Recurrent thalamo-cortical resonance is an observed phenomenon of oscillatory neural activity between the

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...

and various cortical regions of the brain. It is proposed by

Rodolfo Llinas Rodolfo is a given name. Notable people with the name include: * Rodolfo (footballer, born 1992), Brazilian footballer Rodolfo José da Silva Bardella *Rodolfo Albano III, Filipino politician * Rodolfo Vera Quizon Sr. (1928-2012), Filipino actor ...

and others as a theory for the integration of sensory information into the whole of

perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...

in the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

. Thalamocortical oscillation is proposed to be a mechanism of

synchronization Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...

between different cortical regions of the brain, a process known as temporal binding. This is possible through the existence of thalamocortical networks, groupings of thalamic and cortical cells that exhibit oscillatory properties. Thalamocortical oscillation involves the synchronous firing of thalamic and cortical neurons at specific frequencies; in the thalamocortical system, the exact frequencies depend on current brain state and mental activity. Fast frequencies in the

gamma Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter re ...

range are associated with much of conscious thought and active cognition. The thalamus in this system acts as both the gate for sensory input to the cortex as well as the site for feedback from cortical pyramidal cells, implying a processing role in sensory perception in addition to its function in directing information flow. The state of the brain, whether it be conscious, in REM sleep, or

non-rapid eye movement sleep Non-rapid eye movement sleep (NREM), also known as quiescent sleep, is, collectively, sleep stages 1–3, previously known as stages 1–4. Rapid eye movement sleep (REM) is not included. There are distinct electroencephalographic and other cha ...

, changes how sensory information is gated through the thalamus.

Thalamocortical network structure

Thalamocortical networks consist of neurons in both the thalamus and cortex. The thalamic neurons are typically one of three types: thalamocortical, with

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...

s extending into the cortex, reticular, and thalamic

interneuron Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. I ...

s. Thalamocortical neurons (TC) vary significantly in size, which is correlated with the depth to which they project into the cortex. These cells are limited in their outputs and seem to only connect to the

cortical layers The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of a ...

and reticular thalamic neurons. Reticular neurons (RE), on the other hand, are highly interconnected and have their own intrinsic oscillatory properties. These neurons are capable of inhibiting thalamocortical activity via their direct connections to TCs. Corticothalamic neurons are the cortical neurons that TC neurons synapse on. These cells are glutaminergic excitatory cells that exhibit increasing activity as they become more depolarized. This activity is described as "bursting", firing in the gamma range at rates between 20 and 50 Hz.

Thalamic oscillation

The thalamocortical loop starts with oscillatory thalamic cells. These cells receive both sensory input from the body as well as input from feedback pathways in the brain. In a sense, these cells serve to integrate these multiple inputs by changing their inherent oscillatory properties in response to depolarization by these many different inputs. TC neurons exhibit gamma oscillation when depolarized to greater than -45 mV, and the frequency of oscillation is related to the degree of

depolarization In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is ess ...

. This oscillation is caused by the activation of leaky P/Q-type calcium channels found in the

dendrites Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...

of the cells. Because of the leaky channel properties, spontaneous, inherent oscillation can also occur independent of any rhythmic input as well, though the ramifications of this capability are not entirely known and may add nothing but background noise to the thalamocortical loop. The cortex provides feedback to the thalamus through links to dendrites of these thalamocortical cells and serves as the source of constant thalamic oscillation. Oscillatory behavior depends on the conscious/unconscious state of the brain. During active thinking,

electroencephalography Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...

reveals a strong appearance of gamma range oscillation from around 20–50 Hz.

Thalamocortical circuits

Thalamic cells synapse on

apical dendrite An apical dendrite is a dendrite that emerges from the apex of a pyramidal cell. Apical dendrites are one of two primary categories of dendrites, and they distinguish the pyramidal cells from spiny stellate cells in the cortices. Pyramidal cells are ...

s of

pyramidal cells Pyramidal cells, or pyramidal neurons, are a type of multipolar neuron found in areas of the brain including the cerebral cortex, the hippocampus, and the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cor ...

in the cortex. These pyramidal cells reciprocally

synapse In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...

back on thalamic neurons. Each loop is self-contained and modulated by sensory input. Inhibitory

interneurons Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. I ...

both in the cortex and the

reticular nucleus Reticular nucleus may refer to: * Caudal pontine reticular nucleus * Gigantocellular reticular nucleus, a nucleus that innervates the caudal hypoglossal nucleus, and responds to glutamateric stimuli. * Lateral reticular nucleus * Oral pontine retic ...

of the thalamus regulate circuit activity.

Inputs to thalamocortical system

The thalamus gates information into thalamocortical loops based on the source of the signal. There are two major sources for TC input: sensory perception and information about the current mental state. Cortical structures of external events or sensory data are referred to as specific inputs and enter into the ventrobasal thalamus at the "specific" thalamic nuclei. These neurons project to layer IV of the cortex. Similarly, nonspecific inputs provide context from internal state of the brain and enter into intralaminar "non-specific" nuclei in the centrolateral thalamus with axons in layers I and VI. Both types of TC neurons synapse on the pyramidal cortical cells which are thought to integrate the signals. In this way, outside sensory information is introduced into the current context of cognition.

Resonant columns

Studies involving manipulation of slices of

visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and ...

have shown that thalamocortical resonance from stimulated TCs induces the formation of coherent regions of similar electrical activity through vertical layers of the cortex. In essence this means that groupings of activated cortical cells form from the activation of these thalamic cells. These regions are columnar and are physically separated from adjacent resonance columns by areas of inhibited cortex between them. It is not known what the exact function of these columns is, although their formation occurs only when the cortical

white matter White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distribution ...

afferents are stimulated at the gamma frequency range, implying an association with task-focused thought. The regions of inactive cortex that form between cortical columns were determined to be actively inhibited; administration of a GABA_A blocker stops columnar development.

Temporal binding

Thalamocortical resonance is thought to be a potential explanation for coherence of perception in the brain. Temporal coincidence could occur through this mechanism by the integration of both specific and non-specific thalamic nuclei at the pyramidal cortical cell, as they both synapse on its apical dendrites. Feedback from the cortical cell back to the thalamic nuclei then relays the integrated signal. As there are numerous thalamocortical loops throughout the cortex, this process takes place simultaneously across many different regions of the brain during conscious perception. It is this ability to support large-scale synchronized events between remote brain regions that may provide for coherent perception. Altogether, the specific, ventrobasal neurons in the thalamus serve to introduce sensory input to a self-sustained feedback loop that is sustained by the non-specific, centrolateral TCs relaying information about the current cognitive state of the brain.

Relation to brain activity

Thalamocortical oscillation is thought to be responsible for the synchronization of neural activity between different regions of the cortex and is associated with the appearance of specific mental states depending on the frequency range of the most prominent oscillatory activity, gamma most associated with conscious, selective concentration on tasks,

learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, value (personal and cultural), values, attitudes, and preferences. The ability to learn is possessed by humans, animals, and some machine learning, machines ...

(

perceptual Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...

and associative), and

short-term memory Short-term memory (or "primary" or "active memory") is the capacity for holding a small amount of information in an active, readily available state for a short interval. For example, short-term memory holds a phone number that has just been recit ...

Magnetoencephalography Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers. Arrays of SQUIDs (s ...

(MEG) has been used to show that during conscious perception, gamma-band frequency electrical activity and thalamocortical resonance prominently occurs in the human brain. Absence of these gamma-band patterns correlates with nonconscious states and is characterized by the presence of lower-frequency oscillations instead.

Vision

The

lateral geniculate nucleus In neuroanatomy, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral projec ...

, known as the major relay center from the sensory neurons in the eyes to the visual cortex, is found in the thalamus and has thalamocortical oscillatory properties, forming a feedback loop between the thalamus and the visual cortex. Sensory input can be seen to modulate the oscillatory patterns of thalamocortical activity while awake. In the case of vision, stimulation from light sources can be seen to cause direct changes in the amplitude of the thalamocortical oscillations as measured by EEG.

Sleep

Gamma wave thalamocortical oscillation is prominent during REM sleep, similar to the awakened, active brain. Contrary to the conscious state, however, it appears that sensory input may be blocked or gated from interfering with the intrinsic activity of the brain during REM. Measures of bulk electrical signalling in the brain by MEG show no impact of auditory stimuli on the gamma wave patterns; measurements on conscious subjects show a distinct modulation due to the auditory input. In this way, the thalamocortical system acts to gate the brain from external stimuli during REM. Non-rapid eye movement (NREM) sleep differs from REM in that gamma activity is no longer prominent, stepping aside for lower frequency oscillations. While electrical activity at gamma frequencies can occasionally be detected in NREM, it is infrequent and comes in bursts. The exact purpose of its appearance in NREM is not understood. In NREM sleep, thalamocortical oscillatory activity is still present, but the overall frequencies range from the slow (<1 Hz), to the

delta Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, at a river mouth * D ( NATO phonetic alphabet: "Delta") * Delta Air Lines, US * Delta variant of SARS-CoV-2 that causes COVID-19 Delta may also ...

(1–4 Hz), and

theta Theta (, ; uppercase: Θ or ; lowercase: θ or ; grc, ''thē̂ta'' ; Modern: ''thī́ta'' ) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth . In the system of Greek numerals, it has a value of 9. Gr ...

(4–7 Hz) range. Synchronized theta oscillation has additionally been observed in the

hippocampus The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...

during NREM.

Alpha oscillations and attention

Gamma-range oscillations are not the only rhythms associated with conscious thought and activity. Thalamocortical alpha frequency oscillations have been noted in the human

occipital The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...

- parietal cortex. This activity could be originated by the pyramidal neurons in layer IV. It has been shown that alpha rhythms seem to be related to the focus of one's attention: external focus on visual tasks diminishes alpha activity while internal focus as in heavy

working memory Working memory is a cognitive system with a limited capacity that can hold information temporarily. It is important for reasoning and the guidance of decision-making and behavior. Working memory is often used synonymously with short-term memory, ...

tasks show an increase in alpha magnitudes. This is contrary to gamma wave oscillatory frequencies which emerge in selective focus tasks.

Thalamocortical dysrhythmia

Thalamocortical dysrhythmia Thalamocortical dysrhythmia (TCD) is a theoretical framework in which neuroscientists try to explain the positive and negative symptoms induced by neuropsychiatric disorders like Parkinson's Disease, neurogenic pain, tinnitus, visual snow syndrom ...

(TCD) is a proposed explanation for certain cognitive disorders. It occurs upon the disruption of normal gamma-band electrical activity between the cortex and thalamic neurons during awakened, conscious states. This disorder is associated with diseases and conditions such as

neuropathic pain Neuropathic pain is pain caused by damage or disease affecting the somatosensory system. Neuropathic pain may be associated with abnormal sensations called dysesthesia or pain from normally non-painful stimuli (allodynia). It may have continuous ...

tinnitus Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone experiences a faint "normal tinnitus" in a completely quiet room; but it is of concern only if it is bothersome, interferes with normal hearin ...

, and

Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...

and is characterized by the presence of unusually low-frequency resonance in the thalamocortical system. TCD is associated with disruption of many brain functions including

cognition Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thought, ...

, sensory perception, and

motor control Motor control is the regulation of movement in organisms that possess a nervous system. Motor control includes reflexes as well as directed movement. To control movement, the nervous system must integrate multimodal sensory information (both f ...

and occurs when thalamocortical neurons become inappropriately hyperpolarized, allowing T-type calcium channels to activate and the oscillatory properties of the thalamocortical neurons to change. A repeated burst of action potentials occurs at lower frequencies in the 4–10 Hz range. These bursts can be sustained by inhibition from the thalamic reticular nucleus and may cause an activation of cortical regions that are normally inhibited by gamma-band activity during resonance column formation. While the effect of the deviation from normal patterns of gamma oscillatory activity during conscious perception is not entirely settled, it is proposed that the phenomenon can be used to explain chronic pain in cases where there is no specific peripheral nerve damage.

References

{{Reflist, 2 Thalamus