title: Processing of single-photon responses in the mammalian On and Off retinal pathways at the sensitivity limit of vision.
abstract: Visually guided behaviour at its sensitivity limit relies on single-photon responses originating in a small number of rod photoreceptors. For decades, researchers have debated the neural mechanisms and noise sources that underlie this striking sensitivity. To address this question, we need to understand the constraints arising from the retinal output signals provided by distinct retinal ganglion cell types. It has recently been shown in the primate retina that On and Off parasol ganglion cells, the cell types likely to underlie light detection at the absolute visual threshold, differ fundamentally not only in response polarity, but also in the way they handle single-photon responses originating in rods. The On pathway provides the brain with a thresholded, low-noise readout and the Off pathway with a noisy, linear readout. We outline the mechanistic basis of these different coding strategies and analyse their implications for detecting the weakest light signals. We show that high-fidelity, nonlinear signal processing in the On pathway comes with costs: more single-photon responses are lost and their propagation is delayed compared with the Off pathway. On the other hand, the responses of On ganglion cells allow better intensity discrimination compared with the Off ganglion cell responses near visual threshold.This article is part of the themed issue 'Vision in dim light'.
authors.affiliation: Department of Biosciences, University of Helsinki, PO Box 65, 00014 University of Helsinki, Finland.
authors.forename: Daisuke
authors.identifier: http://orcid.org/0000-0002-2599-0827
authors.initials: D
authors.lastname: Takeshita
authors.affiliation: Department of Biosciences, University of Helsinki, PO Box 65, 00014 University of Helsinki, Finland.
authors.forename: Lina
authors.identifier: http://orcid.org/0000-0001-6510-049X
authors.initials: L
authors.lastname: Smeds
authors.affiliation: Department of Biosciences, University of Helsinki, PO Box 65, 00014 University of Helsinki, Finland petri.ala-laurila@helsinki.fi.|Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, PO Box 12200, 00076 Aalto, Finland.
authors.forename: Petri
authors.identifier: http://orcid.org/0000-0002-6139-6825
authors.initials: P
authors.lastname: Ala-Laurila
mesh_terms: D000818:Animals; D008322:Mammals / Q000502:physiology* / Q000523:psychology; D008954:Models, Biological; D010775:Photic Stimulation; D012160:Retina / Q000502:physiology*; D014785:Vision, Ocular; D014795:Visual Pathways; D014796:Visual Perception
keywords: On and Off retinal ganglion cells; linear and nonlinear signal processing; physical limits; scotopic vision; visual sensitivity; visual threshold