Figure 1.
Connectivity of the avian visual system and the isthmo-tectal circuit.
A, illustrates some of the connectivity in the visual pathways in birds. The black arrows show the projections from one structure to the other. The optic tectum (TeO), the nucleus of the basal optic root (nBOR), the nucleus lentiformis mesenscephali (LM) and the ventral geniculate nucleus (GLv) all receive projections from the contralateral retina. The isthmo-optic nucleus (ION), which projects to the retina, GLv, LM, the nucleus rotundus (nRt), the magnocellular and parvocellular portions of nucleus isthmi (Imc, Ipc) and the nucleus semilunaris (SLu) all receive projections from TeO. Several of the nuclei are also interconnected, like LM and nBOR or Imc, Ipc, and SLu. B, illustrates in detail the isthmo-tectal circuit. Imc, Ipc and SLu receive a topographic, excitatory projection from cells in layer 10 of the TeO (blue cells). Ipc and SLu send back excitatory projections to TeO in a topographic manner (green cells). Imc neurons on the other hand are GABAergic [50], [51] and send an ‘antitopographic’ projection to Ipc, SLu or to the deep layers of TeO [52].
Figure 2.
Location, borders and cytoarchitecture of the isthmal complex.
Photomicrographs showing the location and borders of the three isthmal nuclei, the magnocellular and parvocellular portions of nucleus isthmi (Imc, Ipc) and the nucleus semilunaris (SLu) in four species of birds. A–C show the isthmal complex in the three different groups of birds that exhibited a Imc segregated in two layers, the internal subdivision of the Imc (Imc-in) and the external subdivision of the Imc (Imc-ex). A shows a songbird (Passeriformes), the Gouldian Finch (Erythrura gouldiae). B shows a Gruiform, the American Coot (Fulica Americana). C shows a woodpecker (Piciformes), the Yellow-bellied Sapsucker (Sphyrapicus varius); D shows a pigeon (Columbiformes), the Bar-shouldered Dove (Geopelia humeralis). E and F show two species of owls (Strigiformes), the Short-eared Owl (Asio flammeus) and the Northern Hawk Owl (Surnia ulula).
Figure 3.
Location, borders and cytoarchitecture of other visual nuclei.
Photomicrographs of coronal sections showing the location and borders of the different visual nuclei in birds. A, shows the isthmo optic nucleus (ION) in a songbird (Passeriformes) the Spotted Pardalote (Pardalotus punctatus). B shows the nucleus of the basal optic root (nBOR) in an owl (Strigiformes), the Northern Hawk Owl (Surnia ulula). C shows the nucleus lentiformis mesencephali (LM), the ventral part of the geniculate nucleus (GLv) and the nucleus rotundus (nRt) in a Gruiform, the American Coot (Fulica americana). D shows the optic tectum (TeO) in a gallinaceous bird (Galliformes) the Ring-necked Pheasant (Phasianus colchicus).
Figure 4.
Photomicrographs of Nissl stained coronal sections in four of the specimens used in this study. A and C show two of the lowest quality staining used in this study while B and D show sections equivalent to the ones showed in A and C in specimens with good quality of staining, Notice that even in A and C, the borders of visual structures measured in this study, like the nucleus lentiformis mecencephali (LM), the ventral part of the geniculate nucleus (GLv), the nucleus rotundus (nRt), the nucleus of the basal optic root (nBOR) and the optic tectum (TeO), are all clearly discernible. In A, the white arrows show the borders between LM and the nucleus laminaris precommissuralis (LPC) and also the dorsal border of GLv. In C, the white arrows show the border of nBOR. Scales bars = 400 µm.
Figure 5.
Relative size of the magnocellular and parvocellular portions of nucleus isthmi.
Scatterplot of log-transformed volume of the magnocellular and parvocellular portions of nucleus isthmi (Imc or Ipc) plotted as a function of either the log-transformed brain volume minus the volume of the respective nuclei (Imc, A; Ipc, C) or the log-transformed volume of the optic tectum (TeO; Imc, E; Ipc, G) for all species examined (see table S1). The bar graphs show the relative size of each nuclei relative to the brain (Imc, B; Ipc, D) or the TeO (Imc, F; Ipc, H). Values shown in the bar graphs are the means of the residuals derived from the respective regressions show in A, C, E and G.
Figure 6.
Relative size of nucleus semilunaris.
Scatterplot of log-transformed volume of nucleus semilunaris (SLu) plotted as a function of the log-transformed brain volume minus the SLu volume (A) or the log-transformed volume of the optic tectum (TeO; B) for all species examined (see table S1). The bar graph shows the relative size of SLu relative to the brain (B) or the TeO (C). Values shown are the means of the residuals derived from the respective regressions shown in A and C.
Figure 7.
Relative size of other visual nuclei.
Scatterplot of log-transformed volume of different nuclei plotted as a function of the log-transformed brain volume minus the volume of the respective nuclei (A, C, E, G and I). The bar graphs show the relative size each nucleus relative to the brain, represented as the mean of the residuals derived from the respective regressions (B, D, F, H and K). A–B, Scatterplot and bar graph for the isthmo optic nucleus (ION). C–D, Scatterplot and bar graph for the ventral geniculate nucleus (GLv). The white triangles indicate gallinaceous birds and black circles to all other birds studied. E–F, Scatterplot and bar graph for the nucleus of the basal optic root (nBOR). G–H, Scatterplot and bar graph for the nucleus lentiformis mesencephali (LM). The white triangles indicate gallinaceous birds, the open circles indicate hummingbirds and the black circles are all other birds species studied.
Figure 8.
Relative size of optic tectum and nucleus rotundus.
Scatterplot of log-transformed volume of structures of the tectofugal pathway plotted as a function of the log-transformed brain volume minus the volume of the respective nuclei (A and C). The bar graphs show the relative size each nuclei relative to the brain represented as the mean of the residuals derived from the respective regressions (B and D). A–B, Scatterplot and bar graph for the the nucleus rotundus (nRt). C–D. Scatterplot and bar graph for the optic tectum (TeO).
Table 1.
Results of principal component analysis.
Table 2.
Visual nuclei bivariate allometric coefficients.
Table 3.
Maximum likelihood estimates of the evolutionary parameters.