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EDITORIAL FOCUS
use a new technique to obtain new and convincing results about the specificity of cone connections in the monkey retina. Because monkey and human retinas are so alike structurally and functionally, these new results are relevant to understanding the neural basis of human color vision.
Like many others, Sun et al. (2006) used modulation of the color of visual stimuli to discover the pattern of cone inputs, but they made a shrewd choice of the kind of color modulation to use. Figure 1 of their paper shows their technique of modulation around a circle in color space. The idea for these stimuli came from visual psychophysics (Zaidi and Halevy 1991, 1993). Such stimuli cause sinusoidal temporal modulation of the spike rate in retinal ganglion cells; the temporal phase shift of the response with respect to the stimulus tells us what is the preferred color direction around that circle for that ganglion cell. One plane and one direction are not enough to characterize a ganglion cell because we need to know the strength and sign of the inputs from the three cone types: L, M, and S. Thus Sun et al. (2006) used modulation around color circles in three different planes of color space for studying each cell, as detailed in their paper. From their measurements of the phases of responses to these three color circles, they inferred the weighting of the three cone inputs to the ganglion cell, including the signs.
Sun et al. (2006) used their color circle technique to answer a subset of the fundamental question posed above: namely, what is the connectivity pattern from S-cones to ganglion cells? One vexing question recently in the neurophysiology of color vision is, what are the weights of S-cone signals in the parvocellular neurons that mainly receive L-M, or M-L inputs, and in the magnocellular neurons that receive L+M inputs? In older work, Derrington et al. (1984) stated that there was weak or no S-cone input to the L-M parvocellular and to magnocellular neurons at the level of the LGN. Reid and Shapley (2002) confirmed this finding using the direct technique of measurement of the response to cone-isolating stimuli. However, Chatterjee and Callaway (2002) claimed to find substantial S cone input in magnocellular neurons. Sun et al. (2006), using color circle modulation, found, like Derrington et al. and Reid and Shapley, that there was little or no S-cone input either to magnocellular-projecting ganglion cells or to those parvocellular-projecting ganglion cells that respond to L-M inputs. Like other authors, Sun et al.(2006) found evidence for strong, measurable S-cone signals in the small fraction of ganglion cells that receive S-L or S-(L+M) cone inputs.
The confirmation of negligible or very weak S-cone inputs to magno-projecting ganglion cells proves the importance of the specificity of cone connectivity in the primate retina. However, there is a wealth of other experimental evidence, including other new results in the paper of Sun et al. (2006), that reveals other kinds of specificity. In my opinion, a very important question about specificity is, how specific are the L- and M-cone connections in L-M or M-L parvocellular-projecting neurons? The reason I think it is important is that it is related to sensitivity of the L-M neurons to color (DeValois et al. 1966). Reid and Shapley (1992, 2002) found that L- and M-cone inputs were very specific, with each L-M neuron receiving roughly equal and opposite weighted signals from the L- and M-cones, and each cone only connecting with a one sign of input, either excitatory or inhibitory. This finding was supported also by the work of Lee et al. (1998). While arguments have been offered about how the antagonistic surround of L-M cells could be formed by random connections from L and M cones (e.g., Lennie et al. 1991), there is no experimental evidence for such nonspecific connectivity in the monkey retina.
There are other indications of specificity, for instance, the relatively narrow distribution of L/M ratio in magnocellular-projecting retinal ganglion cells. If these ganglion cells sampled from the cone mosaic at random, one would expect that the L/M ratio could be distributed over a wide range of values. Sun et al. (2006) showed that the distribution is fairly narrow, with a mean ratio of 1.6. The findings of specificity in many different physiological measurements do not answer a new question that they raise, namely, how can retinal functional connections be so specific? There is as yet no satisfactory answer to this question, and it is a challenge for future research.
New York University, Center for Neural Science, New York, New York
Address for reprint requests and other correspondence: New York University, Center for Neural Science, New York, New York 10003 (E-mail: shapley{at}cns.nyu.edu)
REFERENCES
Chatterjee S and Callaway EM. S cone contributions to the magnocellular visual pathway in macaque monkey. Neuron 35: 1135–1146, 2002.[CrossRef][ISI][Medline]
Derrington AM, Krauskopf J, and Lennie P. Chromatic mechanisms in lateral geniculate nucleus of macaque. J Physiol 357: 241–265, 1984.
DeValois R, Abramov I, and Jacobs GH. Analysis of response patterns of LGN cells. J Opt Soc Am 56: 966–977, 1966.[Medline]
Lee BB, Kremers J, and Yeh T. Receptive fields of primate retinal ganglion cells studied with a novel technique. Vis Neurosci 15: 161–175, 1998.[CrossRef][ISI][Medline]
Lennie P, Haake PW, and Williams DR. The design of chromatically opponent receptive fields. In: Computational Models of Visual Processing, edited by Landy M and Movshon J, Cambridge, MA: MIT Press, 1991, p. 71–82.
Reid RC and Shapley RM. Spatial structure of cone inputs to receptive fields in primate lateral geniculate nucleus. Nature 356: 716–718, 1992.[CrossRef][Medline]
Reid RC and Shapley RM. Space and time maps of cone photoreceptor signals in macaque lateral geniculate nucleus. J Neurosci 22: 6158–6175, 2002.
Sun H, Smithson H, Zaidi Q, and Lee BB Specificity of cone inputs to macaque retinal ganglion cells. J Neurophysiol 95: 837–849, 2006.
Zaidi Q and Halevy D. Chromatic mechanisms beyond linear opponency. In: From Pigments to Perception, edited by Valberg A and Lee BB. London: Plenum Press, 1991, p. 337–348.
Zaidi Q and Halevy D. Visual mechanisms that signal the direction of color changes. Vision Res 33: 1037–1051, 1993.[CrossRef][ISI][Medline]
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