14  Further exploration of chromatic rod vision

14.1  Contribution of J. J. McCann and J. L. Benton

Soon after the discovery that rod signals may initiate all kinds of hue, a number of research workers contributed to the further exploration of this phenomenon.

Firstly, McCann and Benton (1969) convincingly demonstrated that rods had the ability to interact with the long-wave cones (L-cones) and thereby produce a multicoloured image. This was illustrated by first illuminating a multicoloured paper with a 656 nm monochromatic light at an intensity level just above the colour threshold in order to activate only the L-cone mechanism. Thereafter, they superimposed a monochromatic light of 546 or 450 nm that activated only the rod receptor system. Adding the scotopic light dramatically changed the colour of the display; red, orange, yellow, blue-green, brown, grey and black could be seen in the display.

A more sophisticated and detailed study of this rod-cone interaction colour effect was later reported by McKee, McCann and Benton (1977). To produce the multicoloured display, a transparent photographic picture was taken both through a red and a green filter and then combined. The display was then illuminated with a red 656 nm monochromatic light at an intensity just above the colour threshold and by one of ten monochromatic lights selected from the 420–600 nm region of the spectrum. By increasing, in turn, the intensity level of each of the ten monochromatic lights they demonstrated that the threshold level of the multicoloured image (i.e. the lowest intensity level where the observer reported that he could see a faint image of red, yellow and blue-green) fitted the scotopic luminosity curve.

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The intensity of the 656 nm light was then increased 1.2 log units and the observer asked to adjust the intensity of one of the ten monochromatic lights until the multicoloured image was judged to appear neither too red nor too blue-green. This judgement was termed ‘optimum colour’. Under these conditions the observer saw a larger variety of colours, including whites, reds, oranges, yellows, blue-greens and browns. When the ‘optimum colour’-threshold intensity was plotted as a function of wavelength, the curve again matched the scotopic luminosity function.

Lastly, a 510 nm light was directed either through the centre or periphery of the pupil while the observer adjusted the intensity of the 510 nm light to obtain ‘optimum colour’.

The results showed that the ‘optimum colour’-threshold was obtained at very similar intensity levels under these two conditions. Thus, the Stiles-Crawford effect associated with cone vision was absent, indicating that the 510 nm light stimulated rods only. On the other hand, a Stiles-Crawford effect became apparent when the intensity of the 656 nm light was further increased, so that the intensity of the 510 nm light reached above cone-threshold level before the ‘optimum colour’ threshold was reached.

The authors concluded that L-cones may interact with rods to produce a multicoloured image and that, therefore, rods may be as much a part of colour vision as cones.

A clue to an understanding of this remarkable ability of the rod receptor system was found in an observation made by McCann and Benton (1969). These authors had shown that when a photopic 495 nm test light was added to the 656 nm multicoloured display, the subject observed the same change in colour as obtained with the scotopic test light. The only difference was that the colours obtained with the photopic 495 nm test light appeared somewhat brighter, sharper and slightly more saturated. Accordingly, since the photopic 495 nm test light could reasonably be assumed to activate both the S-wave and M-wave cone receptors, McKee, McCann and Benton

126 chromatic rod vision: a historical account

(1977) concluded that rods may share the colour channels with both the middleand short-wave cones.

14.2  Contribution of P. W. Trezona

Another early contribution to the idea of chromatic rod vision was provided by Trezona (1960, 1970). Her starting point was an exploration of colour after-effects observed following stimulation with a ‘white’ stimulus (illuminant SA). Retinal illumination, field size, time of exposure and the part of the retina stimulated were varied, and the subject, sitting in the dark, simply reported on the colour after-effects seen (Trezona, 1960).

The most significant after-effect observed was the so-called‘after-blueness’ – a blue sensation that lasted only a few seconds. This‘after-blueness’ was observable only when the ‘white’ pre-stimula- tion light had activated receptors outside the rod-free region. Hence, she concluded that it might be triggered by rods.

She found supporting evidence by reviewing previous studies where she, just like Willmer (1961), could point to observations that showed rod activity was associated with blue colour sensations under a variety of conditions. For instance, it had been found that when rod activity was added to a test field that matched the colour of a comparison field, a blue component had to be added to the comparison field in order to restore the match.

As mentioned above, König (1894) did not make any serious attempt to explain the change-over from achromatic to blue rod vision as intensity increased from scotopic to photopic levels. Trezona (1970), on the other hand, attempted to explain this shift in rod colour based on the assumption that rod signals had access to the spectrally opponent yellow-blue cells (the y-b unit), and that they would generate a blue colour sensation only when they reached above the threshold level of the (y-b) unit. Yet, she held that the ‘blue’cones tended to inhibit rod activity and that, as a consequence, the blue-related rod activity would be increasingly inhibited as the intensity increased within the mesopic intensity interval when ‘blue’ test lights were employed.

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14.3  Contribution of C. F. Stromeyer III

A third early contribution to an understanding of chromatic rod vision was provided by Charles F. Stromeyer III (1974a, b). He showed that the McCollough effect (i.e. a form-specific colour after-effect) could be obtained in scotopic vision. The McCollough effect was built up with square-wave gratings (0.75 cycle deg -1), subtending an area of about 30 degrees square. In the first session the subjects were adapted for 20 min to a vertical grating projected in green light and a horizontal grating in magenta (red) light. The patterns were projected in alternate vertical and horizontal orientations and interchanged every second. A tiny fixation light was located in the screen’s centre. After this adaptation procedure, the subject dark adapted for 30 min and was then test stimulated at scotopic intensity levels with a blackwhite test grating with retinal orientation and grating frequency that matched the adapting patterns.

The results showed that after adaptation to the vertical green and horizontal magenta (red) gratings, the vertical and horizontal test grating appeared faintly reddish and greenish, respectively, at the lowest scotopic test level, i.e. 1.8 log units below the cone-rod break measured for the white strips of the test pattern. Also, the subjects reported that the colours became more saturated as the test light was increased within the scotopic intensity range.

The experiment was repeated with a vertical grating projected in orange light and a horizontal grating in blue light. After this colour adaptation, the vertical and horizontal components of the white test pattern appeared, respectively, blue or blue-green and burnt orange or yellowish in scotopic vision.

To explain the McCollough colour after-effect in scotopic vision, Stromeyer (1974a, b) suggested that the adaptation to the coloured grating tended to depress sensitivity in cortical cells tuned to the adapting colour, spatial frequency and orientation. A test pattern similar to the adapting pattern would then produce less activity in these cells, and this reduced response would tend to signal