: thalami; from Greek θάλαμος, "chamber") is a large mass of gray matter on the lateral walls of the third ventricle forming 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, known as the thalamocortical radiations, allowing hub-like exchanges of information.

[3][4] Anatomically, it is a paramedian symmetrical structure of two halves (left and right), within the vertebrate brain, situated between the cerebral cortex and the midbrain.

It forms during embryonic development as the main product of the diencephalon, as first recognized by the Swiss embryologist and anatomist Wilhelm His Sr. in 1893.

[5] The thalamus is a paired structure of gray matter about four centimetres long, located in the forebrain which is superior to the midbrain, near the center of the brain with nerve fibers projecting out to the cerebral cortex in all directions.

[9] In a study of 12 healthy males with average age 17 years, MRI scans showed mean whole thalamus volume 8.68cm

[13][14] Derivatives of the diencephalon include the dorsally-located epithalamus (essentially the habenula and annexes) and the perithalamus (prethalamus) containing the zona incerta and the thalamic reticular nucleus.

Additional structures are the reticular nucleus (which envelops the lateral thalamus), the stratum zonale,[19] and the interthalamic adhesion.

[35] Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits that are believed to be involved with consciousness.

[38] The role of the thalamus in the more anterior pallidal and nigral territories in the basal ganglia system disturbances is recognized but still poorly understood.

The thalamus is functionally connected to the hippocampus[40] as part of the extended hippocampal system at the thalamic anterior nuclei.

[42][43] The thalamic region's connection to the medial temporal lobe provides differentiation of the functioning of recollective and familiarity memory.

"[50] Researchers found that "enhancing MD activity magnified the ability of mice to "think,"[50] driving down by more than 25 percent their error rate in deciding which conflicting sensory stimuli to follow to find the reward.

[54] The thalamus is the largest structure deriving from the embryonic diencephalon, the posterior part of the forebrain situated between the midbrain and the cerebrum.

[59] At the interface between the expression domains of Fez and Otx, the mid-diencephalic organizer (MDO, also called the ZLI organiser) is induced within the thalamic anlage.

Besides its importance as signalling center, the organizer matures into the morphological structure of the zona limitans intrathalamica (ZLI).

[60] In mice, the function of signaling at the MDO has not been addressed directly due to a complete absence of the diencephalon in SHH mutants.

Expression of this hairy-like bHLH transcription factor, which represses Neurogenin but is required for Ascl1, is progressively lost from the caudal thalamus but maintained in the prethalamus and in the stripe of rostral thalamic cells.

People who inherit two short alleles (SERT-ss) have more neurons and a larger volume in the pulvinar and possibly the limbic regions of the thalamus.

Enlargement of the thalamus provides an anatomical basis for why people who inherit two SERT-ss alleles are more vulnerable to major depression, post-traumatic stress disorder, and suicide.

[66] A thalamus damaged by a stroke can lead to thalamic pain syndrome,[67] which involves a one-sided burning or aching sensation often accompanied by mood swings.

[70][71] Thalamic volume loss by atrophy, is also significantly shown in sporadic frontotemporal dementia, noted in the anterior-dorsal thickness.

[72] Microstimulation of the posterior portion of the ventral medial thalamic nucleus can be used to evoke pain, temperature and visceral sensations.