A five-primary photostimulator suitable for studying intrinsically photosensitive retinal ganglion cell functions in humans

Abstract

Intrinsically photosensitive retinal ganglion cells (ipRGCs) can respond to light directly through self-contained photopigment, melanopsin. IpRGCs also receive synaptic inputs from rods and cones. Thus, studying ipRGC functions requires a novel photostimulating method that can account for all of the photoreceptor inputs. Here, we introduced an inexpensive LED-based five-primary photostimulator that can control the excitations of rods, S-, M-, L-cones, and melanopsin-containing ipRGCs in humans at constant background photoreceptor excitation levels, a critical requirement for studying the adaptation behavior of ipRGCs with rod, cone, or melanopsin input. We described the theory and technical aspects (including optics, electronics, software, and calibration) of the five-primary photostimulator. Then we presented two preliminary studies using the photostimulator we have implemented to measure melanopsin-mediated pupil responses and temporal contrast sensitivity function (TCSF). The results showed that the S-cone input to pupil responses was antagonistic to the L-, M- or melanopsin inputs, consistent with an S-OFF and (L + M)-ON response property of primate ipRGCs (Dacey et al., 2005). In addition, the melanopsin-mediated TCSF had a distinctive pattern compared with L + M or S-cone mediated TCSF. Other than controlling individual photoreceptor excitation independently, the five-primary photostimulator has the flexibility in presenting stimuli modulating any combination of photoreceptor excitations, which allows researchers to study the mechanisms by which ipRGCs combine various photoreceptor inputs.

Evidence for an impact of melanopsin activation on unique white perception

“…We developed a five-primary Maxwellian-view photostimulator that is capable of independently controlling the excitation levels of the five different types of photoreceptors (S-cone, M-cone, L-cone, rod, and ipRGC) in the human retina using the silent substitution method [18]. The custom-made device has five different colored LEDs—”red,” “amber,” “green,” “cyan,” and “blue” (dominant wavelength: “red”—632 nm, “amber”—592 nm, “green”—540 nm, “cyan”—488 nm, “blue”—456 nm).…”

“…Details of the instrument and calibration procedures can be found in our previous publication [18,27]. After physical calibration, we conducted an observer calibration using heterochromatic flicker photometry (HFP) at 15 Hz to establish equiluminance of the primaries for each observer.…”

The importance of intrinsically photosensitive retinal ganglion cells and implications for lighting design

“…Additionally, primate ipRGCs responds excitatory to melanopsin activation, rod, L-and M-cone inputs but inhibitory to S-cone inputs [3]. This unique characteristic of its receptive field was shown to appear in pupillary recordings [4,53]. This chromatic opponency of ipRGCs may also be evolved to signal the large spectral changes, from bluish to orangish, produced at dawn and dusk to set the biological clock more precisely [3] and effectively [54].…”

“…It has been reported that humans lacking an outer retina [39] or animals with rods and cones ablated genetically [40] can preserve some light detection functions. In people with normal retinas, melanopsin activation could contribute to brightness discrimination [40], chromatic discrimination [41], color perception [42,43] and contrast sensitivity [4,44]. However, the mechanisms for melanopsin activation affecting conscious visual perception are not well-understood.…”

“…IpRGCs express melanopsin, a photopigment with a peak sensitivity at~482 nm (see Fig. 1 for human photoreceptor spectral sensitivity functions [4][5][6][7]. In addition to intrinsic melanopsin-mediated photoresponses, ipRGCs also receive synaptic inputs from rods and cones.…”