50 
Northcutt 
Figure 19. Summary of experimentally determined ascending pathways in reptiles (A) and amphibians 
(B). Differences are primarily related to increased differentiation of dorsal thalamus and elaboration of 
dorsal ventricular ridge in reptiles. C, cerebellum; DL, dorsolateral nucleus of the dorsal thalamus; DT 
dorsal thalamus; DVR, dorsal ventricular ridge; H, hypothalamus; h, habenular nucleus; IT, isthmal region 
of the tegmentum; M, medulla; MP, medial pallium; OB, olfactory bulb; OC, optic chiasm; OiV, olfactory 
nerve; OP, olfactory peduncle (tracts); OT, optic tectum; P, preoptic area of the hypothalamus; PDT, 
posterior nuclear complex of dorsal thalamus; RE, nucleus reuniens (medialis) ; RO, nucleus rotundus; S, 
septal nuclei; ST, striatum; T, torus semicircularis; VT, ventral thalamus; 1, spinocerebellar and spino- 
thalamic pathways (the most rostral extent of this pathway is presently uncertain in amphibians) ; 2, as- 
cending reticular pathways; 3, ascending auditory pathway; i, ascending thalamo-telencephalic pathways 
(in amphibians these pathways terminate primarily in the striatum; in reptiles they terminate in separate 
sensory zones of the dorsal ventricular ridge as well as in the striatum). 
recipient of efferents from the ridge sensory- 
areas. Similar findings have been reported 
in birds (Karten and Dubbledam, 1973), 
These results argue that, whatever the nature 
of the sensory function of the rostral dorsal 
ventricular ridge, it primarily affects lower 
brain centers by acting through the striatum 
and its descending efferents. Thus, it is likely 
that lesions of the rostral dorsal ventricular 
ridge in reptiles will not result in the out- 
right loss of particular types of behavior. 
It is more likely that such lesions would re- 
sult in rather subtle changes in behavior 
that probably cannot be measured by simple 
learning paradigms or by noting the presence 
or absence of a particular species-typical 
behavior pattern following such lesions. 
If chemoreception did not play a decisive 
role in determining the position of further 
sensory elaboration in the telencephalon — as 
suggested by the close connection of rostral 
dorsal ventricular ridge to the striatum 
rather than to the lateral cortex or vomero- 
nasal target — then what selective pressure (s) 
