Recurrent thalamo-cortical resonance

Thalamocortical circuit diagram depicting both specific/sensory and non-specific intralaminar thalamocortical systems.

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.[2] 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.[2] 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.

Studies involving manipulation of slices of visual cortex have shown that thalamocortical resonance from stimulated TCs induces the formation of coherent regions of similar electrical activity through vertical layers of the cortex.[2] 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 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.

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.[6] 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.

The lateral geniculate nucleus, 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,[7] 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.[11]

Gamma wave thalamocortical oscillation is prominent during REM sleep, similar to the awakened, active brain.[2] 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.[5] 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 (1–4 Hz), and theta (4–7 Hz) range.[12] Synchronized theta oscillation has additionally been observed in the hippocampus during NREM.[12]

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-parietal cortex. This activity could be originated by the pyramidal neurons in layer IV.[3] 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 tasks show an increase in alpha magnitudes.[3] This is contrary to gamma wave oscillatory frequencies which emerge in selective focus tasks.