Scotopic Levels Research Articles

Visual discomfort from flicker: Effects of mean light level and contrast

Uncomfortable images generally have a particular spatial structure, which deviates from a reciprocal relationship between amplitude and spatial frequency (f) in the Fourier domain (1/f). Although flickering patterns with similar temporal structure also appear uncomfortable, the discomfort is affected by not only the amplitude spectrum but also the phase spectrum. Here we examined how discomfort from flicker with differing temporal profiles also varies as a function of the mean light level and luminance contrast of the stimulus. Participants were asked to rate discomfort for a 17° flickering uniform field at different light levels from scotopic to photopic. The flicker waveform was varied with a square wave or random phase spectrum and filtered by modulating the slope of the amplitude spectrum relative to 1/f. At photopic levels, the 1/f square wave flicker appeared most comfortable, whereas the discomfort from the random flicker increased monotonically as the slope of the amplitude spectrum decreased. This special status for the 1/f square wave condition was limited to photopic light levels. At the lower mesopic or scotopic levels, the effect of phase spectrum on the discomfort was diminished, with both phase spectra showing a monotonic change with the slope of the amplitude spectrum. We show that these changes cannot be accounted for by changes in the effective luminance contrast of the stimuli or by the responses from a linear model based on the temporal impulse responses under different light levels. However, discomfort from flicker is robustly correlated with judgments of the perceived naturalness of flicker across different contrasts and mean luminance levels.

Расчет визуального восприятия яркости в условиях низкой освещенности

Mesopic photometry, which studies visual perception of low-level optical radiation, is of great interest today in lighting engineering. It involves investigating human responses to visual observations in low light conditions in the object space, determining the optimum artificial illumination levels in industrial areas, and solving clinical perimetry problems. The estimation procedure for mesopic photometry recommended by the International Commission on Illumination (CIE) is based on computing a combination of photopic (daylight) and scotopic (nighttime) visual perception levels. This procedure being iterative makes it inconvenient to apply in engineering practice, as the number of iterative steps proves to be several dozens on average, exceeding a hundred in certain cases. As a result, the feasibility of using the CIE procedure instead of a purely photopic perception technique becomes questionable. The discrepancy in the results obtained via these methods informs the selection criterion. The paper compares computation results for perceived brightness in photopic and mesopic vision in low luminance conditions. We also establish whether it is possible to find analytical solutions using the CIE procedure. We show that, for radiation of a colour temperature in the range of 950--12000 K, the maximum computational discrepancy between photopic and mesopic vision scenarios lies in the --200--50 % range, while the minimum discrepancy is approximately 5 % for radiation characterised by a colour temperature of approximately 2000 K. We also present analytical solutions for several specific cases according to the CIE procedure

Population receptive fields in V1 enlarge as luminance is reduced from photopic to scotopic levels

Population receptive fields in V1 enlarge as luminance is reduced from photopic to scotopic levels

Contour interaction under photopic and scotopic conditions.

In the present study, we asked whether contour interaction undergoes significant changes for different luminance levels in the central and peripheral visual field. This study included nine normal observers at two laboratories (five at Palacky University Olomouc, Czech Republic and four at the University of Houston, USA). Observers viewed a randomly selected Sloan letter surrounded by four equally spaced bars for several separations measured edge-to-edge in min arc. Stimuli were viewed foveally under photopic and mesopic luminances and between 5° and 12° peripherally for four different background luminances of the display monitors, corresponding to photopic, mesopic, scotopic, and dim scotopic levels. The extent of the contour interaction in the fovea is approximately 20 times smaller than in the periphery. Whereas the magnitude of foveal contour interaction markedly decreases with decreasing luminance, no consistent luminance-induced change occurs in peripheral contour interaction. The extent of contour interaction does not scale with the size of the target letter, either in the fovea or peripherally. The results support a neural origin of contour interaction consistent with the properties of center-surround antagonism.

Motion perception under mesopic vision.

Mesopic and scotopic vision extend over an illuminance range of 106. The goal of the present study was to determine the effect of decreasing light level on the underlying motion mechanism that integrates spatiotemporally separated motion signals. To accomplish this, we took advantage of the phenomenon of visual motion priming, in which the perceived direction of a directionally ambiguous test stimulus is influenced by the directional movement of a preceding priming stimulus. After terminating a drifting priming stimulus, a 180° phase-shifted grating was presented as a test stimulus. The priming and test stimuli were separately presented to the central and peripheral retinas, respectively. The participants judged the perceived direction of this test stimulus at various light levels from photopic to scotopic levels. We found that the effects of motion priming disappeared over 1 log unit of mesopic light levels. When the test stimulus was presented before the offset of the priming stimulus to compensate for the temporal delay in the rod pathway or when both stimuli were presented at the same location in the periphery, a motion-priming effect appeared at mesopic light levels. These results suggest that different temporal characteristics between the cone pathway and rod pathway disturb the function of the putative motion mechanism responsible for the spatiotemporal integration of motion signals, which leads to specific modulation of motion perception over a wide range of mesopic vision.

FMRI activation of LGN and visual cortex under photopic, mesopic and scotopic luminance levels

fMRI activation of LGN and visual cortex under photopic, mesopic and scotopic luminance levels

Night vision in barn owls: Visual acuity and contrast sensitivity under dark adaptation

Barn owls are effective nocturnal predators. We tested their visual performance at low light levels and determined visual acuity and contrast sensitivity of three barn owls by their behavior at stimulus luminances ranging from photopic to fully scotopic levels (23.5 to 1.5 × 10⁻⁶). Contrast sensitivity and visual acuity decreased only slightly from photopic to scotopic conditions. Peak grating acuity was at mesopic (4 × 10⁻² cd/m²) conditions. Barn owls retained a quarter of their maximal acuity when luminance decreased by 5.5 log units. We argue that the visual system of barn owls is designed to yield as much visual acuity under low light conditions as possible, thereby sacrificing resolution at photopic conditions.

Photopic and scotopic multifocal pupillographic responses in age-related macular degeneration

We compared photopic and scotopic multifocal pupillographic stimuli in age-related macular degeneration (AMD). Both eyes of 18 normal and 14 AMD subjects were tested with four stimulus variants presented at photopic and 126 times lower luminances. The multifocal stimuli presented 24 test regions/eye to the central 60°. The stimulus variants had two different check sizes, and when presented either flickered (15Hz) for 266ms, or were steady for 133ms. Mean differences from normal of 5 to 7dB were observed in the central visual field for both photopic and scotopic stimuli (all p<0.00002). The best areas under receiver operating characteristic plots for exudative AMD in the photopic and scotopic conditions were 92.9±8.0 and 90.3±5.7% respectively, and in less severely affected eyes 83.8±9.7% and 76.9±8.2%. Damage recorded at photopic levels was possibly more diffusely distributed across the visual field. Sensitivity and specificity was similar at photopic and scotopic levels.

Mechanistic modeling of vertebrate spatial contrast sensitivity and acuity at low luminance

The validity of the Barten theoretical model for describing the vertebrate spatial contrast sensitivity function (CSF) and acuity at scotopic light levels has been examined. Although this model (which has its basis in signal modulation transfer theory) can successfully describe vertebrate CSF, and its relation to underlying visual neurophysiology at photopic light levels, significant discrepancies between theory and experimental data have been found at scotopic levels. It is shown that in order to describe scotopic CSF, the theory must be modified to account for important mechanistic changes, which occur as cone vision switches to rod vision. These changes are divided into photon management factors [changes in optical performance (for a dilated pupil), quantum efficiency, receptor sampling] and neural factors (changes in spatial integration area, neural noise, and lateral inhibition in the retina). Predictions of both scotopic CSF and acuity obtained from the modified theory were found to be in good agreement with experimental values obtained from the human, macaque, cat, and owl monkey. The last two species have rod densities particularly suited for scotopic conditions.

Motion perception and ageing at scotopic light levels

We investigated how elderly observers perceive scotopic motion. Full-screen drifting sinusoidal gratings generated by CRS VSG-2/3 were presented on a 21-inch Sony monitor for 500 ms. Scotopic conditions were simulated using neutral density filters. Following 30 min dark adaptation, motion detection and speed discrimination thresholds were measured in two groups of young (20–30 years) and older (50–75years) subjects. Motion detection threshold was assessed for two spatial frequencies at both ends of the visible range. Grating speed varied starting from a very low value, according to the method of constant stimuli involving forced choice direction discrimination task. Speed discrimination performance for the same spatial frequencies was studied using the method of single stimuli. Subjects indicated if the grating was moving faster or slower than the mean drift rate of the range. Slow (2.6 deg/s) and faster (9.2 deg/s) drift rates were used. Older subjects, unable to detect slow moving gratings, had higher motion detection threshold than younger subjects especially at low spatial frequency. Speed discrimination worsened for older subjects, more so at the low drift rate. Scotopic CSF did not correlate with the loss demonstrated by older subjects. Our results indicate visual motion processing impairments with age under scotopic conditions.

Mesopic luminance assessed with minimum motion photometry

We measured the relative contribution of rods and cones to luminance across a range of photopic, mesopic, and scotopic adaptation levels and at various retinal eccentricities. We isolated the luminance channel by setting motion-based luminance nulls (minimum motion photometry) using annular stimuli. Luminance nulls between differently colored stimuli require equality in a weighted sum of rod and cone excitations. The relative cone weight increases smoothly from the scotopic range, where rods dominate, to photopic levels, where rod influence becomes negligible. The change from rod to cone vision does not occur uniformly over the visual field. The more peripheral the stimulus location, the higher is the light level required for cones to participate strongly. The relative cone contribution can be described by a sigmoid function of intensity, with two parameters that each depend on the eccentricity and spatial frequency of the stimulus. One parameter determines the "meso-mesopic" luminance--the center of the mesopic range, at which rod and cone contributions are balanced. This increases with eccentricity, reflecting an increase in the meso-mesopic luminance from 0.04 scotopic cd/m(2) at 2° eccentricity to 0.44 scotopic cd/m(2) at 18°. The second parameter represents the slope of the log-log threshold-versus-intensity curve (TVI curve) for rod vision. This parameter inversely scales the width of the mesopic range and increases only slightly with eccentricity (from 0.73 at 2° to 0.78 for vision at 18° off-axis).

A tetrachromatic model for colorimetric use in mesopic vision

AbstractIn this article, a tetrachromatic colorimetric model is proposed. Four monochromatic stimuli are used as primaries instead of three to achieve colorimetric matches in mesopic vision. Tetrachromatic colorimetric matches are assumed in such a way so as to hold in both the photopic and the scotopic levels of adaptation. The fourth primary helps to balance the rod contribution in mesopic vision. The proposed model is based on the experimental observations that (a) trichromatic matches can be achieved in any light level and (b) once a tetrachromatic match is made, it holds at all radiances. The main outcome is that (a) only three channels in human vision system are important for color matching at mesopic and photopic levels and (b) additivity can be preserved at all levels once rod input is taken into account. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2011

Six different roles for crossover inhibition in the retina: Correcting the nonlinearities of synaptic transmission

Early retinal studies categorized ganglion cell behavior as either linear or nonlinear and rectifying as represented by the familiar X- and Y-type ganglion cells in cat. Nonlinear behavior is in large part a consequence of the rectifying nonlinearities inherent in synaptic transmission. These nonlinear signals underlie many special functions in retinal processing, including motion detection, motion in motion, and local edge detection. But linear behavior is also required for some visual processing tasks. For these tasks, the inherently nonlinear signals are "linearized" by "crossover inhibition." Linearization utilizes a circuitry whereby nonlinear ON inhibition adds with nonlinear OFF excitation or ON excitation adds with OFF inhibition to generate a more linear postsynaptic voltage response. Crossover inhibition has now been measured in most bipolar, amacrine, and ganglion cells. Functionally crossover inhibition enhances edge detection, allows ganglion cells to recognize luminance-neutral patterns with their receptive fields, permits ganglion cells to distinguish contrast from luminance, and maintains a more constant conductance during the light response. In some cases, crossover extends the operating range of cone-driven OFF ganglion cells into the scotopic levels. Crossover inhibition is also found in neurons of the lateral geniculate nucleus and V1.

Retinal pathways influence temporal niche

In mammals, light input from the retina entrains central circadian oscillators located in the suprachiasmatic nuclei (SCN). The phase of circadian activity rhythms with respect to the external light:dark cycle is reversed in diurnal and nocturnal species, although the phase of SCN rhythms relative to the light cycle remains unchanged. Neural mechanisms downstream from the SCN are therefore believed to determine diurnality or nocturnality. Here, we report a switch from nocturnal to diurnal entrainment of circadian activity rhythms in double-knockout mice lacking the inner-retinal photopigment melanopsin (OPN4) and RPE65, a key protein used in retinal chromophore recycling. These mice retained only a small amount of rod function. The change in entrainment phase of Rpe65(-/-);Opn4(-/-) mice was accompanied by a reversal of the rhythm of clock gene expression in the SCN and a reversal in acute masking effects of both light and darkness on activity, suggesting that the nocturnal to diurnal switch is due to a change in the neural response to light upstream from the SCN. A switch from nocturnal to diurnal activity rhythms was also found in wild-type mice transferred from standard intensity light:dark cycles to light:dark cycles in which the intensity of the light phase was reduced to scotopic levels. These results reveal a novel mechanism by which changes in retinal input can mediate acute temporal-niche switching.

Effects of acute ethyl alcohol consumption on a psychophysical measure of lateral inhibition in human vision

Acute consumption of ethyl alcohol affects a variety of visual functions. However, there have been few systematic attempts to investigate the neural mechanisms underlying these effects. Here, we employed the Westheimer paradigm to investigate the hypothesis that alcohol reduces lateral inhibition within human “perceptive fields”, the psychophysical analogue of physiological receptive fields. Westheimer functions obtained under alcohol and no-alcohol conditions at photopic, mesopic, and scotopic levels of adaptation showed changes consistent with an alcohol-induced decrease in lateral inhibition. We conclude that this decrease in lateral inhibition may be responsible for some of the changes in visual perception that result from alcohol consumption.

The initial ocular following responses elicited by apparent-motion stimuli: Reversal by inter-stimulus intervals

Transient apparent-motion stimuli, consisting of single 1/4-wavelength steps applied to square-wave gratings lacking the fundamental (“missing fundamental stimulus”) and to sinusoidal gratings, were used to elicit ocular following responses (OFRs) in humans. As previously reported [Sheliga, B. M., Chen, K. J., FitzGibbon, E. J., & Miles, F. A. (2005). Initial ocular following in humans: a response to first-order motion energy. Vision Research, in press], the earliest OFRs were strongly dependent on the motion of the major Fourier component, consistent with early spatio-temporal filtering prior to motion detection, as in the well-known energy model of motion analysis. Introducing inter-stimulus intervals (ISIs) of 10–200 ms, during which the screen was gray with the same mean luminance, reversed the initial direction of the OFR, the peak reversed responses (with ISIs of 20–40 ms) being substantially greater than the non-reversed responses (with an ISI of 0 ms). When the mean luminance was reduced to scotopic levels, reversals now occurred only with ISIs ⩾60 ms and the peak reversed responses (with ISIs of 60–100 ms) were substantially smaller than the non-reversed responses (with an ISI of 0 ms). These findings are consistent with the idea that initial OFRs are mediated by first-order motion-energy-sensing mechanisms that receive a visual input whose temporal impulse response function is strongly biphasic in photopic conditions and almost monophasic in scotopic conditions.

Vergleich verschiedener Pupillometrieverfahren unter mesopischen und skotopischen Bedingungen

The aim of the study was to compare different methods for pupillometry, namely the Goldmann perimeter (gp), the Colvard pupillometer (cp) and the Procyon Video pupillometer (pvp). For the pvp three different illuminations were available: mesopic high, mesopic low, and scotopic. The size of the pupil was measured in 100 eyes (50 healthy subjects) with the three different methods. We examined 29 females (58 %) and 21 males (42 %) with an average age of 25.16 years, ranging from 18 to 30 years. For the Goldmann perimeter, a mean pupil diameter of 4.39 mm +/- 0.62 mm was found under mesopic conditions (1.40 lux). For the Colvard pupillometer for scotopic conditions (0 lux), a mean pupil diameter of 6.80 mm +/- 0.81 mm was found. For pvp the pupil diameter ranged from 7.06 mm +/- 0.71 mm for scotopic (0.04 lux), over 6.24 mm +/- 0.80 mm for mesopic low (0.40 lux) to 4.65 mm +/- 0.73 mm for mesopic high conditions (4.00 lux). The comparison of the results showed a high correlation between the Goldmann perimeter and the Procyon Video Pupillometer for mesopic high with a minimum difference of 0.25 +/- 0.69 mm. By addition of 2.67 mm to the mesopic measurement of the Goldmann perimeter, the results for the Procyon Video pupillometer at the scotopic level, by addition of 2.4 mm the scotopic measurement of the Colvard pupillometer could be achieved.

Generality of rod hue biases with smaller, brighter, and photopically specified stimuli

This study tests the generality of previously demonstrated rod hue biases (red and blue biases at shorter wavelengths and a green bias at longer wavelengths) that cause the loci of the three spectral unique hues to shift to longer wavelengths. We found rod hue biases for 2-deg targets to be generally similar in magnitude and light-level dependence to those observed for 7.4-deg targets (the size most often studied) when measured at 7-deg eccentricity. The largest effects for both test sizes occurred at the lowest light levels tested, 1 log scotopic troland. All three rod hue biases were found with 0.6-deg targets, but were not reliably measurable at the lowest light levels and were reduced in magnitude and consistency across observers. The largest rod hue biases all occurred at the same scotopic light level, which corresponds to different photopic light levels for the three hue biases, because of differences in photopic and scotopic spectral sensitivity. This suggests that no single photopic light level will produce such large effects for all three rod hue biases. Finally, when the rod influence on a specific unique-hue locus was measured using photopically (rather than scotopically) constant stimuli, rod hue biases were still found but were more variable in magnitude and incidence across observers. We conclude that the rod hue biases we have previously described can be found with smaller stimuli, at somewhat higher light levels, and under photopically constant conditions, although our prior conditions tend to produce larger, more reliable rod hue biases.

Light adaptation in motion direction judgments.

We examined the time course of light adaptation in the visual motion system. Subjects judged the direction of a two-frame apparent-motion display, with the two frames separated by a 50-ms interstimulus interval of the same mean luminance. The phase of the first frame was randomly determined on each trial. The grating presented in the second frame was phase shifted either leftward or rightward by pi/2 with respect to the grating in the first frame. At some variable point during the first frame, the mean luminance of the pattern increased or decreased by 1-3 log units. Mean luminance levels varied from scotopic or low mesopic to photopic levels. We found that the perceived direction of motion depended jointly on the luminance level of the first frame grating and the time at which the shift in average luminance occurs. When the average luminance increases from scotopic or mesopic to photopic levels at least 0.5 s before the offset of the first frame, motion in the 3pi/2 direction is perceived. When average luminance decreases to low mesopic or scotopic levels, motion in the pi/2 direction is perceived if the change occurs 1.0 s or more before first frame offset, depending on the size of the luminance step. Thus light adaptation in the visual motion system is essentially complete within 1 s. This suggests a rapid change in the shape (biphasic or monophasic) of the temporal impulse response functions that feed into a first-order motion mechanism.

Rod temporal channels

Mechanisms underlying rod temporal contrast sensitivity have been considered in terms of a fast retinal signal predominating at mesopic levels and a slower retinal signal predominating at scotopic levels. Here we use a small signal masking method, which has previously been used to delineate the cone-mediated cortical temporal channels, to investigate their rod-mediated cortical counterparts. The results suggest that there are three different rod-mediated cortical temporal channels, one which is lowpass and two which are bandpass. These mechanisms co-exist at all light levels and their relative sensitivity depend on the stimulus spatio-temporal frequency.