Retinal output changes qualitatively with every change in ambient illuminance

Abstract

The collective activity pattern of retinal ganglion cells, the retinal code, underlies higher visual processing. How does the ambient illuminance of the visual scene influence this retinal output? We recorded from isolated mouse and pig retina and from mouse dorsal lateral geniculate nucleus in vivo at up to seven ambient light levels covering the scotopic to photopic regimes. Across each luminance transition, most ganglion cells exhibited qualitative response changes, whereas they maintained stable responses within each luminance. We commonly observed the appearance and disappearance of ON responses in OFF cells and vice versa. Such qualitative response changes occurred for a variety of stimuli, including full-field and localized contrast steps and naturalistic movies. Our results suggest that the retinal code is not fixed but varies with every change of ambient luminance. This finding raises questions about signal processing within the retina and has implications for visual processing in higher brain areas.

Figure 1. Overview of experimental paradigm.

(a) Light stimuli were gray scale images on a gray background. The full-field step stimulus had a Weber contrast of ±66%. (b) Absolute intensity of stimuli, converted to R* rod⁻¹ s⁻¹, as a function of the ambient luminance set by neutral density (ND) filters.

Figure 2. Early and delayed anti-preferred responses.

(a) Responses (firing rate) of a single OFF ganglion cell to white and black full-field contrast steps (average firing rate to 45 repetitions at each of five different light levels, ND8 to ND4). (b) Histogram of response latencies (time-to-peak) in OFF cells (left column) and ON cells (right column), measured from responses of all units at all light levels, to both black and white full-field steps.

Figure 3. Summary of luminance-dependent response types.

(a) Fraction of OFF cells displaying no, early and delayed ON responses at each luminance level to white and black full-field step stimuli. (b) Fraction of units with stable and changing responses. Cells were defined as “stable” if they had the same response type (no, early, delayed, both) at all compared light levels, both to the black step and to the white step. All other cells were “changing”. (c,d) Same statistics as in a and b for OFF responses in ON cells. Numbers: Units included in the analysis due to their reliable responses at these light levels (see Methods).

Figure 4. Responses (firing rate) of two ON ganglion cells.

Stimulus was a 2-s white or black full-field step, presented at different ambient light levels. (a) Many ON ganglion cells were strongly suppressed by OFF stimuli. (b) ON ganglion cell with asymmetric and changing OFF responses.

Figure 5. Stability of responses at individual light levels.

(a) Experimental protocol. Colored bars label the last 5 min at each light level. (b) Raster plot of the responses of a single unit to all 730 presentations of the black full-field step (50 repetitions during each 15 min sequence, 5 repetitions during each 1 min sequence). Even quick luminance changes are immediately reflected in a different response pattern (see magnification). Colored bars mark the same experimental sections as in a. (c) Average spike rate of the responses marked by colored bars in a and b.

Figure 6. Responses recorded from individual ganglion cells.

(a–d) Results from one PV5 ganglion cell. (a) Maximum-intensity projection of a confocal stack, showing the neurobiotin-filled PV5 ganglion cell and the regions of interests (ROIs) used for analyzing stratification level. (b) Fluorescence intensity profile along z-axis of ROI 4. Blue: DAPI label. Red: ChAT label, Black: Neurobiotin label. Stratification levels of cell and ChAT bands: peaks of their intensity profiles. Borders of inner nuclear layer (INL) and ganglion cell layer (GCL): where the intensity profile dropped below 67% of its peak. (c) Left: Stratification measurements for each ROI, as in b. Right: Conversion of stratification relative to ChAT bands. (d) This OFF-stratifying cell had pronounced delayed ON responses from ND7 to ND5. (e) Dendritic stratification level (black, mean ± s.e.m., relative to the ChAT bands, red) of individually recorded cells. For right-most cell, ChAT staining was not successful. Most cells had luminance-dependent response changes.

Figure 7. Luminance-dependent qualitative response changes in the dorsal lateral geniculate nucleus (dLGN).

(a) Recording locations in the dLGN, highlighted on the left). Middle: reconstructed positions (colored dots) of recording sites in three rostro-caudal positions relative to bregma. Reconstruction was based on DiI labeling of electrode shanks (right). Blue line: maximum depth of recording electrode. Brain schematics based on Paxinos et al. (b) Absolute stimulus intensities used for the in-vivo experiments (black) in comparison to the intensities used during in-vitro experiments (gray, see Fig. 1b). Note that the stimulus range is extended to higher intensities. (c) Fraction of light-responsive units in the dLGN with changing or stable responses. Conventions as in Fig. 3b. (d) A single ON unit from the dLGN that has both changing and asymmetric OFF responses at different ambient light levels.

Figure 8. Luminance-dependent response changes to small localized disk stimuli.

(a) Percentage of units with stable or changing responses across different luminance levels. Conventions as in Fig. 3b, but combining both ON and OFF cells. (b) Histogram showing how much of the disk stimulus was contained within the receptive field center, as determined by a binary checkerboard flicker stimulus. (c) Example unit changing its responses to localized stimulation of the receptive field center. Right: Overlap of the disk stimulus (red) with the receptive field (blue ellipse shows 2.5-sigma of Gaussian fit). (d) Example unit that had stable responses to the disk stimulus, but changing responses to the full-field step at the ND6/5 luminance transition.