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P.B. Kruger, H. Rutman, F.J. Rucker, W.H. Swanson; Accommodation Does Not Respond to Change in the Ratio of L–/M–Cone Excitations . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4571.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: There is uncertainty whether accommodation responds to the ratio of long–wavelength (L–) and middle–wavelength (M–) cone excitations or to the ratio of L– and M–cone contrasts (Flitcroft, 1990, Visual Neuroscience 5:547–555; Rucker & Kruger, 2004, Vision Research 44:2931–2944). We tested the former hypothesis using white and monochromatic red or green stimuli, with L– and M–cone contrasts held constant. Methods: Accommodation was monitored continuously while observers (24) viewed a vertical sine–wave grating (2.7 cycles/deg.; 0.56 contrast) in a Badal optical system. The grating target remained stationary (2 D) during trials lasting 20.5 sec. and was viewed through a pinhole pupil (0.75 mm). The target was illuminated by equal–energy white light (tungsten + Kodak Wratten 80A; 158 td) during the first half of the trial; then illumination changed to red (620 nm with 10 nm bandwidth; 175 td), or green (520 nm with 10 nm bandwidth; 156 td), or remained white during the second half of the trial. When illumination changed from white to red, M–cone excitation decreased (–64%) and L–cone excitation increased (50%). Conversely, when illumination changed from white to green, M–cone excitation increased (24%) and L–cone excitation decreased (–14%). The three conditions were presented six times each in random order. L–cone and M–cone contrasts were held constant (0.56) by viewing through a pinhole pupil. Mean response was calculated for the first and second half of each trial, and the change in mean accommodative response was compared among the three conditions. Results: The change in mean accommodative response for the three conditions was not statistically different (F=2.36; p>0.10; ANOVA). None of the observers accommodated in the expected direction (i.e. an increase in accommodation in red light, and a decrease in accommodation in green light), but four observers accommodated in the opposite direction. Conclusions: The mean level of accommodation does not respond to change in the ratio of L– and M–cone excitations. Instead, accommodation responds to change in the ratio of L– and M–cone contrasts. Comparison of cone–contrasts rather than cone–excitations ensures that the chromatic signal from longitudinal chromatic aberration remains veridical despite large changes in the wavelength composition of broadband illumination.
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