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 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 beco ...

, neurogenic pain,

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 ...

, visual snow syndrome,

schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social w ...

obsessive–compulsive disorder Obsessive–compulsive disorder (OCD) is a mental and behavioral disorder in which an individual has intrusive thoughts and/or feels the need to perform certain routines repeatedly to the extent where it induces distress or impairs general ...

, depressive disorder and

epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrica ...

. In TCD, normal thalamocortical resonance is disrupted by changes in the behaviour of

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...

in the thalamus.
TCD can be treated with

neurosurgical Neurosurgery or neurological surgery, known in common parlance as brain surgery, is the medical specialty concerned with the surgical treatment of disorders which affect any portion of the nervous system including the brain, spinal cord and ...

methods like the central lateral

thalamotomy Thalamotomy ( el, θάλαμος, thalamus, lit =chamber; el, τομή , tomē, lit=cut, slice) is a surgical procedure in which an opening is made into the thalamus to improve the overall brain function in patients. First introduced in the 1950 ...

, which due to its invasiveness is only used on patients that have proven resistant to conventional therapies.

Background

At the base of the theory lies diminished excitatory or increased inhibitory input at the thalamic level. This leads to a switch of the thalamocortical neurons from tonic to burst firing and subsequently entrains thalamic and cortical areas with pathological oscillations at around 5 Hz.

Evidence

Evidence for TCD comes from Magnetoencephalography (MEG), and

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 neocorte ...

(EEG) recordings on the scalp as well as

local field potential Local field potentials (LFP) are transient electrical signals generated in nervous and other tissues by the summed and synchronous electrical activity of the individual cells (e.g. neurons) in that tissue. LFP are "extracellular" signals, meaning ...

(LFP) recordings in the patients' thalamus during surgery. Analysing the power spectra reveals increased coherence as well as increased bicoherence in the power spectra in the theta band compared to healthy controls. This indicates a close coupling of cortex and thalamus in the generation of the pathological theta rhythmicity. The thalamic loss of input or gated activity allows the frequency of the thalamo-cortical column to slow into the theta or delta band, and this defeats the

lateral inhibition In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction ...

, so faster Gamma band activity appears surrounding the area of slower alpha seen in the theta band, with the theta associated with negative symptoms and the Gamma for positive symptoms. This is documented in Tinnitus (phantom sound) and phantom pain, as well as Parkinsonism and recently even in depression (see current work by Dirk DeRidder, MD, PhD). The thalamocoherence was identified by machine learning, with significant differentiation of each of these clinical entities from normal by the presence of the dysrhythmia, and with the specific disorder differentiated by the spatial/topographic networks involved. It was also proposed that

psychotic disorders Psychosis is a condition of the mind that results in difficulties determining what is real and what is not real. Symptoms may include delusions and hallucinations, among other features. Additional symptoms are incoherent speech and behavior t ...

present in Parkinson disease‐

dementia with Lewy bodies Dementia with Lewy bodies (DLB) is a type of dementia characterized by changes in sleep, behavior, cognition, movement, and regulation of automatic bodily functions. Memory loss is not always an early symptom. The disease worsens over time an ...

depend on thalamic abnormal rhythms.

Therapy

While it is not clear how this happens in detail, surgical intervention by means of lesioning small parts of the central lateral thalamic areas has proven successful as a therapy for Parkinson's Disease as well as neurogenic pain. Neurofeedback, where the brain is trained to emphasise and de-emphasise brain wave frequencies, amplitudes and coherence can be an effective noninvasive therapy.

Background

Evidence

Therapy

References

Further reading