Random Wiring in the Midget Pathway of Primate Retina

Abstract

The present study addresses the questions of how topographically organized neuronal populations are connected, and whether there is anatomical evidence for color-selective wiring in retinal pathways for red-green color vision. The connectivity of OFF midget bipolar and OFF midget ganglion cells was studied in the peripheral retina of dichromatic ("red-green color blind") and trichromatic ("color normal") marmosets (Callithrix jacchus). Midget bipolar cells were identified immunohistochemically. Midget ganglion cells were retrogradely labeled from the lateral geniculate nucleus and photofilled. Comparable results were obtained from all retinas studied. Between 3 and 16 bipolar terminals converge onto each ganglion cell. Nearly all bipolar terminals investigated show regions of colocalization (areas of presumed synaptic contacts) with ganglion cell dendrites. This contact area makes up ϳ14% of the axon surface area for a typical midget bipolar cell. The output from individual midget bipolar axons is often shared between midget ganglion cells so that, on average, Ͻ70% of the axon terminal area of a midget bipolar cell shows overlap with the dendritic field of a given midget ganglion cell. We conclude that there is no morphological evidence of red-green color selectivity in the connections between midget bipolar and midget ganglion cell mosaics. Furthermore, the results suggest that convergence is based on local interactions between axons and dendrites rather than cell-by-cell recognition between members of each mosaic.

Specificity of M and L Cone Inputs to Receptive Fields in the Parvocellular Pathway: Random Wiring with Functional Bias

“…Figure 4 A shows a drawing of a midget ganglion cell at 13°eccentricity (Jusuf et al, 2006b). The cell drawing is superimposed on a drawing of the cone pedicle mosaic close to this eccentricity (Fig.…”

“…The random wiring hypothesis predicts that, in peripheral retina, there will be mixing of input from L and M cones to the receptive field center (Lennie et al, 1991;Mullen and Kingdom, 1996;Calkins and Sterling, 1999;Diller et al, 2004), and this is supported by anatomical and physiological observations (Diller et al, 2004;Jusuf et al, 2006b) (but see Solomon et al, 2005). It is therefore unclear why many PC cells in peripheral retina show cone-opponent signals Solomon et al, 2005).…”

Chromatic Properties of Horizontal and Ganglion Cell Responses Follow a Dual Gradient in Cone Opsin Expression

“…Thus, horizontal and ganglion cells might simply propagate signals from all of the overlying cones and reflect the predictions from immunostaining (Diller et al, 2004;Jusuf et al, 2006) or they might collect selectively from one cone type, for example, to smooth out the gradients. Another possible mechanism for smoothing the gradient would be if synaptic gains on signals from M and S cones varied with retinal location.…”

Signal Pathways in the Electroretinogram

“…This is caused by the antagonistic input to receptive field centres and surrounds which receive differently weighted L-and M-cone inputs 3 . It is not clear whether the receptive field centres and surrounds are cone selective or receive a mixture of L-and M-cone input (Buzas et al, 2006;Jusuf et al, 2006). Another difference compared to the magnocellular pathway is that there is psychophysical evidence that L-and M-cones have about equal input independent of their densities in the retina (Krauskopf, 2000;Kremers et al, 2000) and state of adaptation .…”