Organization of ADVR 
127 
/TYPE 3 
Figure 7. Two types of axons which can be 
traced from the lateral forebrain bundle 
into ADVR in rapid Golgi preparations. 
They are described in the text. 
1 and type 3 axons can be obtained using 
degeneration techniques. Figure 9 shows a 
system of degenerated axons which runs 
rostrally in the forebrain bundles following 
a unilateral transection of the midbrain. 
They extend into the ADVR in two fascicles, 
one runs behind the anterior commissure 
(PCF) and one turns dorsad rostral to the 
commissure. These axons are about 2[jl in 
caliber and course rectilinearly into zones C 
and D. Comparable but less dense projections 
go to ADVR contralateral to the lesion. These 
experiments suggest that type 1 axons origi- 
nate somewhere in the brainstem caudal to 
the diencephalon, but their precise origin 
is not known. 
Figure 10 shows sections from a snake 
which had a stereotaxic lesion of the thala- 
mus, Lesions of this type and control lesions 
of surrounding structures demonstrate that 
cells in the caudal part of the dorsal thalmus 
project by way of the lateral forebrain 
bundle to the rostral part of tht ipsilateral 
ADVR. The axons are of thin caliber and 
bear small varicosities. They are distributed 
throughout zones B, C, and D. The area of 
the thalamus involved in the lesions includes 
the nucleus lentiformis thalami, which re- 
ceives auditory projections from the poste- 
rior colliculi (Ulinski, 1977a), and the cen- 
tral thalamus, which receives visual infor- 
mation from the optic tectum (Ulinski, 
19776). ADVR in snakes, thus, receives 
thalamic sensory projections comparable to 
those demonstrated in birds and other rep- 
tiles (Karten, 1968 ; Karten and Hodos, 1970 ; 
Hall and Ebner, 1970; Fritz, 1974, 1975). 
These thalamic afferents are type 3 axons. 
ORGANIZATION OF ADVR 
Figure 11 summarizes my current concept 
of the organization of the snake ADVR. Four 
populations of neurons are present, arranged 
in concentric zones. Each population has 
characteristic connections. As far as is 
known, zone A neurons are influenced pri- 
marily by the axons of zone B neurons. Zone 
B neurons form multicellular units which are 
strongly influenced by type 3 axons, including 
those of thalamic auditory and visual nuclei. 
These axons are arranged so that they influ- 
ence neurons throughout large regions of 
zone B. Zone C and zone D neurons are 
strongly influenced by type 1 and type 3 
axons coursing radially. Neurons in each 
zone interact with neurons in adjacent zones. 
Finally, zone D neurons project to the stria- 
tum and to PDVR. One virtue of this scheme 
is that it provides an orderly approach for 
conducting further experiments on the ana- 
tomy and functions of ADVR. 
ACKNOWLEDGMENT 
This research is supported by PHS Grant NS 
12518. 
