6  The duplicity theory of Polyak

After a comprehensive review of the anatomical and histological research on the organs of vision from the classical antiquity to his own time, Polyak (1941) concluded that this research had played a relatively insignificant role in forming hypotheses and theories of the function of the visual system. A major reason was that the retina of humans and other primates had never been examined in any detail byanalytical histological methods such as those of Golgi and Ehrlich. These methods, although very time consuming, were preferable, since they permitted the staining of only a few nerve cells at a time from the bewildering mass of tissues. Indeed, in successfully treated preparations, the individual neurons could be stained completely, revealing the whole nerve cell with all its extensions and branches extending as far as the finest terminals. Furthermore, he held that a disclosure of the structural details of the retina of primates would make possible interpretations related to the many psychophysical functions already established in humans.

Polyak, therefore, completed a very impressive histological investigation on the retina of macaques and chimpanzees, using the Golgi method. He also occasionally used retinas from humans. His comprehensive and seminal research work (summarized in 1941; second impression, 1948) has long been recognized as a classic contribution to our knowledge of the retinal structure and function in primates.

His results confirmed two of the basic findings of Schultze (1866): (1) the retina of primates contained two morphologically clearly ­differentiated varieties of photoreceptors, the rods and cones, and (2) the structure of cones did not differ essentially within any of the retinal regions. They all appeared to belong to the same variety, possessing

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the same structure and synaptic relationships. Polyak, therefore, like Schultze, presumed that they were functionally ­equivalent.

Yet, Polyak found that the cone receptors in different regions of the primate retina had somewhat different structures and that they, therefore, probably also differed somewhat with regard to function. Thus, Polyak held that the cones in the foveal centre were farthest advanced in their structure and biochemical differentiation and capable of the most elaborate and diverse responses, and that the responses gradually became more crude and summary in character as the ora serrata was approached.

6.1  Trichromacy of colour vision explained by three types of bipolar cell

Schultze had explained trichromacy, demonstrated by Maxwell (1860), by suggesting that different spectral lights affected different outgoing fibres connected to the inner segment of each cone in a qualitatively different manner. Polyak, however, did not agree with Schultze that the mechanism of trichromacy was located in each cone receptor. Using the Golgi technique, he could find no evidence of three distinct varieties of minute structures or organelles that could serve as chromatic analyzers in the cone receptors of primates. He therefore, at variance with Schultze, presumed that the cone response, over short time intervals, was rather uniform and homogeneous, spreading over the entire cone cell, including all its extensions. To explain the close connection between wavelength of light and hue sensation, he suggested that at different times the photosensitive cone substance would generate different kinds of dynamic changes subserving different kinds of colour sensations when activated by different wavelengths or different wavelength combinations.

But if each cone may mediate hundreds of different kinds of uniform colour-related reactions in response to different wavelengths, then how could the trichromacy of colour vision be explained? Obviously, the numerous colour-related cone excitations had to be reduced to three basic processes somewhere in the visual pathway.

64development of the duplicity theory from 1930–1966

Polyak pointed to the bipolar cells as promising candidates for performing this analyzing function. These neurons were positioned between the cones and the ganglion cells and therefore had to be regarded as the sole centripetal transmitter of the cone impulses. Also, Polyak had found the number of bipolar cell types in the primate retinas corresponded closely to the number of fibres postulated by the three-component theory of Young (1802). Polyak named four varieties: the mop, brush, flat and midget bipolar, but was uncertain whether the brush and flat bipolar cells actually represented different types.

To explain their analyzing functions, Polyak suggested, as the most probable hypothesis, that although each bipolar variety had preserved its original universal responsiveness to the visual spectrum, to some extent, each of them had become especially responsive to a particular part of the spectrum. Presuming that the cones had their maximum sensitivity in the long-wave region of the spectrum, Polyak suggested that the midget bipolar cells, being exclusively connected to cones, were chiefly responsive to cone impulses corresponding to this spectral region, while they were moderately responsive to the middle region, and least, if responsive at all to the short-wave region. The responsiveness of the mop bipolar cells, on the other hand, was assumed to be just the reverse, since he had discovered that this bipolar variety was chiefly connected to the rods that were known to be most sensitive to relatively short wavelengths. Finally, the brush and flat bipolar cells were assumed to be chiefly concerned with transmission of impulses corresponding to the middle part and less so to the two ends of the spectrum. As can be seen, Polyak made use of three types of bipolar cell, instead of three types of cone, to satisfy the requirement of the three-component theory.

The next problem confronting Polyak was to account for the apparent synthesis of the different colour-related processes. Obviously, since mixing the three primaries may produce colour sensations that are appreciated as a single quality, some merging and combination of the visual responses have to take place subsequent to the analyzing process.

66development of the duplicity theory from 1930–1966

received from the three or four different kinds of bipolar cells. This presumption, of course, clearly violated the specific fibre-energy doctrine of Helmholtz (1896) and also appeared to be in conflict with electrophysiological recordings from single nerve fibres where only one kind of electrical activity had been recorded (Graham & Hartline, 1935). In opposition to the doctrine of Helmholtz, however, Polyak stressed the possibility that the mode of reaction of a nerve fibre might be ‘plastic’. Thus, he suggested that a great variety of bioelectrical and chemical processes of different kinds might take place in the neuro-plasma of a nerve fibre while eliciting substantially the same outward electrical manifestation registered by the microelectrode. Different qualities might also be coded by special groupings of the impulses in time, for example, by rhythmical groupings not yet known. Furthermore, in defence of his colour theory, Polyak pointed to the fact that the retina had a very complex structure and was made up of basically the same nerve cells as found in the brain, where evidently different qualities could emerge. He did admit, however, that little was yet known with regard to the ability of the same neuron or nerve fibre to generate and to conduct different modalities and qualities of dynamical changes.

6.4  Applications of Polyak’s colour theory

Polyak held that his colour theory, because of its general character, was applicable to a number of other colour theories in addition to Young’s trichromatic colour theory. The basic requirements of Hering’s opponent colour theory, for example, could be satisfied by the assumption that transmission of each dissimilationassimilation process (the red-green, yellow-blue and white-black antagonistic processes) was carried out by one of the three bipolar varieties.

Furthermore, Polyak argued that his colour theory provided new insight into the material causes of various forms of colourblindness: protanopia, deuteranopia, tritanopia, various forms of anomalous trichromatism, monochromatism, and also partial and complete blindness. Instead of failures of cones, one, two or all three