A linear chromatic mechanism drives the pupillary response

Previous studies have shown that a chromatic mechanism can drive pupil resp onses. The aim of this research was to clarify whether a linear or nonlinea r chromatic mechanism drives pupillary responses by using test stimuli of v arious colours that are defined in cone contrast space. The pupil and accom modation responses evoked by these test stimuli were continuously and simul taneously objectively measured by photorefraction. The results with isochro matic and isoluminant stimuli showed that (lie accommodative level remained approximately constant (< 0.25 D change in mean level) even when the concu rrent pupillary response was large (ca. 0.30mm). The pupillary response to an isoluminant grating was sustained, delayed by ca. 60 ms) and larger in a mplitude than that for a isochromatic uniform stimulus, which supports prev ious work suggesting that the chromatic mechanism contributes to the pupill ary response. In a second experiment, selected chromatic test gratings were used and isoresponse contours in cone contrast space were obtained. The re sults showed that the isoresponse contour in cone contrast space is well de scribed (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 is subtracted from that of the middle wavelength c one and vice versa, drives pupillary responses.