Retinogeniculate connections: a balancing act between connection specificity and receptive field diversity

Retinogeniculate connections are one of the most striking examples of connection specificity within the visual pathway. In almost every connection there is one dominant afferent cell per geniculate cell, and both afferent and geniculate cells have very similar receptive fields. The remarkable specificity and strength of retinogeniculate connections have inspired comparisons of the lateral geniculate nucleus (LGN) with a simple relay that connects the retina with the visual cortex. However, because each retinal ganglion cell diverges to innervate multiple cells in the LGN, most geniculate cells must receive additional inputs from other retinal afferents that are not the dominant ones. These additional afferents make weaker connections and their receptive fields are not as perfectly matched with the geniculate target as the dominant afferent. We argue that these 'match imperfections' are important to create receptive field diversity among the cells that represent each point of visual space in the LGN. We propose that the convergence of dominant and weak retinal afferents in the LGN multiplexes the array of retinal ganglion cells by creating receptive fields that have a richer range of positions, sizes and response time courses than those available at the ganglion cell layer of the retina.