Investigation of the-visual processes mediating the pupillary light responses

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To establish the pupillometry as a noninvasive and objective probe for visual processing in humans, the present study investigated the visual mechanism mediating the pupillary response evoked by chromatic changes. The present findings can be summarized as follows. (1) The properties of the chromatic pupillary response are similar in several aspects to those of the perceptual responses (I.e., detection thresholds). Both responses are driven by cone-opponent signals and at least partially mediated by an (L-M) chromatic process. The cone-opponent interaction in the (L-M) process is subtractive and the weight for L-cone signals is almost identical to that for M-cone signals. (2) The temporal response properties of the pupillary response are partially similar to those of the retinal ganglion cells (PC cells) mediating color detection in that the chromatic pupillary response exhibits a sustained (as well as transient) response wave form. (3) The sensitivity of the pupillary response can be reduced in a similar way to the perceptual response by color-contrast adaptation. This finding suggests that cortical chromatic processes contribute to the pupillary response. These results improve the understanding of the functional properties of the pupillary visual process and suggest that the visual signals driving the pupillary response come from PC cells in the retina via some cortical areas where the visual signals would be transformed substantially. In addition, the present results showed that the pupillometry can be used to investigate, chromatic responses at various contrast levels ranging from near-threshold to at least 2 log units above threshold.In summary, the present study provided converging evidence that the pupillometry can be a useful and promising probe for visual processing in humans.