from our senses, and travel to many subcortical structures, with the primary structure for the bulk of sensory input being the thalamus, and then these signals travel into the cortex, generally flowing through layers from the top superficial layers (starting with Layer 1) to the bottom deep layers (ending with Layer 6) some of the signals travel in loops back to subcortical areas and back to Layers 1-4. The Thalamus with its Matrix and Core neurons is the major player for these columnar cortical loops, but it's not the only one.
Let's examine the visual system in more detail from sensory input to subcortical areas, including the Thalamus (The LGN - Lateral Geniculate Nuclei part of the Thalamus) and to the Neocortex, through the Layers from upper layers to lower layers (layer 6) and back to the Thalamus and then returning again the Neocortex Layers (Layers 1-4), and we will look at the other Visual signal inputs to Layers 1 & 2.
Light enters the eye, hitting the retina where photoreceptors convert it into waves of neurotransmitters that cause spikes, that inhibit nearby neurons, trigger downstream spikes, that trigger more waves of neurotransmitters, and the process repeats.
Signals from the retina eventually reach the Thalamus and some travel to V1 in addition some signals from Retina directly project to layer 1 of the primary visual cortex, contributing to modulation of visual processing based on retinal feedback. The Retina influences layer 1 and 2 by adjusting visual processing based on eye movements and retinal adaptation.
Please note that while the retina does send some information directly to retinal projections to layer 1 of V1 this is not a primary pathway for visual information. The major route for visual information is via the lateral geniculate nucleus (LGN) of the thalamus to the primary visual cortex.
Some Visual signals travel to the Superior colliculus: This midbrain structure receives visual and auditory inputs and projects to layer 1 of various sensory cortices, influencing attentional shifts and orienting towards stimuli.
The Superior colliculus affects layers 1 and 2 by modulating visual attention and in addition it modulates auditory and somatosensory experiences.
In V1, the lateral geniculate nucleus (LGN) fibers (in the Thalamus) initially connect to the superficial layers, specifically layer 1 (molecular layer) and layer 2 (external granular layer).
However, the first and second layers (molecular and external granular layers) receive input from different sources.
Thalamocortical fibers: These originate from non-LGN thalamic nuclei and contribute to various functions like attention, arousal, and multisensory integration.
Corticocortical fibers: These connect the primary visual cortex with other visual areas and non-visual areas involved in processing visual information.
Layers 1 and 2 receive input from other thalamic nuclei, other cortical areas, and brainstem structures, contributing to broader functions beyond strictly visual processing.
Layer 1 (molecular layer): Receives feedback from higher cortical areas and modulates activity in deeper layers. This outermost layer primarily receives afferent fibers from various brain regions, including other cortical areas, thalamic nuclei besides the LGN (such as the intralaminar nuclei), and brainstem structures. These fibers modulate the activity of the underlying layers involved in sensory processing.
Layer 2 (external granular layer): Receives input from other cortical areas and subcortical structures, contributing to multisensory integration.
Feedforward connections: These ascend from lower visual areas like the lateral geniculate nucleus (LGN) to layer 2.
The main visual input from LGN targets layers 4 and 6. While the thalamus, LGN matrix, and core neurons play a crucial role in shaping visual processing, their primary targets are the granular layers (4-6) of the primary visual cortex (V1 not to be confused with layer 1, Area V1 is a section of the brain that has all 6 layers in it). These layers are considered granular layers due to the abundance of granular cells present.
Some of the signals travel through the optic nerve to the thalamus, where the (LGN) processes them and relays them to the primary visual cortex (V1) in layers 4-6.
Layers 4 (granular layer) and 6 (multiform layer): Receive direct input from the LGN and process basic visual features.
Layers 3 and 5: Integrate information from granular layers and project it to higher-order visual areas.
Visual information from the LGN (lateral geniculate nucleus) of the thalamus indeed travels to the primary visual cortex (V1). However, it doesn't directly terminate in layers 1 and 2 of V1. Instead, it primarily targets layers 4 and 6 of V1
Information then flows downwards to the deeper layers, primarily the granular layers 4 and 5. These layers contain densely packed granular cells responsible for processing basic visual features like edges, orientations, and colors.
Processed information reaches the deepest layer, layer 6 (multiform layer). Here, neurons integrate signals from granular layers and project them to other brain regions for further processing. Some projections also head back to the thalamus, but this is a feedback loop for modulation, not the main output pathway.
TC Neurons also known as Thalamic Matrix Neurons and Thalamic Core Neurons
1. Thalamic Matrix Neurons:
These neurons originate from non-specific thalamic nuclei, meaning they aren't dedicated solely to visual processing.
They project broadly to multiple cortical areas, including layer 1 of V1.
Their functions in layer 1 include:
Modulating cortical excitability: They can increase or decrease the overall activity of V1 neurons, influencing how strongly the cortex responds to visual stimuli.
Regulating sleep and wakefulness: Matrix neurons play a role in regulating arousal states, which can affect visual processing.
Contributing to multisensory integration: They can integrate visual information with input from other senses, like hearing or touch.
2. Thalamic Core Neurons:
These neurons come from specific thalamic nuclei, such as the LGN, that are devoted to relaying sensory information.
While their primary projections target deeper layers of V1, they also send a smaller number of fibers to layer 1.
Their functions in layer 1 are less well understood, but they might contribute to:
Fine-tuning visual processing: They could provide additional feedback or modulatory signals to refine the processing of visual features.
Enhancing attention: They might help focus attention on specific aspects of visual stimuli.
Visualizing Thalamic Input to Layer 1:
Thalamic input to different layers of the primary visual cortex
Cortical Column & Subcortical Feedback loops
Major subcortical areas involved in loops from the 6th layer of the Neocortex to the Subcortical Area to Layers 1 and 2 of the Cortex include:
The Thalamus: This is the most prominent example. Layer 6 cells in sensory cortices (visual, auditory, somatosensory) project back to specific thalamic nuclei. These nuclei then relay processed information back to layer 1 and deeper layers of the same sensory cortex, modulating and sharpening the processing of incoming stimuli.
The Superior colliculus: Plays a similar role in visual and auditory attentional control. Layer 6 projections from sensory cortices reach the superior colliculus, influencing which stimuli receive attentional focus. The superior colliculus then projects back to layer 1 of sensory cortices, affecting how those stimuli are processed.
The Basal ganglia: Layer 6 projections from motor and premotor cortices reach the striatum in the basal ganglia. The striatum then interacts with other basal ganglia structures and projects back to layer 1 and deeper layers of motor and premotor areas, modulating motor planning and execution based on reward and reinforcement signals.
Some subcortical areas receive signals from layer 6 but are not involved in direct loops back to layer 1.
Brainstem nuclei: Layer 6 projections to structures like the raphe nuclei (modulating arousal) and locus coeruleus (modulating attention) influence cortical information processing without necessarily forming direct feedback loops.
Hippocampus: While some hippocampal connections can contribute to feedback loops in specific contexts, most Layer 6 projections provide contextual information without returning directly to layers 1 and 2.
Layer 6 cells in sensory cortices (visual, auditory, somatosensory) project to specific thalamic nuclei related to the modality.
The thalamic nuclei then integrate incoming information and project back to the prefrontal cortex, specifically areas like the dorsolateral prefrontal cortex (DLPFC) and ventral prefrontal cortex (VLPFC).
This loop allows the prefrontal cortex to access and interpret sensory information for decision-making, attentional control, and working memory.
The thalamic loop to the DLPFC is crucial for working memory tasks, as it allows the prefrontal cortex to maintain and manipulate information.
Attentional control: These loops enable the prefrontal cortex to prioritize and focus on relevant sensory information while filtering out distractions. Thalamic nuclei act as "gates" modulating the flow of sensory information to the prefrontal cortex, allowing it to direct attention toward specific stimuli.
Sensory working memory: Prefrontal areas like the dorsolateral prefrontal cortex (DLPFC) utilize these loops to hold onto processed sensory information for short periods. The thalamus helps maintain this information by continuously refreshing the signal from the sensory cortices.
Sensory-driven decision-making: By integrating sensory information with internal goals and past experiences, the prefrontal cortex can guide decisions based on the perceived world. For example, visual cues about an approaching car might trigger a decision to cross the street based on its speed and distance.
Multisensory integration: Different sensory-prefrontal loops allow the brain to combine information from various senses (vision, hearing, touch) to create a cohesive understanding of the environment. Thalamic nuclei can integrate these multimodal signals before sending them to the prefrontal cortex for further processing.
Layer 6 cells in the amygdala and other limbic regions project to various subcortical areas like the ventral tegmental area (VTA) and hypothalamus.
These subcortical structures process the emotional salience of stimuli and relay signals back to the prefrontal cortex, particularly the VLPFC and orbitofrontal cortex (OFC).
This loop allows the prefrontal cortex to integrate emotional information with cognitive processing, influencing decision-making, reward-based learning, and emotional regulation.
The limbic-VLPFC loop plays a role in decision-making under uncertainty, where emotional cues influence choices.
Basal Ganglia-Prefrontal Loops:
Layer 6 cells in the motor and premotor cortices project to the striatum within the basal ganglia.
The striatum works with other basal ganglia structures to evaluate actions and their potential consequences.
Feedback signals then loop back to the prefrontal cortex, particularly the DLPFC and ACC, influencing motor planning, action selection, and decision-making related to actions.
The basal ganglia-DLPFC loop is involved in planning complex actions and choosing between competing options.
These loops are often not "closed-loop" circuits but rather involve multiple relays and interactions within other brain regions.
Components of the Loop:
Sensory Cortices: Layer 6 pyramidal cells in areas like the primary visual cortex, auditory cortex, or somatosensory cortex project to specific thalamic nuclei for their respective modality.
Thalamic Nuclei: These nuclei, like the pulvinar for visual processing or the medial geniculate nucleus for auditory processing, receive and refine sensory information before sending it to the prefrontal cortex.
Prefrontal Cortex: Areas like the DLPFC, VLPFC, and posterior parietal cortex receive processed sensory information from the thalamus and integrate it with other cognitive processes for various decision-making and attentional tasks.
While driving, the visual-prefrontal loop might help you focus on the road ahead while ignoring passing billboards. The thalamus would filter out irrelevant visual information while maintaining the important details (traffic lights, pedestrians).
In a crowded bar, the auditory-prefrontal loop allows you to focus on a specific conversation despite background noise. The thalamus would suppress the background noise and amplify the relevant speech signals for the prefrontal cortex to process.
When navigating a new city, the multisensory-prefrontal loop integrates visual landmarks (streets, buildings) with tactile cues (uneven terrain) and auditory information (traffic sounds) to help you form a mental map and make decisions about direction.
Feedforward and Feedback across the brain.
In the brain, information functionally flows in two great directions: feedforward and feedback. These pathways work together to create a dynamic and intricate processing system for sensations, thoughts, and actions.
(If this is a new concept for you, you can imagine feedback & feedforward signals in the brain as like a horizontal flow vs the more vertical flow of information associated with Cortical Columns in the earlier part of the discussion, although neither flow is strictly horizontal or vertical, just broadly speaking you can think of it as a flow across the brain such as from back to front (feedforward) or from front to back (feedback))
Processed visual information also flows between cortical columns to other visual areas within the temporal lobe for more complex analysis and perception. For example V1 (also known as Brodmann area 17 or the striate cortex) is just one area within the visual cortex. Processed visual information from V1 then flows to V2 (Brodmann area 18), which is a separate visual area located adjacent to V1 in the occipital lobe.
Sensory information in V1 travels from the surface inwards. The superficial layers receive initial input, then deeper layers progressively integrate and refine the information before sending it out for further processing or feedback.
Feedforward: Imagine information like a river flowing downstream. Feedforward processing is like this river, carrying signals from one brain region to another in a sequential, unidirectional manner.
Feedforward direction: Starts from lower-level (such as the rear of the head) areas, like sensory cortices, and progresses towards higher-level areas, like association areas or the prefrontal cortex (in the front of the head)
Feedforward function: Initial processing and analysis of incoming information. In the visual system, for example, feedforward pathways carry visual signals from the retina to the primary visual cortex (V1), where basic features like edges and colors are extracted.
Feedback: Think of feedback like a tributary flowing back into the main river. Feedback processing sends signals from higher-level areas back down to lower-level areas, modifying and refining the flow of information.
Feedback direction: Starts from higher-level areas (that might be in the front of the head like the Frontal Lobes) and loops back down towards lower-level areas (that might be towards the rear of the head like the Parietal Lobes)
Feedback function: Modulates and refines processing based on context, previous experience, and other information. In the visual system, feedback from higher areas can influence how V1 neurons respond to specific stimuli, allowing for attention-based focusing and interpretation of the visual scene.
The interplay between feedforward and feedback:
These two pathways constantly interact, creating a dynamic loop for processing information.
Feedforward provides the initial analysis, while feedback refines and enriches this analysis with context and additional information.
This loop allows the brain to continuously update its interpretation of the world based on both incoming stimuli and internal states.
Here are some examples of how feedforward and feedback work together in different brain functions:
Movement: Feedforward pathways generate motor commands to initiate movement, while feedback from proprioceptors and visual cues adjust the movement in real-time.
Perception: Feedforward processes like edge detection in V1 are modulated by feedback from higher areas involved in object recognition and attention, allowing us to perceive specific objects within the visual scene.
Learning: Feedforward pathways carry new information, while feedback strengthens or weakens connections between neurons based on learning and experience.
Understanding feedforward and feedback interconnected pathways work together to create a dynamic and constantly evolving system that underlie all our thoughts, actions, and perceptions.
Feedforward and feedback pathways can be both horizontal and vertical depending on the brain regions involved. While V1 to PFC is a vertical connection, V1 to V2 or PFC to other prefrontal areas are horizontal connections within their respective hemispheres.
The important distinction is not the orientation, but the direction of information flow and the functional relationship between the connected areas.
Different types of neurons contribute to these pathways, each with its specific function and pathway location.
Feedforward pathways can originate from any sensory or lower-level processing area and project to higher-level areas for subsequent processing.
These pathways aren't always perfectly aligned due to the complex structure of the brain. While some might have axons pointing directly from V1 to PFC, others might involve intermediary relays through multiple brain regions.
It's crucial to consider the functional hierarchy, not just the direction of connections. For example, V1 to PFC would be feedforward even if the axons take a slightly roundabout route through other visual areas.
Excitatory pyramidal neurons: These are the main workhorses of feedforward projections, sending their axons to higher-level areas. IT neurons are a type of pyramidal neuron and contribute substantially to this pathway.
Other glutamatergic neurons: While pyramidal IT neurons are prominent, other neurons using the excitatory neurotransmitter glutamate can also participate in feedforward projections, especially within local circuits.
Neurons communicate through both dendrites and axons in both directions. Dendrites receive signals, while axons send signals.
Feedback loops can be more intricate than just front-to-back travel. They can exist within the same area (local feedback) or loop back from higher levels to lower levels across various brain regions (long-range feedback).
The key characteristic of feedback is the modulatory effect on lower-level processing. It influences how incoming information is analyzed and interpreted based on context and prior experience.
Modulatory neurons: These include IT neurons and other types of neurons that use various neurotransmitters like GABA (inhibitory), dopamine, and serotonin. They project back to lower-level areas, influencing the processing of incoming information.
Interneurons: These are local circuit neurons that connect within a brain region, playing a crucial role in both feedforward and feedback processing by modulating the activity of other neurons. IT neurons can also function as interneurons within specific brain areas.
IT neuron roles in feedforward and feedback
Intratelencephalic (IT) neurons play a significant role in both feedforward and feedback pathways. However, their role in feedforward and feedback pathways predominantly relates to their modulatory functions through inhibitory neurotransmitters like GABA. They influence the activity of other neurons, shaping how information is processed and propagated through the brain.
Heterogeneity: IT neurons are a diverse population, and not all of them express the same neurotransmitters. Most research indicates that a significant portion of IT neurons are indeed GABAergic, meaning they use GABA as their primary neurotransmitter.
Glutamatergic IT neurons: However, a smaller subset of IT neurons does express glutamate, the main excitatory neurotransmitter in the brain. These glutamatergic IT neurons participate in excitatory projections within specific brain regions, contributing to feedforward information flow.
GABAergic IT neurons: These neurons primarily function in feedback pathways. They project back to lower-level areas, including sensory cortices, and modulate the processing of incoming information through inhibitory signals. This modulation can sharpen responses to specific stimuli, suppress irrelevant information, and integrate feedback from higher areas.
Relationship between neurotransmitter and pathway:
While there's a general trend of GABAergic IT neurons being more involved in feedback and glutamatergic IT neurons participating in feedforward pathways, it's not a definitive rule. The specific function and projection target of an IT neuron depend on its location within the brain circuit and its specific connections with other neurons.
Some GABAergic IT neurons can also contribute to feedforward projections within local circuits, and some glutamatergic IT neurons might participate in feedback loops depending on their connectivity patterns.
In summary, the relationship between IT neuron neurotransmitter type and its role in feedforward or feedback pathways is complex and not simply a one-to-one mapping. While generalizations exist, understanding the specific connections and circuit architecture within a brain region provides a more nuanced picture of individual IT neuron function.
PT Neurons or Layer 5 Pyramidal Cell Neurons
PT Neurons or Layer 5 pyramidal cells play a significant role in feedforward projections, especially for long-range connections between brain regions. They represent the main output layer of the cortex, projecting their axons to various higher-level areas for further processing.
The proportion of layer 5 cells involved in feedforward connections can differ. In some regions like the primary visual cortex, a large majority of layer 5 pyramidal cells project primarily outwards. In other regions like the prefrontal cortex, the proportion might be closer to half.
Additionally, some layer 5 neurons can have both feedforward and local (horizontal) projections within the same area, contributing to both forward information flow and intra-regional processing.
Compared to feedforward pathways, layer 5 pyramidal cells contribute less to feedback connections. However, they do participate in some feedback loops, particularly those with shorter distances within a lobe or hemisphere.
The main players in feedback pathways are often different neuronal types, including interneurons and other cell types using modulatory neurotransmitters like GABA.
Layer 5 pyramidal cells can indirectly influence feedback loops through their interaction with interneurons and other feedback-projecting neurons.
How much do layer 5 pyramidal cells participate in feedforward connections? Feedforward: Moderate to high. A substantial portion of layer 5 pyramidal cells, depending on the brain region, participates in feedforward projections.
How much do layer 5 pyramidal cells participate in feedback connections? Feedback: Low to moderate. Layer 5 pyramidal cells contribute less directly to feedback compared to other neuronal types, but some still participate in shorter-range feedback loops.
CT Neurons or Layer 6 Pyramidal Cells
Limited Direct Involvement: Unlike layer 5 cells, layer 6 cells generally play a smaller role in long-range feedforward projections to higher-level areas. Their primary targets are often other cortical layers within the same region or nearby regions.
Local Feedforward: Within their local circuit, some layer 6 neurons can project to deeper layers like layer 5, influencing the information relayed outwards through feedforward pathways. This contributes to shaping the information flow within the region before it reaches higher levels.
Indirect Contribution: Although not directly involved in long-range feedforward projections, layer 6 cells can indirectly influence them through their interactions with other neurons, particularly with layer 5 pyramidal cells. They can modulate the activity of layer 5 cells, which ultimately contribute to the feedforward output of the region.
Significant Contribution: Layer 6 cells play a more prominent role in feedback pathways compared to feedforward. They project heavily back to lower layers, including sensory cortices and deeper granular layers (layer 4), modulating the processing of incoming information.
Contextual Modulation: Feedback from layer 6 cells can incorporate context and top-down information from higher areas, influencing how lower layers respond to sensory stimuli and integrate information. This allows for more refined and contextual interpretation of the incoming data.
Integration of Feedback Loops: While some layer 6 cells project directly back to lower layers, others participate in more complex feedback loops involving multiple brain regions. They can integrate feedback from higher areas and interact with other feedback-modulating neurons to tailor the information flow within the circuit.
Layer 6 & Layer 5 Comparison:
Compared to Layer 5 cells, Layer 6 cells contribute less directly to long-range feedforward pathways (compared to layer 5), but they can influence them indirectly and participate in local feedforward within their region.
Their primary contribution lies in feedback pathways, where they heavily project back to lower layers, modulating incoming information and incorporating context from higher areas.
While it's true that layer 6 primarily contributes to feedback pathways, some layer 6 neurons do participate in feedforward signaling, especially within local circuits.
Their diverse projections and involvement in feedback loops make them crucial for integrating top-down information and refining the processing of sensory and other types of information.
What percentage of Layer 6 cells connect to the thalamus?
Determining the exact percentage of layer 6 cells that connect to the thalamus across the entire brain is difficult due to the complex and diverse nature of brain anatomy and the variability between different brain regions. However, we can provide some insights based on the available research:
Variability between Regions:
The proportion of layer 6 cells projecting to the thalamus significantly varies depending on the specific brain region. Some regions, like the primary visual cortex and the primary somatosensory cortex, have a relatively high percentage of corticothalamic projections, with estimates ranging from 30 to 50% of layer 6 cells connecting to the thalamus.
Other regions, like the prefrontal cortex and the temporal lobe, have a lower percentage, with estimates falling closer to 10 to 20%. This variability reflects the functional differences between brain regions and their specific communication needs with the thalamus.
Factors Influencing Percentage:
Cortical regions heavily reliant on thalamic input for sensory processing tend to have a higher percentage of layer 6 cells projecting back to the thalamus. This facilitates a feedback loop for refining and modulating sensory information.
The presence of strong anatomical pathways between a specific cortical region and the thalamic nuclei it interacts with also influences the proportion of layer 6 cells involved in these connections.
While layer 6 primarily consists of pyramidal cells, they can exhibit functional heterogeneity. Specific subpopulations might be more or less likely to project to the thalamus depending on their role in the local circuit.
Precisely pinpointing a single percentage for the entire brain is not possible due to regional variations.
Estimates based on different brain regions range from 10 to 50% of layer 6 cells connecting to the thalamus.
Factors like function, anatomical connections, and cell type heterogeneity contribute to this variability.
Non-thalamic Layer 6 Connections:
Layer 1 and 2: Studies suggest that Layer 6 connections to layers 1 and 2 are relatively sparse, likely constituting under 5% of their total projections. These superficial layers receive more substantial input from other sources like subcortical structures and other cortical areas.
Layer 3: Connections to Layer 3 might be slightly more prevalent, possibly reaching 10-15% of Layer 6 outputs. Layer 3 plays a crucial role in integrating information from various sources, and Layer 6 projections might contribute to this integration process.
Layer 4: Estimates for Layer 4 connections vary depending on the brain region, but in general, they might be comparable to
Delta of Non-thalamic Layer 6 Connections
Layer 3: potentially ranging from 10-15%. Layer 4 receives significant thalamic input and processes specific features of sensory information, making it a potential target for non-thalamic Layer 6 projections.
Layer 5: Non-thalamic Layer 6 connections to Layer 5 are likely the most abundant among the deeper layers, potentially reaching 20-30% of Layer 6 outputs. Layer 5 is the main output layer of the cortex, and Layer 6 projections can influence downstream processing via this crucial pathway.
Thalamocortical Feedback Loops and Non-thalamic Connections
While thalamic feedback loops primarily target layers 1 and 2, non-thalamic Layer 6 connections to these layers still exist.
These may contribute to local modulation and integration of information within the cortical column.
The absence of thalamic input doesn't preclude Layer 6 from communicating with superficial layers. Other sources like feedback from higher cortical areas and subcortical structures can also influence layers 1 and 2.
Adjacent Column and Regional Communication
When communicating with nearby columns or regions, Layer 6 cells primarily target deeper layers, although the specific distribution varies:
Layer 5: Due to its role as the main output layer, Layer 5 often receives a significant portion of Layer 6 projections within and across columns, potentially exceeding 30% in some cases.
Layer 4: Depending on the brain region and functional context, Layer 4 can also be a prominent target for Layer 6 projections, potentially receiving 20-25% in some areas.
Layers 1-3: Connections to these superficial layers are less prevalent but still present, likely contributing to local integration and modulation within connected columns.
Keep in mind this is a simplified model of the flow of information in the Brain
For example, while the roles of intratelencephalic (IT) and pyramidal tract (PT) neurons in feedforward and feedback pathways are appropriately noted, their functions can be even more diverse and context-dependent than the generalizations provided. In addition the functions of Thalamic Matrix and Core Neurons are much more complex and varied than I have described. The complex nature of thalamocortical feedback loops, including their involvement in various cortical functions goes beyond just sensory processing.
Cognitive Functions: Thalamocortical interactions are crucial in various cognitive processes including attention, memory, and executive functions. The thalamus acts as a relay and integrative center, modulating the flow of information to the cortex based on cognitive demands and task relevance.
Consciousness and Arousal: The thalamus, particularly the intralaminar and midline nuclei, plays a vital role in maintaining states of consciousness and arousal. Thalamocortical dynamics are crucial in the transition between wakefulness and sleep, as well as in maintaining alertness.
Motor Control and Coordination: Beyond sensory processing, thalamocortical circuits are deeply involved in motor control. The motor and premotor cortical areas interact with the thalamus to refine and coordinate motor commands, contributing to precise and smooth execution of movements.
Emotional Processing and Regulation: Thalamocortical loops also participate in the processing of emotional information. The limbic thalamic nuclei have connections with various cortical areas involved in emotional regulation and response.
Learning and Plasticity: The thalamus is not just a passive relay station but plays an active role in learning and synaptic plasticity. Thalamocortical loops can modulate synaptic strength and efficacy, thereby influencing learning processes and memory consolidation.
Integration of Multisensory Information: The thalamus contributes to the integration of information from different sensory modalities, ensuring a cohesive perception of the environment.
To learn about the precise flow of information in cortico thalamo cortical loops I recommend that you watch the following video:
Watch "Naoki Yamawaki: Untangling the cortico-thalamo-cortical loop" for a deep analysis of the(2021)
Here is the corresponding PDF of the paper "Untangling the cortico-thalamo-cortical loop" that is discussed in that video https://europepmc.org/backend/ptpmcrender.fcgi?accid=PMC9006917&blobtype=pdf
In the grand schema of cortical dynamics and neurocircuitry, PT cells serve as a bridge between cognitive and sensory processing and the physical execution of motor actions.
They are integral to the feedback and feedforward loops within the cortex, demonstrating the interconnected nature of sensory processing, cognitive function, and motor output.
The role of Pyramidal Tract (PT) cells, particularly those in layer 5 of the motor cortex, is central to understanding how motor signals are generated and transmitted to various parts of the body, including the eyes, mouth, larynx, neck, hands, feet, and overall body movement. These cells are crucial in the orchestration of precise and coordinated motor activities.
Integration of PT Cells with Sensory Processing and Neural Circuitry:
Sensory Information Processing:
The cerebral cortex processes sensory information through a network of neural circuits involving both feedforward and feedback mechanisms.
PT cells, while primarily associated with motor output, also receive inputs from various sensory areas of the brain. This input is critical for coordinating motor responses with sensory stimuli.
Feedback and Feedforward Mechanisms:
In the context of cortical dynamics, feedforward pathways are typically thought of as sensory pathways, moving from primary sensory areas to higher cognitive areas.
Feedback mechanisms involve higher cognitive areas influencing sensory processing, for example, by modulating attention or expectation. PT cells contribute to these feedback mechanisms by adjusting motor responses based on cognitive inputs like decision-making or memory.
Cognitive Function Integration:
Higher cognitive functions, such as attention, memory, and decision-making, are integrated within the cortical network and have direct implications for motor control.
PT cells represent the final output pathway of this integrated process, where decisions and cognitive evaluations are transformed into motor actions.
The Role of PT Cells in Cortical Circuitry:
Motor Output as a Result of Cortical Processing:
The ultimate output of cortical processing, especially in terms of interaction and response to the environment, often manifests as motor activity controlled by PT cells.
Motor actions, from speech to limb movement, are the result of complex cognitive processes, including the integration of sensory information.
Interaction with Sensory Cortices:
PT cells interact with sensory cortices, not in isolation, but as part of a feedback loop where sensory inputs inform motor actions, and motor outputs, in turn, influence sensory perception (e.g., moving eyes to enhance visual perception).
Higher-Order Cognitive Functions:
Cognitive processes like attention, memory, and decision-making influence the activity of PT cells. For instance, deciding to move a hand involves layer 5 PT cells in the motor cortex, which is informed by a host of sensory and cognitive inputs.
Modulatory Role of PT Cells:
PT cells can modulate sensory processing indirectly. For instance, motor actions initiated by PT cells can alter sensory inputs (like turning the head changes auditory inputs), which feeds back into the cortical sensory processing areas.
PT Cells and Layer 5 Pyramidal Cells in Motor Output:
Structure and Location: Layer 5 PT cells, including Betz cells, are distinctive due to their large size and pyramidal shape. Located in the motor cortex, these cells have long axons that extend through the brainstem to the spinal cord, forming part of the corticospinal tract.
Direct Motor Control: These cells directly project to the spinal cord, where they synapse with motor neurons. This direct pathway allows for precise and skilled voluntary movements, enabling fine motor skills.
Integration with Sensory Feedback: PT cells do not operate in isolation. They integrate sensory feedback to refine motor commands, ensuring movements are precise and contextually appropriate. This integration involves a complex network of neural circuits, including sensory inputs and cognitive processing centers.
Motor Output Connections to the Spinal Cord:
Direct and Indirect Pathways: The direct corticospinal pathway allows PT cells to connect directly with motor neurons in the spinal cord. In contrast, the indirect pathway involves synapses with interneurons, facilitating broader and more complex motor patterns.
Role in Reflexive and Voluntary Actions: These connections are not just for deliberate actions but also play a role in reflexive responses, where quick, involuntary movements are required.
Voluntary Movement: PT cells are integral in initiating and controlling voluntary movements, ranging from speaking and facial expressions to walking and hand movements.
Motor Learning and Adaptation: These cells are involved in motor learning, allowing for the refinement and improvement of motor skills over time.
Postural Control and Coordination: They play a role in maintaining posture and balance, ensuring coordinated movements during various physical activities.
Clinical and Technological Relevance:
Neurological Conditions: Dysfunction or damage to PT cells or their pathways can lead to motor impairments, as seen in conditions like stroke, cerebral palsy, and spinal cord injuries.
Brain-Computer Interfaces (BCI): Research in BCIs focuses on utilizing the signals from PT cells to control external devices, aiding individuals with motor impairments.
Advanced Prosthetics: Understanding PT cell pathways is crucial in developing more sophisticated prosthetic limbs that can mimic natural movements.
Integrating Sensory Inputs and Cognitive Processes:
Sensory-Motor Integration: PT cells are part of a larger network that integrates sensory information with motor outputs, ensuring that movements are adapted to the current environmental context.
Cognitive Influence: Cognitive processes like planning, decision-making, and attention directly influence the activity of PT cells, further integrating motor actions with overall behavioral goals.
In summary, the role of PT cells, particularly those in layer 5 of the motor cortex, is pivotal in the generation and control of motor signals. These cells form a critical bridge between the brain's decision-making processes, sensory feedback, and the execution of complex motor actions.
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