Abstract

Previous studies have shown that the isthmotectal system constitutes a mechanism of visual-spatial attention in the midbrain of vertebrates. In this circuit, the nucleus isthmi parvocellularis (Ipc) sends feedback signals to the optic tectum (TeO) enhancing the transmission of retinal visual inputs from selected tectal locations to higher visual areas. In pigeons, the retino-tectal map has an increased representation of two retinal areas with higher ganglion cell density, the central fovea and the red field. Purpose: in this study we proposed to study the reciprocal projections between the Ipc and TeO to elucidate whether the representations of these retinal specialisations are also enhanced in Ipc. Methods: we made multiple injections of the retrograde tracer biotinilated dextran amine in TeO (n=21) finding the position of traced neurons in both the Ipc and the retina. Additionally, we performed electrophysiological recordings (n=22) of visually evoked potentials in Ipc and determined the position of their visual receptive fields. Finally, using these data we calculated the magnification factor for each retinal region in Ipc, comparing the distance between two points in Ipc with the distance between their respective loci, in both the retina and the visual field. Results: our results show that in the Ipc the retinal specialisations have a significantly higher magnification factor when compared to the periphery of the visual field, indicating that these regions have an enlarged representation in this nucleus. Conclusion: these results suggest that the circuit Ipc-Teo may enhance the visual processing of stimuli in all parts of the visual field, especially within regions perceived at high spatial resolution, particularly relevant during behaviours such as foraging and detection of predators