30development of the basic ideas of the duplicity theory

been reckoned among its founders. We will, therefore, give an account of their contributions.

3.9  The duplicity theory of Parinaud

In harmony with Schultze’s duplicity theory (1866), Parinaud assumed that the human retina contained two different types of photoreceptor, rods and cones, and that the cones gave rise to brightness, form and colour sensations, while the rods, being solely responsible for night vision, gave rise to achromatic sensation only. Also, under mesopic conditions, where rods and cones are activated simultaneously,­

the rod activity was assumed to influence both colour and form sensations by desaturating cone-mediated colours and increasing their brightness (like adding white light to a colour) and reducingacuity performance (Parinaud, 1885, p. 822).

In contrast to Schultze (1866), however, but in agreement with Newton (1675, 1730), Parinaud held that the essential cause of the different colour qualities had to be found in the brain and not in the optic nerve (Parinaud, 1884a, p. 242). Furthermore, he held that the rods were mediators of diffuse brightness sensation only.

Yet, his most important theoretical contribution, no doubt influenced by the remarkable discoveries of Boll and Kühne, was his far-reaching photochemical presumptions:

1.  The initial light reaction of the rods is photochemical in nature, i.e. the nerve impulses are triggered by light acting on the photopigment rhodopsin. The initial light reaction of the cones, on the other hand, is of a physical nature (Parinaud, 1884b, 1885, p. 822).

2.  Rhodopsin is broken down by light and regenerates in the dark. The amount of decomposition increases when the wavelength of the light stimulus decreases from red to blue, but then remains approximately constant in the violet part of the spectrum (Parinaud, 1884b).

3.  The sensitivity of the eye increases with the amount of rhodopsin (Parinaud, 1885, p. 823).

With this last assumption, he explained both the sensitivity increase of the eye with dark adaptation and the sensitivity decrease

the schultze tradition 31

with light adaptation, suggesting that only the rods, not the cones, adapted to changes in light levels (Parinaud, 1885, p. 823).

It appears that Parinaud arrived at his theory of rod and cone functions by his study of ‘héméralopie’ (note that in French ‘héméralopie’ means night blindness, while in English ‘hemeralopia’ means day blindness). By this term, he meant night blindness (in English ‘nyctalopia’) – a defect that reduces the sensitivity of the eye markedly. He explained this defect with the assumption that rhodopsin in the rods of the night blind was non-functioning (Parinaud, 1881). Thereby, the normal form and colour vision, and the lack of sensitivity increase during dark adaptation of thenight blind could be accounted for. Moreover, he could explain the excellent acuity and colour discrimination within the rod-free fovea of humans and also the excellent acuity performance of pigeons and hens that lacked rhodopsin.

Furthermore, his photochemical presumptions could easily explain the change in size of the so-called photochromatic interval (the intensity interval between light and colour thresholds) obtained psychophysically in the extrafoveal retina in a dark-adapted state. Thus, in accordance with his predictions, the photochromatic interval was found to increase from the deep red to the blue part of the spectrum (bleaching effect increased) and to be absent when the measurements were made within the rod-free fovea.

Moreover, the deterioration of acuity performance observed when a test field is moved away from the central fovea (a major problem for Schultze) was accounted for by Parinaud with his assumptions that cones were the receptors responsible for form discrimination and that their number decreased with eccentricity (Parinaud, 1884a, p. 242, 1885, p. 822).

Finally, Parinaud (1884b) pointed out that his model also provided explanations for several other important psychophysical observations not adequately accounted for previously, e.g. thePurkinje phenomenon (see Purkinje, 1825) as well as difficulties encountered by photometric measurements. He did not formulate

32 development of the basic ideas of the duplicity theory

his arguments in any detail, however, probably because he found the explanations based on his model to be straightforward.

3.10  König: rhodopsin is the mediator

of night vision – a conclusive proof

Parinaud’s investigation of nyctalopia strongly indicated that rhodopsin was the photopigment responsible for night vision in humans. Even more compelling evidence in support of this suggestion was provided by König (1894). Using a spectrophotometer, he measured the absorption coefficient of the rhodopsin of the human eye over the spectrum from 640 nm to 420 nm and then compared the spectral absorption obtained with the spectral sensitivity measured psychophysically under scotopic conditions, in trichromats, dichromats and rod monochromats. The values of the two measurements corresponded excellently between 640 nm and 500 nm. Between 500 nm and 420 nm the spectral sensitivity was somewhat less than that calculated from the spectral absorption curve. This discrepancy could, however, be reasonably accounted for by the absorption of the macular and the lens pigments of the eye.

3.11  The duplicity theory of König

Having provided conclusive evidence that rhodopsin mediated night vision, König (1894) proceeded to formulate his special version of the duplicity theory by seven simple theses as stated below:

1.  There is no visual purple in the central fovea or in any cone receptor. (The same conclusion had previously been reached by Kühne, 1877b, 1879.)

2.  The colourless sensation obtained extrafoveally at the absolute threshold for all wavelengths (except for red-related wavelengths) is due to the decomposition of a small amount of rhodopsin.

3.  Stronger decomposition of rhodopsin produces visual yellow, which is responsible for the primary blue colour sensation.

4.  The unknown visual substances responsible for the ‘red’ and ‘green’ primaries are (like visual yellow) less sensitive to light than rhodopsin.

the schultze tradition 33

5.  In total colour-blindness (i.e. rod monochromacy) there is no photopigment other than rhodopsin, and the visual yellow cannot be sufficiently decomposed by light to activate the primary blue sensation.

6.  The Purkinje phenomenon and the breakdown of Newton’s colourmixture laws, found when the test intensity is increased, may be explained by the relative sensitivity change between rhodopsin andvisual yellow. (In scotopic vision only rhodopsin may be activated by light and hence only achromatic sensations are obtained.)

7.  The physiological process underlying ‘white’ sensation is not an increase of the process underlying the sensation of grey (die Grauempfindung).

The last thesis appears somewhat cryptic, but is probably intended to mean that two different processes underlie achromatic sensation: one generated by cones and the other by rods.

Theoretically, thesis 6 was most important, since it highlighted the limitation of the colour-mixture laws proposed by Newton (1730). From Newton’s laws one would expect that when two lights match, with respect to hue, saturation and brightness, they would continue to match when the light level is proportionally reduced or increased, since the number of rays of each colour would change in the same proportion (see Grassmann, 1853; Helmholtz, 1855, p. 24).

The prediction that the match would hold when light intensity is changed also followed from the colour theory of Young-Helmholtz (Helmholtz, 1896). Clearly, quantum absorption of the three photopigments would be expected to remain equated when the intensity of two matched colour fields is reduced or increased in the same proportion.

Yet, late in the nineteenth century, the colour-mixture laws of Newton were found to break down under extrafoveal conditions (see von Kries, 1929). The additivity failure obtained could, however, be easily explained by the finding that the extrafoveal retina has four, instead of three, elementary receptor functions, and that only rhodopsin in rods may be activated in night vision. Thus, a colour match between a test and comparison field obtained in the extrafovea in a light-adapted state (i.e. under photopic conditions) would break down under mesopic and scotopic conditions if, under these