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Manuel Spitschan, Jack Ryan, David H Brainard, Geoffrey K. Aguirre; Dependence of melanopsin-mediated human pupil responses upon light level and stimulus masking. Invest. Ophthalmol. Vis. Sci. 2016;57(12):4562.
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© ARVO (1962-2015); The Authors (2016-present)
We measured how signals from melanopsin (Mel) drive the human pupil response as a function of overall light level, and what components of the response are altered by stimulus masking.
We created Mel-directed light steps (method of silent substitution, +42% Mel contrast, nominally silent for the LMS cones). After removing residual luminance and chromatic stimulus components with psychophysical nulling, three observers viewed these steps (5.5 sec, 250 msec half-cosine window at on/offset) as modulations of a spatially uniform annular field (5° id, 27.5° od, CIE x=0.50, y=0.40) at five logarithmically-spaced photopic light levels. The viewing eye was pharmacologically dilated and consensual pupil responses were recorded in the other eye. Step onset time was jittered across the 45 second trials.
At 70 and 240 cd/m2, there was a sustained pupil constriction in response to the Mel-directed steps that persisted beyond step offset until the end of the trial. Repeat measurements at these light levels replicated the effect. The sustained response was not clearly different from zero at 20, 895, and 2780 cd/m2. At all light levels, there was also a transient pupil constriction at both onset and offset of the Mel-directed step. To address the possibility that the observed responses were in fact due to residual stimulation of the cones and/or awareness of the stimulus, the measurements were repeated at 240 cd/m2 with added cone noise (8 Hz noise uniformly distributed between ±3% L+M+S and ±3% L–M contrast). This noise masked the visibility of the Mel-directed steps, but not that of control L+M+S cone-directed steps (+42% contrast). This manipulation greatly reduced the transient responses to the Mel-directed steps, but left the sustained response unaltered. Transient responses, however, remained for the L+M+S cone-directed steps, despite the noise.
Mel-directed contrast steps presented on mid-photopic backgrounds evoke sustained pupil responses even when masked by cone noise. The magnitude of sustained response depends strongly on background light level, with maximal response observed at 70 and 240 cd/m2. This dependence could be due to properties of the melanopsin-containing retinal ganglion cells and/or to the properties of the pupil control system.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
a) Time average pupil responses as a function of light level in three subjects; b) Average pupil time series at 240 cd/m2.
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