Rejection of the biophoton hypothesis on the origin of photoreceptor dark noise

Abstract

It has been suggested that retinal “dark light” is caused by photons emitted by the retina itself. The authors show that the “biophoton” radiation from the retina can be detected, but its intensity is ≥100 times lower than necessary to produce the measured physiological noise understood to arise from the spontaneous activation of rhodopsin.

The mechanism of photon-like dark noise in rod photoreceptors

“…1). Govardovskii et al (2019) provide compelling evidence and analysis that reject the hypothesis that UPE is the mechanism of rod photon-like dark noise in frogs (Rana bidibunda) and sterlets (sturgeons; Acipenser ruthenus) at room temperature. They do so by directly measuring UPE in isolated retinas from the two species, and comparing this with recordings of spontaneous photon-like events in rods under the same conditions and SPRs of rods to carefully measured illumination.…”

“…Such light could, in turn, be captured by a rhodopsin, triggering phototransduction. For the experiments of Govardovskii et al (2019), converting the measured UPE into a full intrinsic (volumetric) unit requires assumptions about the subcellular source. The authors consider the case most favorable to the hypothesis proposed by Salari et al (2016) in which lipid peroxidation in the outer segments, where rhodopsin resides, gives rise to UPE and is the cause of the photon-like current events.…”

“…No doubt this was achieved by evolutionary selection pressure on the rhodopsin molecule itself, on the biochemistry of rod phototransduction, and on retinal cell types and circuitry (Pugh, 2018). The findings of Govardovskii et al (2019) indirectly lend support to the long-standing idea that the spontaneous activation of rhodopsin arises from an intrinsic susceptibility of the rhodopsin chromophore in situ to thermal activation (Luo et al, 2011), a susceptibility that the evolution of rhodopsin was unable to eliminate, despite greatly lowering its rate relative to the rate of thermal isomerization in vitro (Kim et al, 2003). Amazingly, this greatly reduced thermal isomerization rate still dictates the absolute sensitivity of night vision by setting a floor of noise, the Eigengrau, of the entire visual system (Hecht et al, 1942;Barlow, 1956;Naarendorp et al, 2010).…”

“…Amazingly, this greatly reduced thermal isomerization rate still dictates the absolute sensitivity of night vision by setting a floor of noise, the Eigengrau, of the entire visual system (Hecht et al, 1942;Barlow, 1956;Naarendorp et al, 2010). While the work of Govardovskii et al (2019) makes it clear that this floor of noise is not set by the UPE, it nonetheless focuses attention on an interesting, ubiquitous photonic feature of biological systems. showing four rods and three cones.…”

Spectral and Thermal Properties of Rhodopsins: Closely Related but not Tightly Coupled

“…They proposed that the events are in fact responses to real photons, "biophotons", emitted by the retinal tissue. However, Govardovskii and coworkers (2019) [37] showed by direct measurements that biophoton emission rates in frog and sterlet retina are >100-fold too low to account for the discrete rod events recorded in the same species.…”