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Dingcai Cao, Pablo Barrionuevo, Nathaniel Nicandro, J Jason McAnany, Andrew Zele, Paul Gamlin; Nonlinear Pupil Responses to Rod and Cone Inputs. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2674.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the loci of nonlinear processes in pupil responses to rod and cone inputs
A four-primary photostimulating method was implemented in a Ganzfeld (Diagnosys ColorDome) to generate rod or cone isolating stimuli at 0.13 and 1.0 photopic cd/m2 (or 0.10 and 0.82 scotopic cd/m2). The stimuli were modulated according to a single sinusoidal waveform (single sinewave condition, 12% Michelson contrast) at one temporal frequency of 4, 5, 8 or 9 Hz, or to the sum of two sinusoidal waveforms at different frequencies (beat condition, frequency pair 4+5 Hz, or 8+9 Hz, generating a beat frequency of 1 Hz). The component frequencies were chosen to minimize the melanopsin photoresponse of ipRGC such that pupil response was primarily driven by photoreceptor inputs. Each modulation was either the rod or cone isolating modulation. The pupil sizes were recorded using an Eyelink II (SR Research) eyetracker in three observers. Fourier analysis was used to derive the amplitudes and phases of the pupil responses.
At both light levels, the pupil response to the single sinewave modulation that isolated rod or cone excitations was minimal at each component frequency. For the beat condition, when modulation was restricted to a single photoreceptor type, there was a pronounced pupil response to the beat frequency (1Hz) for the 4+5 Hz frequency pair but not for the 8+9Hz frequency pair for all three observers. However, when one component modulated rod excitations and the other component modulated cone excitations, there was no response to the beat frequency for two observers.
There is nonlinearity in the pupil response to rod and cone inputs at mesopic light levels. Finding a beat response for modulation components restricted to a single photoreceptor type, but not for components with two photoreceptor types, suggests that the location of nonlinear processing in the pupil response occurs at a site earlier than where the rod and cone signals are combined, that is, at the photoreceptor level.
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