Second tectofugal pathway in a songbird (Taeniopygia guttata) revisited: Tectal and lateral pontine projections to the posterior thalamus, thence to the intermediate nidopallium.

J Martin Wild,Andrea H Gaede

doi:10.1002/cne.23886

J Martin Wild, Andrea H Gaede

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https://doi.org/10.1002/cne.23886

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Abstract

Birds are almost always said to have two visual pathways from the retina to the telencephalon: thalamofugal terminating in the Wulst, and tectofugal terminating in the entopallium. Often ignored is a second tectofugal pathway that terminates in the nidopallium medial to and separate from the entopallium (e.g., Gamlin and Cohen [1986] J Comp Neurol 250:296-310). Using standard tract-tracing and electroanatomical techniques, we extend earlier evidence of a second tectofugal pathway in songbirds (Wild [1994] J Comp Neurol 349:512-535), by showing that visual projections to nucleus uvaeformis (Uva) of the posterior thalamus in zebra finches extend farther rostrally than to Uva, as generally recognized in the context of the song control system. Projections to "rUva" resulted from injections of biotinylated dextran amine into the lateral pontine nucleus (PL), and led to extensive retrograde labeling of tectal neurons, predominantly in layer 13. Injections in rUva also resulted in extensive retrograde labeling of predominantly layer 13 tectal neurons, retrograde labeling of PL neurons, and anterograde labeling of PL. It thus appears that some tectal neurons could project to rUva and PL via branched axons. Ascending projections of rUva terminated throughout a visually responsive region of the intermediate nidopallium (NI) lying between the nucleus interface medially and the entopallium laterally. Lastly, as shown by Clarke in pigeons ([1977] J Comp Neurol 174:535-552), we found that PL projects to caudal cerebellar folia.

Highlights

Birds are almost always said to have two visual pathways from the retina to the telencephalon: thalamofugal terminating in the Wulst, and tectofugal terminating in the entopallium
Stereotaxic coordinates for recordings of visual responses in the posterior thalamus were A0.2–A0.35, L1.5–1.7, and D4.4–4.6
In pigeons ‘DLPr’ was used by Gamlin and Cohen (1986) to describe a nucleus lying immediately rostral to DLPc, and in parallel fashion we have given the name rUva to the rostral extension of Uva in the zebra finch

Summary

Introduction

Birds are almost always said to have two visual pathways from the retina to the telencephalon: thalamofugal terminating in the Wulst, and tectofugal terminating in the entopallium. Tract tracing revealed novel aspects of the second tectofugal pathway, including a rostrally extensive tectal projection to rUva, an input from the lateral pontine nucleus, and projections to the rostrolateral intermediate nidopallium The evidence suggests this is a visual pathway separate from the first tectofugal pathway. Tectofugal and thalamofugal visual pathways have been well described and reviewed in the avian literature (e.g., Karten and Revzin, 1966; Karten and Hodos, 1970; Karten et al, 1973; Engelage and Bischof, 1993; Shimizu and Bowers, 1999; Shimizu and Karten, 2003; Shimizu et al, 2008; 2010), a much less well recognized, second tectofugal visual pathway was clearly demonstrated in pigeons by Gamlin and Cohen (1986) and in finches by Wild (1994) This pathway originates in several deep laminae of the optic tectum, but primarily in lamina 13. Other lamina 13 cells originate the better known first tectofugal projection to the thalamic nucleus rotundus (Rt) and thence to the entopallium (E: Karten and Revzin, 1966; Karten and Hodos, 1970; Benowitz and Karten, 1976; Karten et al, 1997; Marin et al, 2003; Fredes et al, 2010)

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