M-CONE AND L-CONE IN MACAQUE FOVEA CONNECT TO MIDGET GANGLION-CELLS BY DIFFERENT NUMBERS OF EXCITATORY SYNAPSES

VISUAL acuity depends on the fine-grained neural image set by the fove al cone mosaic(1-3). To preserve this spatial detail, cones transmit t hrough non-divergent pathways: cone-->midget bipolar cell-->midget gan glion cell. Adequate gain must be established along each pathway; cros stalk and sources of variation between pathways must be minimized. The se requirements raise fundamental questions regarding the synaptic con nections: (1) how many synapses from bipolar to ganglion cell transmit a cone signal and with what degree of crosstalk between adjacent path ways; (2) how accurately these connections are reproduced across the m osaic; and (3) whether the midget circuits for middle (M) and long (L) wavelength sensitive cones are the same. We report here that the midg et ganglion cell collects without crosstalk either 28 +/- 14 or 47 +/- 3 midget bipolar synapses. Two cone types are defined by this differe nce; being about equal in number and distributing randomly in small cl usters of like type, they are probably M and L.