THE existence of a posterolateral thalamic relay nucleus for pain and
temperature sensation was postulated in 1911, on the basis of the stro
ke-induced analgesia and thermanaesthesia found paradoxically in patie
nts with thalamic pain syndrome(1). Pain or temperature sensations can
be evoked in humans by electrical stimulation in a vaguely defined re
gion of the posterolateral thalamus(2,3). Here we use anterograde trac
ing and single unit recordings to demonstrate that there is a distinct
nucleus in the posterior thalamus of the macaque monkey that receives
a dense, topographic input from spinothalamic lamina I neurons and in
which almost all neurons are nociceptive- or thermoreceptive-specific
. Immunohistochemical staining showed that this nucleus is defined by
a dense calbindin-positive fibre plexus in the macaque, so we applied
the same staining method to sections of human thalamus. We found a nea
rly identical fibre plexus localized within a distinct nucleus that is
cytoarchitectonically homologous to the lamina I relay nucleus in the
macaque thalamus. The stereotaxic coordinates of this nucleus and its
location relative to the main somatosensory representation fit clinic
al descriptions of the pain-producing region in humans. We conclude th
at this is a specific thalamic nucleus for pain and temperature sensat
ion in both monkey and human.