Pupil responses to foveal exchange of monochromatic lights.

Temporal exchanges of equiluminant monochromatic lights of different spectral distributions produced a momentary constriction of the pupil in man. This is not a stimulus artifact because exchanges of two lights with identical distributions in the same apparatus produced no response. Responses evoked by rod signals were successfully obviated by presenting the foveal stimulus inside a large rod saturating annulus. The amplitude of the response varied systematically with stimulus wavelength. The exchange of a standard light to either shorter or longer wavelength lights produced a momentary constriction of the pupil; the greater the wavelength difference (between them); the larger the constriction. This ability to respond to exchanges of one spectral distribution for another is not a consequence of chromatic aberration or chromatic differences in magnification. Chromatic exchanges between lights of equal chromatic aberration do not produce identical pupillary response in deuteranopes: The exchange 560 nm→650 nm produced no pupil response, while the 560 nm→498 nm exchange produced a sizable response. Exchange of equally luminant heterochromatic lights evoked a response with 50 ms longer latency than the same amplitude constriction evoked by a step increase in luminance of a homochromatic light. The homochromatic contrast needed to evoke the same constriction as a given equal luminance heterochromatic exchange closely follows the homochromatic contrast which matched the residual flicker in flicker photometry of that same wavelength pair.