Thalamotelencephalic organization in birds

Investigation of thalamo-telencephalic connections reveals correspondences between the avian and mammalian thalamic subdivisions (which may or may not mean true homologies). Based mainly on hodological comparisons, the avian thalamus possesses the principal anatomical and functional subdivisions characteristic for mammals. The current review is focused on a comparative analysis of intralaminar, midline and mediodorsal nuclei. There is evidence for matching subdivisions in the case of midline thalamic and mediodorsal nuclei within the avian dorsal thalamic zone, whereas such correspondence is evident, if less complete, in the case of the intralaminar nuclei. Thalamic connections are also relevant to the debated issue of the avian 'prefrontal' cortex. From the current study it is suggested that the prefrontal analogue regions of the bird may spread across the rostrocaudal extent of telencephalon, the rostral nidopallial/mesopallial region (formerly known as medial neostriatum/hyperstriatum) being one subdivision, receiving direct input from the paraventricular thalamic nucleus homologue of midline thalamic region (the medial juxtaventricular region of the nucleus dorsomedialis posterior). Hodological evidence from the current study and other reports argues for the possibility that the area corticoidea dorsolateralis might be hodologically comparable to the cingulate cortex, receiving input from a mediodorsal thalamic-relevant subdivision (lateral subdivision of nucleus dorsomedialis anterior, and medial aspect of nucleus dorsolateralis pars medialis), which also projects on the caudal nidopallium close to (but not coextensive with) the nidopallium caudolaterale, another potential analogue of avian prefrontal cortex. The rostral dorsolateral aspect of nucleus dorsomedialis anterior thalami and the dorsal aspect of nucleus dorsolateralis pars medialis are partially comparable to the mammalian intralaminar nuclei, sharing connections to non-limbic 'corticoid' areas (the Wulst), and the reticular thalamic nuclei.