Assessing rod, cone, and melanopsin contributions to human pupil flicker responses

Abstract

Purpose: We determined the relative contributions of rods, cones, and melanopsin to pupil responses in humans using temporal sinusoidal stimulation for light levels spanning the low mesopic to photopic range.

Methods: A four-primary Ganzfeld photostimulator controlled flicker stimulations at seven light levels (-2.7 to 2 log cd/m(2)) and five frequencies (0.5-8 Hz). Pupil diameter was measured using a high-resolution eye tracker. Three kinds of sinusoidal photoreceptor modulations were generated using silent substitution: rod modulation, cone modulation, and combined rod and cone modulation in phase (experiment 1) or cone phase shifted (experiment 2) from a fixed rod phase. The melanopsin excitation was computed for each condition. A vector sum model was used to estimate the relative contribution of rods, cones, and melanopsin to the pupil response.

Results: From experiment 1, the pupil frequency response peaked at 1 Hz at two mesopic light levels for the three modulation conditions. Analyzing the rod-cone phase difference for the combined modulations (experiment 2) identified a V-shaped response amplitude with a minimum between 135° and 180°. The pupil response phases increased as cone modulation phase increased. The pupil amplitude increased with increasing light level for cone, and combined (in-phase rod and cone) modulation, but not for the rod modulation.

Conclusions: These results demonstrate that cone- and rod-pathway contributions are more predominant than melanopsin contribution to the phasic pupil response. The combined rod, cone, and melanopsin inputs to the phasic state of the pupil light reflex follow linear summation.

Keywords: flicker; melanopsin; pupil light reflex; silent-substitution.

Figure 1

(A) Sequential presentation of the stimuli. Each session started from the background adaptation period. If more than one session were run in a block, only one dark adaptation was performed. (B) Pupil trace in temporal domain. The pupil response clearly showed a sinusoidal pattern. A sinusoidal trace at 1 Hz is added for reference (thin gray line). (C) Frequency spectrum of the pupil amplitude response (B), the highest component is for the stimulation frequency. In this case, the parameters of the sinusoidal stimulation were: frequency = 1 Hz, luminance of the background = 1 photopic cd/m², and rod-cone phase = 45°.

Figure 2

Predicted amplitude and phase relationship of the rod, cone, and melanopsin contributions to the phasic pupil response at 1 Hz. A temporal sinusoidal photoresponse (A) can be represented in a polar plot as a vector, where the signal magnitude is the vector length and the signal phase is the vector angle. (B) The vector sum of the photoresponses represents the output of the ipRGC. (C) Predictions of the vector summation model together with the rod, cone, and melanopsin predictions based on “winner-takes-all” mechanisms. The black dots (real data from experiment 2) clearly follow the vector summation prediction. The red lines indicate the pupil response for the ipRGC output vector in the polar plot (B).

Figure 3

Pupil frequency response obtained in experiment 1. Amplitude and phase data are shown for rod, cone, and combined stimuli at two mesopic light levels. Each column represents the data for each observer.

Figure 4

Relative pupil amplitude (upper panels) and phase (lower panels) as a function of light levels for the rod stimuli, cone stimuli, and combined rod and cone stimuli in phase.

Figure 5

Pupil responses for combined rod and cone modulation condition of experiment 2. Data belong to four low mesopic light levels. Amplitude and phase of the pupil data are plotted in function of the cone phases (respect to the rod phase). Error bars are standard error. The lines show the optimizations of the vectorial summation model.

Figure 6

Pupil responses for combined rod and cone modulation condition of experiment 2. Data belong to one mesopic and two photopic light levels. Same format as Figure 5.

Figure 7

Parameter relations of the vectorial summation model applied to data of experiment 2. The relative amplitudes (A) and the delays (B) of rods and melanopsin with respect to cones in all the light levels were determined from the model fits. Note that the total luminance range spans 4.7 log units, while the total illuminance range is reduced to 4 log units (see Table) due to the large changes in baseline pupil size.