= Habenula
The \b[habenula] is a small nuclear complex of the <epithalamus> at the dorsal posterior wall of the <third ventricle>, adjacent to the dorsal <thalamus> and <pineal gland>. In mammals, its <medial habenula> and <lateral habenula> are distinct divisions with different circuits. The name denotes neural tissue, rather than a hormone-secreting gland.<Habenula in NLM MeSH>[D019262 scope note: location, habenular nuclei and epithalamic membership.]{ref}
The distinction between divisions matters when assigning a measured function. In a reward-location reversal task, recordings from the <lateral habenula> of monkeys showed responses opposing those of <dopamine neurons>: many lateral habenular cells were more active when an outcome was worse than expected. Weak electrical stimulation of the <lateral habenula> inhibited recorded <dopamine neurons>.<Lateral habenula as a source of negative reward signals in dopamine neurons by Masayuki Matsumoto and Okihide Hikosaka>[Nature447,1111–1115(2007), abstract and reward-reversal/stimulation experiments.]{ref}
For example, if a cue that formerly predicted a reward ceases to do so, the experiment asks whether neural activity follows the new outcome relationship rather than merely the cue's appearance. A change after reversal supports an outcome-related signal. It does not establish that every <neuron> in the <habenula> encodes disappointment, nor does electrical stimulation establish a direct single-synapse pathway. Keeping the whole complex, its divisions and the recorded cell populations distinct makes the anatomical term useful when comparing experiments.
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