Evidence for a visual subsector within the zona incerta

Here we examine the patterns of connections between the zona incerta (ZI) of the thalamus and the major visual centers of the rat brain, namely the retina, dorsal lateral geniculate nucleus (LGd), superficial layers of the superior colliculus (SCs), and occipital cortex (Ocl). Injections of the tracers biotinylated dextran or cholera toxin subunit b were made into each of these centers, as well as ZI itself, by using stereotaxic coordinates. Rat brains were then aldehyde-fixed and processed using standard methods. We show that the retina, LGd, SCs, and Ocl all have connections with ZI; moreover, that each of these connections make a very distinct territory or subsector within the most lateral ZI regions. This subsector of connectivity with the visual centers does not respect the well-defined cytoarchitectonic sectors of ZI, being made up of small zones in the dorsal, ventral, and caudal sectors. Often, a distinctive "horse-shoe" pattern is evident, particularly after retinal and Ocl injections. Tracer injections into topographically distinct regions of the LGd. SCs, or Ocl results in no shift in the spatial location of labelling within ZI; after each injection, labelling is always seen within the lateral edge of the nucleus. Labelled terminals and cells are seen after LGd and SCs injections, while only labelled terminals are seen after retinal and Ocl injections. Although the precise function of this novel visual subsector is not known, these early findings suggest that ZI may be in a position to integrate visual information together with the other somatosensory, motor, and visceral information that it receives.