Binocular visual responses in cells of the rat dLGN.

In the mammalian visual system the output of the retina reaches the cerebral cortex by means of a synaptic link within the thalamus, the dorsal lateral geniculate nucleus (dLGN). In higher mammals this structure is visibly laminated, such that input from the two eyes remains segregated, binocular responses in essence being seen first in the cerebral cortex. In the rat this segregation is less obvious. With only around 3-10% of retinal ganglion cells projecting axons to the ipsilateral dLGN, the dLGN may be considered basically monocular; however, these ipsilaterally projecting axons contact cells in a region described as the 'hidden lamina', whose physiological properties have not been well described. In the anatomical literature, there is some debate as to the possibility of cross-over between the terminations of the two eyes. Here, a population of cells physiologically receiving input from the ipsilateral eye is described--surprisingly, the majority (63%) had powerful, excitatory input from both eyes, suggesting a simple form of binocular integration at a stage earlier than previously described for other, more 'visually developed' species, in which thalamic binocular integration is complex.