  Most types of retinal neurons are spatially positioned in non-random
patterns, termed retinal mosaics. Several developmental mechanisms are thought
to be important in the formation of these mosaics. Most evidence to date
suggests that homotypic constraints within a type of neuron are dominant, and
that heterotypic interactions between different types of neuron are rare. In an
analysis of macaque H1 and H2 horizontal cell mosaics, W\"assle et al. (2000)
suggested that the high regularity index of the combined H1 and H2 mosaic might
be caused by heterotypic interactions during development. Here we use computer
modelling to suggest that the high regularity index of the combined H1 and H2
mosaic is a by-product of the basic constraint that two neurons cannot occupy
the same space. The spatial arrangement of type A and type B horizontal cells
in cat retina also follow this same principle.
