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types of bipolar cell might be absent or somehow impaired, or they might be incompletely differentiated into clear-cut types. Without bipolar cells the visual impulses from cones would be blocked and complete blindness would result.

6.5  Common pathways of rods and cones

Polyak’s histological investigation of the primate retina also resulted in profound new insights into the function of the rod system. Schultze (1866) had presumed that the rod impulses were transmitted independently­ of the cone impulses to the brain. Strong supporting evidence had been provided by Ramon y Cajal (1937). Presuming, like Schultze, that there were two fundamentally different receptor types, rods and cones, that functioned under quite different conditions mediating qualitatively different colour sensations, Cajal predicted that there had to be two different types of bipolar cell in the retina: those connected solely to rods and those connected solely to cones. Indeed, he assumed that there had to be separate rod and cone pathways all the way from the photoreceptors to the brain (see Cajal, 1937, p. 384, Fig. 47 and pp. 393–395). In accord with his prediction, Cajal, in a series of histological studies on both fish and mammals, found that the rod and cone receptors were connected to different types of bipolar cell.

Polyak (1948), however, strongly rejected Cajal’s idea that the rod and cone impulses travelled along separate routes to the brain. In fact, he found that all the bipolar and ganglion cell varieties, except the midget bipolar variety that was exclusively connected to the cones, were common to both the rod and cone systems. By examining Cajal’s own histological results, he also pointed out that these results indicated common rod and cone pathways, since the rod bipolar cells shown could scarcely avoid contact with cone pedicles, at least in the central region of the retina. Indeed, Cajal, in an earlier paper, had suggested that the rod and cone pathways to the bipolar cells were not completely separated from each other, but that each pathway was dominated by either rod or cone impulses (see Cajal, 1894, p. 129).

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6.6  Explanations of acuity and sensitivity differences between rods and cones

Schultze (1866) in his formulation of the duplicity theory could give no adequate explanation of the difference in acuity performance­ obtained psychophysically between the rod and cone receptor systems. Since the rod receptors were closely packed, more slender and also much more numerous than the cones, Schultze had expected, contrary to the facts, that rods would generally show better acuity performance than the cones.

A solution to this paradox was found when the Golgi method was introduced to reveal fine structural relationships in the retina. Thus, the histological investigations of both Cajal and Polyak revealed that the rod system was arranged in spatially larger functional units than the cone system. Polyak’s investigation was the most important one, since he investigated retinas of primates and also discovered the so-called midget cone system, i.e. a monosynaptic relationship between cones and bipolar cells and between bipolar and ganglion cells, affording a ‘private’ conducting cone system in the central fovea of the primate. Polyak held that such a system would be best suited to serve as a mechanism for the most delicate space resolving and localizing functions. Its degree of refinement would depend on the cone gradient only and, hence, peak in the centre of the fovea.

A complication was introduced in that the midget system was strictly monosynaptic only in the central foveal region becoming gradually coarser out towards the orea serrata, since polysynaptic connections increased collecting impulses from several cones into the functional units. Hence, acuity would no longer depend on cone gradient alone, but also upon the synaptic relations of the midget system.

The reason for the relatively low acuity performance of the rod system was clearly revealed when Polyak showed that in the central part of the retina several rods were connected to a functional unit and that the rod system, even more than the midget cone system, displayed an increasing coarsening of the functional unit from the foveal region

70development of the duplicity theory from 1930–1966

towards the orea serrata. Thus, he could provide explanation both for Schultze’s paradox of the observed acuity ­difference between the rod and cone systems, and the decrease in acuity performance with eccentricity.

The structural arrangement and synaptic connections of the primate retina shown by Polyak by the Golgi method also provided new insight into the light sensitivity difference between the rod and cone systems. Since many more rods than cones were connected to a given functional unit, more rod impulses could be combined and, thereby, make the rod system the most sensitive. Hence, ­photochemical differences between the cone and rod receptor type could no longer be regarded as the sole explanatory factor of the greater rod sensitivity.

6.7  The functional potentials of the synaptic arrangement

Even more important, the histological investigation of the retina with the Golgi method revealed that the retinal neural tissue was not composed of intimately related neurons in continuous nerve nets ornerve webs, but was made up of individual nerve cells only in contact with each other through synapses. This profound insight into the structure of the neural system had been revealed by Cajal in the late 1880s (see Cajal, 1937), but it was not until Polyak (1941) published his great work that the full functional potential of the primate retina became apparent. Thus, Polyak revealed that the neural structure of the primate retina was much more complicated than previously held. Both the bipolar and ganglion cells were composed of several varieties woven into an intricate synaptic pattern. Such a structure, of course, made possible much more varied reactions than a neuron network or neuron web composed of neurons of the same or similar kinds. In particular, Polyak pointed to the possibility that the location and number of synaptic contacts might affect both the kind and magnitude of an input received by a neuron, and that horizontal andamacrine cells might be involved in facilitation and inhibition of the

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centripetal­ impulses. Also, he stressed the interesting possibility that the output reactions of a given neuron, both with regard to quantity and quality, might depend not so much upon the input as upon its own innate disposition, as illustrated by the bioelectrical output of the photoreceptors elicited by the light energy input.

Polyak’s theory of rod and cone functions, although highly speculative, was based on far more accurate histological data than the theory of Schultze. His results strongly indicated that some of the basic assumptions of Schultze had to be reformulated. Thus, he could find no evidence of three distinct varieties of minute structures that could serve as chromatic analyzers in the cone receptors and also showed that several of the bipolar and ganglion cell varieties were connected to both the rod and cone systems. Indeed, Polyak’s own functional interpretation of the histological structure of the retina of the primate made it seem possible that colour vision, sensitivity regulation and acuity performances to a large extent were controlled by bipolar, horizontal, amacrine and ganglion cells and their synaptic relations.

Milestones in the further development of our understanding of the function of rods and cones are represented by the works of H. K. Hartline, S. W. Kuffler and R. Granit.