A linear chromatic mechanism drives the pupillary response

Previous studies have shown that a chromatic mechanism can drive pupil responses. The aim of this research was to clarify whether a linear or non-linear chromatic mechanism drives pupillary responses by using various colours of test stimuli defined in cone-contrast space. Pupil and accommodation responses evoked by these test stimuli were continuously and simultaneously measured objectively by photorefraction. The results with isochromatic and isoluminant stimuli showed that the accommodative level remained approximately constant (<0.25D change in mean level) even when the concurrent pupillary responses was large (~0.30mm). The pupillary response to an isoluminant grating was sustained, delayed (by ~60ms) and larger in amplitude than that for a isochromatic uniform stimulus, which supports previous work suggesting that the chromatic mechanism contributes to the pupillary response. In a second experiment, selected chromatic test gratings were utilized and iso-response contours in cone-contrast space were obtained. The results showed that the iso-response contour in cone contrast space is well described (r^2=0.99) by a straight line with a positive slope. The results indicate that a L-M linear chromatic mechanism, whereby a signal from the Long-wavelength cone (L-cone) is subtracted from that of the Middle-wavelength cone (M-cone) and vice versa, drives pupillary responses.