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Techniques to control the phototactic behaviour of fish have expanded with progress in LED lights. However, the phototaxis direction of fish could be reversible at some light intensities, and thus it is necessary to evaluate the light-intensity levels that will induce repulsion or attraction behaviour to understand the transition. This study determined the light intensities of white LED light required to induce repulsion or attraction behaviour from a dark place, and the degree of dark-adapted visual sensitivity in juvenile of two salmon species. Oncorhynchus keta smolts showed negative phototaxis under intense light intensity, but positive phototaxis was not observed. The range of light intensities under which O. masou exhibited positive phototaxis changed with the life stage (from pre-smolts to smolts). Notably, the light intensities that elicited positive phototaxis were relatively low and narrow for pre-smolts, whereas smolts responded to a greater range of intensities. Positive phototaxis disappeared in O. masou pre-smolts under intense light intensity, but not in O. masou smolts under the most-intense light intensity tested here. Negative phototaxis was not observed in O. masou pre-smolts or smolts. The appropriate light intensity indicated here may be used to either guide juvenile O. masou from the dam reservoir to fishways or bypass channels, or to repel O. keta smolts from the water intakes of agricultural diversion weirs or hydropower dams.

Forest-based scenic spots have received widespread attention for their landscape aesthetics and ecological value, but the rapid growth of tourism and landscape exploitation make it challenging to balance human recreational needs, landscape quality, and ecological stability. This study aimed to evaluate, quantify, and grade landscape quality from the point of view of subjective human aesthetic needs as well as objective landscape visual sensitivity. After the selection of high-quality landscape viewpoints as sources, the minimum cumulative resistance (MCR) model was used to screen the optimal paths for connectivity among viewpoints with consideration of ecological resistance. High-quality landscape viewpoints, optimal paths, and ecological-resistance surfaces constructed the landscape network for sustainable development. The results showed that the landscape quality of viewpoints in Tianmeng Scenic Spot was not good; only 32.4% of these viewpoints had good performance of both landscape aesthetics and landscape visual sensitivity. In the analysis of ecological-resistance surfaces, the proportion of very-high resistance areas and high resistance areas was 32.9%, and these were mostly distributed in the main tourist roads and their buffer areas around the northwest of Tianmeng Mountain Scenic Spot. Eleven landscape core viewpoints and six secondary viewpoints, all with high landscape aesthetics, were selected as high-quality visual landscapes, and then based on the ecological-resistance surfaces, 11 core landscape-dissemination paths and 6 secondary landscape-dissemination paths were identified, respectively, using the MCR model. This method could provide scientific decision support to enhance the effectiveness of viewpoints as well as sustainable landscape planning for development.

How the human visual system determines the lightness of a surface, that is, its whiteness, blackness, or grayness, remains, like vision in general, a mystery. In fact, we have not been able to create a machine that can determine, through an artificial vision system, whether an object is white, black, or gray. Although the photoreceptors in the eye are driven by light, the light reflected by a surface does not reveal its shade of gray. Depending upon the level of illumination, a surface of any shade of gray can reflect any amount of light. This book ties together over thirty years of the author's own research on lightness, and presents an historical review of empirical work on lightness, covering the past 150 years of research on images ranging from the simple to the complex. The book also describes and analyzes the many theories of lightness — including the author's own — showing what each can and cannot explain. The book highlights the forgotten work conducted in the first third of the 20th century, describing several crucial experiments and examining the nearly unknown work of the Hungarian gestalt theorist, Lajos Kardos. The book's review also includes a survey of the pattern of lightness errors made by humans, many of which result in delightful illusions. It argues that because these errors are not random, but systematic, they are the signature of our visual software, and so provide a powerful tool that can reveal how lightness is computed. Based on this argument and the concepts of anchoring, grouping, and frames of reference, the book presents a new theoretical framework that explains an unprecedented array of lightness errors.

Ethambutol is a common cause of drug-related optic neuropathy.Prediction of the onset of ethambutol-induced optic neuropathy and consequent drug withdrawal may be an effective method to stop visual loss.Previous studies have shown that structural injury to the optic nerve occurred earlier than the damage to visual function.Therefore,we decided to detect structural biomarkers marking visual field loss in early stage ethambutol-induced optic neuropathy.The thickness of peripapillary retinal nerve fiber layer,macular thickness and visual sensitivity loss would be observed in 11 ethambutol-induced optic neuropathy patients（22 eyes） using optical coherence tomography.Twenty-four healthy age-and sex-matched participants（48 eyes） were used as controls.Results demonstrated that the temporal peripapillary retinal nerve fiber layer thickness and average macular thickness were thinner in patients with ethambutol-induced optic neuropathy compared with healthy controls.The average macular thickness was strongly positively correlated with central visual sensitivity loss（r2=0.878,P=0.000）.These findings suggest that optical coherence tomography can be used to efficiently screen patients.Macular thickness loss could be a potential factor for predicting the onset of ethambutol-induced optic neuropathy.

Human vision is exquisitely sensitive—a dark-adapted observer is capable of reliably detecting the absorption of a few quanta of light. Such sensitivity requires that the sensory receptors of the retina, rod photoreceptors, generate a reliable signal when single photons are absorbed. In addition, the retina must be able to extract this information and relay it to higher visual centres under conditions where very few rods signal single-photon responses while the majority generate only noise. Critical to signal transmission are mechanistic optimizations within rods and their dedicated retinal circuits that enhance the discriminability of single-photon responses by mitigating photoreceptor and synaptic noise. We describe behavioural experiments over the past century that have led to the appreciation of high sensitivity near absolute visual threshold. We further consider mechanisms within rod photoreceptors and dedicated rod circuits that act to extract single-photon responses from cellular noise. We highlight how these studies have shaped our understanding of brain function and point out several unresolved questions in the processing of light near the visual threshold. This article is part of the themed issue ‘Vision in dim light’.

Vision is critical for most vertebrates, including fish. One challenge that aquatic habitats pose is the high variability in spectral properties depending on depth and the inherent optical properties of the water. By altering opsin gene expression and chromophore usage, cichlid fish modulate visual sensitivities to maximize sensory input from the available light in their respective habitat. Thyroid hormone (TH) has been proposed to play a role in governing adaptive diversification in visual sensitivity in Nicaraguan Midas cichlids, which evolved in less than 4000 generations. As suggested by indirect measurements of TH levels (i.e., expression of deiodinases), populations adapted to short wavelength light in clear lakes have lower TH levels than ones inhabiting turbid lakes enriched in long-wavelength light. We experimentally manipulated TH levels by exposing 2-week-old Midas cichlids to exogenous TH or a TH inhibitor and measured opsin gene expression and chromophore usage (via cyp27c1 expression). Although exogenous TH induces long-wavelength sensitivity by changing opsin gene expression and chromophore usage in a concerted manner, THinhibited fish exhibit a visual phenotype with sensitivities shifted to shorter wavelengths. Tinkering with TH levels in eyes results in concerted phenotypic changes that can provide a rapid mechanism of adaptation to novel light environments.

Midas cichlid fish are a Central American species flock containing 13 described species that has been dated to only a few thousand years old, a historical timescale infrequently associated with speciation. Their radiation involved the colonization of several clear water crater lakes from two turbid great lakes. Therefore, Midas cichlids have been subjected to widely varying photic conditions during their radiation. Being a primary signal relay for information from the environment to the organism, the visual system is under continuing selective pressure and a prime organ system for accumulating adaptive changes during speciation, particularly in the case of dramatic shifts in photic conditions. Here, we characterize the full visual system of Midas cichlids at organismal and genetic levels, to determine what types of adaptive changes evolved within the short time span of their radiation. We show that Midas cichlids have a diverse visual system with unexpectedly high intra- and interspecific variation in color vision sensitivity and lens transmittance. Midas cichlid populations in the clear crater lakes have convergently evolved visual sensitivities shifted toward shorter wavelengths compared with the ancestral populations from the turbid great lakes. This divergence in sensitivity is driven by changes in chromophore usage, differential opsin expression, opsin coexpression, and to a lesser degree by opsin coding sequence variation. The visual system of Midas cichlids has the evolutionary capacity to rapidly integrate multiple adaptations to changing light environments. Our data may indicate that, in early stages of divergence, changes in opsin regulation could precede changes in opsin coding sequence evolution.

Pupil size primarily changes to regulate the amount of light entering the retina, optimizing the balance between visual acuity and sensitivity for effective visual processing. However, research directly examining the relationship between pupil size and visual processing has been limited. While a few studies have recorded pupil size and EEG signals to investigate the role of pupil size in visual processing, these studies have predominantly focused on the domain of visual sensitivity. Causal effects of pupil size on visual acuity, therefore, remain poorly understood. By manipulating peripheral background luminance levels and target stimulus contrast while simultaneously recording pupillometry and EEG signals, we examined how absolute pupil size affects visual discrimination and visually evoked potentials (VEP) in a task using optotype mimicking the Snellen eye chart, the most common assessment of visual acuity. Our findings indicate that both higher background luminance levels and higher target contrast were associated with improved target discrimination and faster correct reaction times. Moreover, while higher contrast visual stimuli evoked larger VEPs, the effects of pupil size on VEPs were not significant. Additionally, we did not observe inter-individual correlations between absolute pupil size and discrimination performance or VEP amplitude. Together, our results demonstrate that absolute pupil size, regulated by global luminance level, played a functional role in enhancing visual discrimination performance in an optotype discrimination task. The differential VEP effects of pupil size compared to those of stimulus contrast further suggested distinct neural mechanisms involved in facilitating visual acuity under small pupils.

This study evaluates differences in the visual field performance when wearing the Defocus Incorporated Multiple Segments (DIMS) spectacle lens compared to wearing a conventional single vision (SV) spectacle lens. Twenty-one children aged 9-14 years with spherical equivalent refraction (SER) between -1.13D to -4.75D were recruited. Mid-peripheral near visual acuity (NVA) under room lighting condition (500 lux ±10%) was measured using DIMS and SV lenses, respectively. Automated static perimetry (Zeiss, Humphrey Visual Field HFA 750i) with SITA Fast 30-2 protocol was used to investigate the visual field sensitivity. During the test, the study lens (Plano DIMS or SV lens) were inserted into the lens holder in front of the trial lenses with each child's compensated prescription. Three children were not able to complete the reliable visual tests due to fixation losses (>20%) or high false positive rate (>15%) while 18 children successfully completed the test. The mean visual field sensitivity was 29.2 ± 3.7 decibels (dB) and 29.3 ± 3.5 dB when wearing DIMS and SV lens, respectively. The mean sensitivity differences between DIMS and SV lens among 76 locations ranged from -2.4 ± 3.9 dB to 1.6 ± 3.9 dB. No statistically significant difference in sensitivity was observed across 76 locations within the central 30 between DIMS and SV lens (Wilcoxon signed rank test with bonferroni correction for multiple comparisons,  > 0.00065). Compared to SV lens, 0.05 logarithm of minimal angle of resolution (logMAR) reduction in mid-peripheral NVA in all 4 quadrants (Superior, Temporal, Inferior and nasal,  < 0.05) was noted with the DIMS lens (  = 18). However, no statistically significant correlation was found between the mid-peripheral NVA and visual sensitivity at the specific locations. Although the mid-peripheral NVA was slightly reduced using DIMS lens, wearing DIMS lens did not change the children's visual sensitivity to detect the static stimulus within 30 visual field when compared to wearing SV lens.

Throughout the lifespan, humans exhibit varying abilities in perceiving color and luminance with sensitivity peaking at different stages. The interaction between luminance and color perception is likely influenced by the distinct developmental trajectories of the respective visual psychophysical channels. This study aimed to investigate how chromatic noise affects luminance discrimination thresholds in adolescents, young adults, and elderly individuals. Sixty participants with no visual complaints (20/20 or corrected binocular visual acuity, and no indication of color vision impairment in the Ishihara pseudoisochromatic plate test) were divided into three age groups: adolescents (15.7 ± 0.8 years), young adults (20–40 years), and elderly adults (60 + years). Participants underwent a luminance contrast discrimination task with chromatic noise masking using a mosaic stimulus, where four chromatic noise protocols were applied (protan, deutan, tritan, and a no-noise protocol). The results showed that luminance contrast thresholds were significantly elevated with the addition of chromatic noise in all groups compared to those without chromatic masking noise, but adults exhibited smaller differences in thresholds between the conditions with and without noise compared to participants in the adolescent and elderly groups ( p  < 0.05). The intergroup comparisons revealed that young adults had the lowest thresholds, followed by adolescents and elderly individuals ( p  ≤ 0.01). Elderly participants exhibited higher luminance thresholds than young adults in all chromatic noise conditions, especially under the tritan protocol. These findings suggest that the maturation of luminance and color interaction is consolidated after adolescence, with sensitivity peaking of the mechanisms of color-luminance interaction in adulthood and declining in the elderly. The study provides insights into the developmental and aging processes of color-luminance interaction mechanisms, highlighting the continued maturation of color processing mechanisms in adolescence and their subsequent decline with age.