The zeta cell: a new ganglion cell type in cat retina

We define a new morphological type of ganglion cell in cat retina by using intracellular staining in vitro. The zeta cell has a small soma, slender axon, and compact, tufted, unistratified dendritic arbor. Dendritic fields were intermediate in size among cat ganglion cells, typically twice the diameter of beta cell fields. They were smallest in the nasal visual streak (<280 microm diameter), especially near the area centralis (60-150 microm diameter), and largest in the nonstreak periphery (maximum diameter 570 microm). Fields sizes were symmetric about the nasotemporal raphe except near the visual streak, where nasal fields were smaller than temporal ones. Zeta-cell dendrites ramified near the boundary between sublaminae a and b (OFF and ON sublayers) of the inner plexiform layer, occupying the narrow gap separating the dendrites of ON and OFF alpha cells. There was no evidence for separate ON and OFF types of zeta cell. Retrograde labeling studies revealed that both nasally and temporally located zeta cells project to the contralateral superior colliculus, whereas few project to the ipsilateral colliculus or to any subdivision of the dorsal lateral geniculate nucleus. The zeta cell's morphology and projection patterns suggest that it corresponds to the ON-OFF phasic W-cell (also known as the local edge detector) of physiological studies. Zeta cells have particularly small dendritic fields in the visual streak, presumably because they are disproportionately represented in the streak in comparison with other ganglion cell types. These conditions are consistent with optimal spatial resolution along the retinal projection of the visual horizon rather than principally at the center of gaze. Strong commonalities with similar ganglion cell types in ferret, rabbit, and monkey suggest that "zeta-like" cells may be a universal feature of the mammalian retina.