To elucidate the specific properties of retinae with grouped
photoreceptors the neural pathways in the outer and inner plexiform layer
were studied. Photoreceptor bundles in this species consist of more than
100 rods and up to 50 cones, and are usually regarded as functional units.
Golgi impregnation in thick and thin sections and light microscopy were
used to identify bipolar cell types linking photoreceptors to amacrine
and/or ganglion cells. Nine different types were distinguished based
on their dendritic morphology and the position of the axon terminal in the
inner plexiform layer. Small cells have dendritic fields smaller than the
diameter of a photoreceptor bundle and are contacted mostly by cones. The
dendritic field size of bushy cells matches that of a photoreceptor
bundle; they are contacted mainly by rods. Flat cells receive about equal
input from rods and cones; their dendritic field size exceeds the bundle
diameter. Within the three major classes there are subtypes addressing
three sublaminae of the inner plexiform layer, the proximal On-centre
region (sl b), the distal Off-centre region (sl a) and a central sublayer
(sl c) probably with transient activity. These observations suggest that
cone vision has a spatial acuity better than the “bundle
grain”. In rod dominated vision the resolution matches that of the
bundles; for this pathway, the hypothesis of the bundle as a functional
unit is confirmed. The mesopic flat cell pathway has a resolution inferior
to the “bundle grain”; it may therefore be dedicated to
movement detection.