Most textbooks will tell you that the thalamus is a "relay"
that simply relays signals from auditory, somatic, visceral and
visual regions of the peripheral nervous system to the cerebral
cortex. The real picture is more complicated, and the commonly
accepted function of the thalamus nowadays is that it modulates,
in addition to relaying, sensory signals to and from cortex.
It is common to classify thalamic nuclei as either "relay
nuclei" or "association nuclei" on the basis of the source of
their driving inputs, whether they are subcortical or cortical.
Relay nuclei receive their driving inputs from subcortical
sources including ascending afferents (medial lemniscus for
somatosensory information, optic tract for visual information,
etc...) and project predominantly to primary sensory cortical
areas. On the other hand, association nuclei receive their
driving inputs from other cortical areas. (See Sherman and
Guillery's "Exploring the Thalamus", 2002)
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