On the basis of immunohistochemical data and the topography of its thalamic projections, the PpD was considered to be an equivalent to the pregeniculate nucleus in other mammals. 
In primates, the IGL is included as part of the pregeniculate nucleus (PGN), a cell group located mediodorsally to the dorsal lateral geniculate nucleus.
The intergeniculate leaflet (IGL), homolog of the primate pregeniculate nucleus, modulates circadian rhythms.
Four subdivisions become obvious: the main portion, the perireticular nucleus, the medial subnucleus, and the pregeniculate nucleus.
Taking advantage of the segregation of different classes of ganglion cell fibres in the optic tract, we investigated the ganglion cell class that projects to the pregeniculate nucleus (PGN) in the normal macaque monkey (Macaca mulatta and Macaca fascicularis) and following long-standing removal of striate cortex in one hemisphere.
Intraocular injections of tritiated proline were used to test the hypothesis that unilateral removal of all visual cortical areas results in increased distribution of retinal terminals in the pregeniculate nucleus (PGN) of the thalamus in monkeys.
Saccade-related activity recorded in the primate pregeniculate nucleus, and its anatomical connections with the pretectal nucleus of the optic tract (NOT) and superior colliculus (SC), suggest that it plays a role in visual-ocular motor integration.
The cholinergic and histaminergic projections have important neuromodulatory functions in the ascending visual pathways, so we compared the pattern and mode of innervation of the two projections in the lateral geniculate complex (dorsal lateral geniculate nucleus and pregeniculate nucleus) of the macaque monkey. A dense plexus of thin, highly branched ChAT-immunoreactive axons laden with varicosities was found in all layers of the dLGN including the koniocellular laminae and in the pregeniculate nucleus. Histamine-immunoreactive axons were distributed homogeneously throughout all laminar and interlaminar zones of the dLGN, but were denser in the pregeniculate nucleus than in the dLGN.
Perireticular cells form clusters which are in continuity with the main portion, globus pallidus, ganglionic eminence and pregeniculate nucleus. The main portion and the pregeniculate nucleus appearing as prominent greys are dramatically reduced in size later on.
In species studied, we found a region in the pregeniculate nucleus containing both NPY immunopositive cells and substance P immunopositive fibres that we identified as the IGL.
The NOT sent lighter, but consistent, projections to other visual and oculomotor-related areas including, from rostral to caudal, the ipsilateral pregeniculate nucleus, the contralateral NOT, the lateral and medial terminal nuclei of the accessory optic system bilaterally, the ipsilateral dorsolateral pontine nucleus, the ipsilateral nucleus prepositus hypoglossi, and the ipsilateral medial vestibular nucleus. The NOT received input from the contralateral NOT, the lateral terminal nuclei bilaterally, and the ipsilateral pregeniculate nucleus.
The lateral geniculate nucleus in the monkey and human is quite different from that in rodents, but contains an area in the pregeniculate nucleus that receives bilateral retinal projections in the monkey and is characterized in both the monkey and human by a population of NPY+ neurons and a plexus of enkephalin- and substance P-containing axons.
In the macaque monkey and human brain, a large portion of the pregeniculate nucleus contains NPY-IR neurons indicating that this is the primate homologue of the rodent IGL.
Using peroxidase-antiperoxidase immunocytochemistry, we demonstrated a dense accumulation of substance P-like (SP) immunoreactive terminals in the internal layer of the pregeniculate nucleus and in the pretectal olivary nucleus of the Japanese monkey.
Area LIP has been found to project to the pregeniculate nucleus, the zona incerta, the anterior pretectal nucleus, and the superior colliculus.
Labels were found in both subdivisions of the LGb: the dorsal lateral geniculate nucleus (DLGn) and the pregeniculate nucleus (PGn).
A third change, but in the opposite direction, was found in the pregeniculate nucleus (PGN) where the area of retinal terminals appeared enlarged.
In addition, a class of capsular neurons is found in the circumgeniculate capsule between layer 6 and the pregeniculate nucleus.
An electron microscope study of the ultrastructure of the pregeniculate nucleus of the monkey (Macaca mulatta) shows it to contain three neuronal types and four varieties of presynaptic terminals. Such a pre- and postsynaptic terminal always forms part of the in series or triadic configuration of terminals occasionally observed in the pregeniculate nucleus.U.
A study of the pregeniculate nucleus of the monkey (Macaca mulatta), in Klüver-Barrera-stained, and Golgi-impregnated material confirmed, with reservations, that the nucleus is bilaminar.
The lateral geniculate body is very variable in size in all respects among the prosimian species, that is, in the dorsoventral shift along the lateral surface of the thalamus, the cellular differentiation of the lateral geniculate nucleus into 5 or 6 layers, the topographical position of the pregeniculate nucleus and the degree of inversion.
Other subcortical targets of one or more visual cortical areas were the basal ganglia, claustrum, zona incerta, one or more of the intralaminar nuclei, lateral posterior nucleus, pregeniculate nucleus, dorsal lateral geniculate nucleus, and pontine nuclei. The projections to the reticular nucleus, pregeniculate nucleus, dorsal lateral geniculate nucleus, the inferior and a portion of the superior division of the pulvinar and the superior colliculus were found to be topographically organized. The targets of the subcortical projections were compared with those of the retina, as revealed by autoradiographic methods following tritiated proline injections of the eye and were found to overlap to varying extents in the superior colliculus, pretectum and dorsal lateral geniculate nucleus and to be segregated in the pregeniculate nucleus.
Other terminations were in subcortical structures which primarily receive input from visual area: these included the pregeniculate nucleus, the medial and central divisions of the inferior pulvinar nucleus, two loci in the superior pulvinar complex, the pretectum and the superior colliculus.
Single units were recorded in the pregeniculate nucleus (PGN).
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