Imran, Ayesha2025-10-072025-10-072025-10-07http://hdl.handle.net/10138/602092Abstract: Background: How do our senses and sensory system, especially the retina, receive the incoming light stimuli, and where do the perceptual interactions arise along the pathways from the retina to the brain to drive visual perception? The exact answers to these questions are quite difficult, especially due to the complexity of neural circuits. Quantification of these complexities becomes easier to assess at the sensitivity limit of vision, where signals emerge from a sparse set of photons and are transmitted through the retina via a well-known neural circuitry. Furthermore, visual perception gets close to the absolute limits set by physics in these conditions as shown already by classic studies of human psychophysics. Objectives: This hybrid systematic review aims to provide a comprehensive overview of the human visual system at the sensory threshold, and the study questions are the following: • How do rods detect single photons? • How are the resulting single-photon signals reliably transmitted across the retinal circuitry at the sensitivity limit? • How are the signals produced by the absorption of a few photons coded in the pattern of activity sent from the retina to the brain at visual threshold? • How are the visual stimuli translated into perception at the sensitivity limit? Methods: Following the “The Preferred Reporting Items for Systematic reviews and Meta-Analyses” (PRISMA) 2020 guidelines, search terms were selected, and inclusion and exclusion criteria were applied so that the most relevant information could be obtained to provide an overview of the study questions. Studies that discuss only photopic and mesopic vision, invertebrate retina, cone phototransduction, intrinsically photosensitive retinal ganglion cells (ipRGCs) or melanopsin-containing retinal ganglion cells (mRGCs), and intraocular lenses were excluded. PubMed was used as a search engine using the keywords “human visual perception”, “vision in dim light”, “single-photon”, “rod phototransduction”, “rod bipolar pathway threshold”, and “retinal ganglion cells”. Key study characteristics are explained, and the risk of biases in these key articles is explained using an automation tool ChatGPT-4 to follow the Cochrane Collaboration and AMSTAR II tool. The study also contains one psychophysical experiment to quantify the visual perception threshold. This experiment was performed on a dark-adapted human subject. The visual threshold was determined following the 2-alternative forced-choice (2AFC) protocol. Results: A total of 100 articles were selected for systematic review after applying inclusion and exclusion criteria. In the scotopic vision, rods are mainly active and can generate a response with only a single photon absorption, but the signal generated is weak and the presence of noise hinders the reliable detection of a single photon response. The presence of thresholding nonlinear synapses in the retinal rod bipolar pathway discards some of the noise and improves the signal-to-noise ratio at the expense of loss of a significant fraction of single-photon responses. Psychophysics experiment suggests that there is a detection threshold in the retina and in the higher-order brain, and our results are consistent with previous findings (Kilpeläinen et al., 2024). Discussion: Results of this systematic review suggest that the evolutionary changes have made our visual system quite sensitive so that even the faintest flashes of light can be perceived. However, our brain trades off this sensitivity with reliability. Thus, the only goal is not the visual perception of single photons, but instead to discriminate between the dim lights of different intensities, approaching the theoretical limits set by physics. In the included studies, the risk of biases such as selection bias, performance bias, and attrition bias was assessed. There is a possibility of the presence of risk of biases, inconsistency and imprecision in this study. A high risk of selection and performance biases was found in (Hecht et al., 1942; Umino et al., 2012; Sakurai et al., 2011; Whitlock & Lamb, 1999; Tsukamoto & Omi, 2013; Sassoè-Pognetto et al., 1994; Berntson et al., 2004; Sakitt, 1972; Tinsley et al., 2016).engIn Copyright 1.0Human visual perception, vision in dim light, single-photon, rod phototransduction, rod bipolar pathway, sensory threshold, retinal ganglion cellsURN:NBN:fi:hulib-202510074141pro gradu -tutkielmaNeurotiedeNeuroscienceNeurovetenskapNeurotieteen maisteriohjelmaMaster's Programme in NeuroscienceMagisterprogrammet i neurovetenskap