Vision under mesopic and scotopic illumination

Abstract

Evidence has accumulated that rod activation under mesopic and scotopic light levels alters visual perception and performance. Here we review the most recent developments in the measurement of rod and cone contributions to mesopic color perception and temporal processing, with a focus on data measured using a four-primary photostimulator method that independently controls rod and cone excitations. We discuss the findings in the context of rod inputs to the three primary retinogeniculate pathways to understand rod contributions to mesopic vision. Additionally, we present evidence that hue perception is possible under scotopic, pure rod-mediated conditions that involves cortical mechanisms.

Keywords: color; cones; mesopic; photopic; rods; scotopic; temporal; vision.

FIGURE 1

Visual functions under scotopic, mesopic and photopic illumination. The luminance level (log cd.m^-2) is the reference for estimating pupil diameter using the empirical relation between pupil diameter (mm) and field luminance (Crawford, 1936; Pokorny and Smith, 1997). The reported values align approximately with the photopic retinal illuminance (photopic Troland is shown in bold) of the measurement conditions in the referenced study. When rod and cone values are both reported, rod values are italicized. Note that the reported values can vary with the observer, stimulus and measurement condition; see References for details of the experimental conditions: ¹Estimate of the light level under starlight and full moon from Smith et al. (1994); ²CIE (1994): the transition values are complexly dependent on the viewing conditions (for example, see estimates of the rod saturation range); ³Crawford (1936); Pokorny and Smith (1997); ⁴Km_sc/Km_ph = 2.49; ⁵Cone threshold estimated at the cornea (Koenig and Hofer, 2011); ⁶Aguilar and Stiles (1954), Shapiro (2002), Kremers et al. (2009). ⁷Pokorny et al. (2006); ⁸Rod data from Conner and MacLeod (1977); Hess and Nordby (1986), Sun et al. (2001a); cone data from de Lange (1954); Kelly (1961); ⁹Time to peak (tp ms) of the impulse response function from Cao et al. (2007); ¹⁰Campbell and Green (1965); Hess and Nordby (1986); ¹¹Barlow (1958).

FIGURE 2

Neural pathways related to mesopic vision. Rod signals input to all three primary retinogeniculate pathways, namely the magnocellular (MC), parvocellular (PC), and koniocellular (KC) pathways. Only rod inputs to the MC pathway are shown in this schematic. Green circles indicate chemical synapses. The red zig–zags indicate electrical synapses. Retinal layers are indicated on the left: OS, Outer Segment; ONL, Outer Nuclear Layer; OPL, Outer Plexiform Layer; INL, Inner Nuclear Layer; IPL, Inner Plexiform Layer; GCL, Ganglion Cell Layer; NFL, Nerve Fiber Layer.

FIGURE 3

Chromaticity shifts in the color appearance of cone signals due to rod excitation. Data for one observer (JP) plotted in a relative cone Troland space at 2 photopic Td (A) and 10 photopic Td (B). The arrows indicate the effect of increased rod excitation on the direction of the color shift from the stimulus chromaticities (unfilled circles) to the matching chromaticities (filled circles). Ellipses show the chromaticities of eight non-dark appearing basic colors. Adapted from Cao et al. (2005).

FIGURE 4

The cone contrasts that perceptually match a rod signal are linear as a function of rod contrast. The data were measured at 2, 10, and 100 Td (observer IS). The dashed lines are fits based on a physiologically plausible model. Adapted from Cao et al. (2008a).

FIGURE 5

Surface color perception under scotopic illumination reveals relational hue percepts mediated exclusively via the rod pathway. Symbols show the reported color names from four trichromatic observers of the gray OSA-UCS color sample as a function of photopic illuminance. Symbol size refers to the lightness of the OSA-UCS sample, with lightness increasing with increasing symbol size. Light levels below about -2.25 log Lux are solely mediated by rods; the colored shaded areas demarcate the blue–green–gray and red–orange color categories used by the observers (samples below threshold were black). Adapted from Pokorny et al. (2006).

FIGURE 6

Post-receptoral rod signaling weights in the MC, PC, and KC pathways depend on the temporal properties of the rod signal. Data were measured using a perceptual matching paradigm at 5 photopic Td (two observers; darker and lighter columns). Adapted from Zele et al. (2013).