Non-linearities in the Rod and Cone Photoreceptor Inputs to the Afferent Pupil Light Response

Abstract

Purpose: To assess the nature and extent of non-linear processes in pupil responses using rod- and cone-isolating visual beat stimuli. Methods: A four-primary photostimulating method based on the principle of silent substitution was implemented to generate rod or cone isolating and combined sinusoidal stimuli at a single component frequency (1, 4, 5, 8, or 9 Hz) or a 1 Hz beat frequency (frequency pairs: 4 + 5, 8 + 9 Hz). The component frequencies were chosen to minimize the melanopsin photoresponse of intrinsically photosensitive retinal ganglion cells (ipRGCs) such that the pupil response was primarily driven by outer retinal photoreceptor inputs. Full-field (Ganzfeld) pupil responses and electroretinograms (ERGs) were recorded to the same stimuli at two mesopic light levels (-0.9 and 0 log cd/m²). Fourier analysis was used to derive the amplitudes and phases of the pupil and ERG responses. Results: For the beat frequency condition, when modulation was restricted to the same photoreceptor type at the higher mesopic level (0 log cd/m²), there was a pronounced pupil response to the 1 Hz beat frequency with the 4 + 5 Hz frequency pair and rare beat responses for the 8 + 9 Hz frequency pair. At the lower mesopic level there were few and inconsistent beat responses. When one component modulated the rod excitation and the other component modulated the cone excitation, responses to the beat frequency were rare and lower than the 1 Hz component frequency condition responses. These results were confirmed by ERG recordings. Conclusions: There is non-linearity in both the pupil response and electroretinogram to rod and cone inputs at mesopic light levels. The presence of a beat response for modulation components restricted to a single photoreceptor type, but not for components with cross-photoreceptor types, indicates that the location of a non-linear process in the pupil pathway occurs at a retinal site earlier than where the rod and cone signals are combined, that is, at the photoreceptor level.

Keywords: ERG analysis; beats; mesopic light level; non-linearity; photoreceptors cells; pupil; retina.

Figure 1

Beats in the pupillary responses. (A) Linear system frequency response for combined sinusoidal stimulations at 4 and 5 Hz (top panel; temporal profile in the inset). Middle and bottom panels contain responses of S1 to Rod & Cone 4 Hz + Rod & Cone 5 Hz in the temporal and frequency domains, respectively. Averaged beat responses across participants are shown for combined 4 and 5 Hz (squares), and combined 8 and 9 Hz (triangles) in comparison with pupil responses for 1 Hz stimulation (circles) for same-photoreceptor type condition (B), and Cross-photoreceptor type condition (C). Error bars represent SEM.

Figure 2

Data for the same photoreceptor type condition for individual participants; S1 (left column), S2 (middle column), and S3 (right column). Upper panels contains data for 0 log cd/m², lower panels for −0.9 log cd/m². Error bars represents SEM. For this condition thee three participants elicited beat responses. S1's beat responses were elicited in all cases except for Cone 8 Hz + Cone 9 Hz at −0.9 log cd/m². For S2 (middle column), beat responses were elicited in all conditions for 4 + 5 Hz pair at 0 log cd/m² and only for Rod & Cone condition at −0.9 log cd/m². For 8 + 9 Hz pair no beat responses were obtained for Cone condition at 0 log cd/m². For S3 (right column), no beat responses were elicited at −0.9 log cd/m² or for 8 + 9 Hz stimuli at 0 log cd/m².

Figure 3

Data for the cross photoreceptor type condition for individual participants; S1 (red circles), S2 (orange triangles), and S3 (light orange squares). Dashed lines represents the Rod & Cone 1 Hz single frequency data for each subject. Upper panel contains data for 0 log cd/m², lower panel for −0.9 log cd/m². Error bars represents SEM. Cross-photoreceptor type results were rare and reduced with respect to Rod & Cone 1 Hz data. S1's pupillary recordings showed cross beat responses for both light levels. For S2 beat responses were obtained for Rod 8 Hz combined with Cone 9 Hz at both light levels, and for combination of Cone 4 Hz and Rod 5 Hz at 0 log cd/m². For S3, beat responses were obtained when Cone 4 Hz was combined with Rod 5 Hz at the lower light level. Beat amplitudes for S2 and S3 were comparable to results for Rod & Cone 1 Hz stimulus.

Figure 4

(A) Polar plot of individual results for the three participants (S1: circles, S2: squares, and S3: triangles) for beat responses (black symbols) and single 1 Hz responses (gray symbols), (B) This panel shows that beat phase caused by rectification is shifted 90° with respect to single frequency phase.

Figure 5

Beats in ERG recordings. (A) Frequency beat response of S1 to Rod & Cone 4 Hz + Rod & Cone 5 Hz condition. (B) Averaged beat responses for combined 4 and 5 Hz (squares) in comparison with results for single 1 Hz stimulation. Error bars represent SEM.