Photometric Compliance of Tablet Screens and Retro-Illuminated Acuity Charts As Visual Acuity Measurement Devices

Mobile technology is increasingly used to measure visual acuity. Standards for chart-based acuity tests specify photometric requirements for luminance, optotype contrast and luminance uniformity. Manufacturers provide some photometric data but little is known about tablet performance for visual acuity testing. This study photometrically characterised seven tablet computers (iPad, Apple inc.) and three ETDRS (Early Treatment Diabetic Retinopathy Study) visual acuity charts with room lights on and off, and compared findings with visual acuity measurement standards. Tablet screen luminance and contrast were measured using nine points across a black and white checkerboard test screen at five arbitrary brightness levels. ETDRS optotypes and adjacent white background luminance and contrast were measured. All seven tablets (room lights off) exceeded the most stringent requirement for mean luminance (≥ 120 cd/m²) providing the nominal brightness setting was above 50%. All exceeded contrast requirement (Weber ≥ 90%) regardless of brightness setting, and five were marginally below the required luminance uniformity threshold (L_min/L_max ≥ 80%). Re-assessing three tablets with room lights on made little difference to mean luminance or contrast, and improved luminance uniformity to exceed the threshold. The three EDTRS charts (room lights off) had adequate mean luminance (≥ 120 cd/m²) and Weber contrast (≥ 90%), but all three charts failed to meet the luminance uniformity standard (L_min/L_max ≥ 80%). Two charts were operating beyond manufacturer’s recommended lamp replacement schedule. With room lights on, chart mean luminance and Weber contrast increased, but two charts still had inadequate luminance uniformity. Tablet computers showed less inter-device variability, higher contrast, and better luminance uniformity than charts in both lights-on and lights-off environments, providing brightness setting was >50%. Overall, iPad tablets matched or marginally out-performed ETDRS charts in terms of photometric compliance with high contrast acuity standards.     

Evidence for the role of retinal receptors R 7/8 in the orientation behaviour of the fly

The height orientation of flying houseflies Musca domestica has been analyzed:

1. The luminance threshold of the orientation behaviour has been determined. It corresponds to the luminance threshold needed for the optomotor response in the torque released by the receptors R 7/8 (Eckert, 1973).
2. The direction of the E-vector of the linearly polarized stimulating light has been varied at a luminance just above threshold. It was found that the ability of the fly to fixate is dependent upon this parameter.
3. The rhabdomeres R 1–6 and/or 7/8 have been stimulated selectively and the threshold of the height orientation response has been measured under the different conditions of stimulation. It has been found that the threshold of luminance, when all receptors are stimulated, is almost identical to the threshold when only the receptors R 7/8 are stimulated. If the receptors R 1–6 are stimulated specifically the response threshold is rised by 1 to 2 decades of illuminance, as compared to the specific stimulation of R 7/8.

It is concluded that the results of all experiments are in accordance with the hypothesis, that the receptors R 7/8 are necessary for the orientation behaviour.     

Correlation versus gradient type motion detectors: the pros and cons

Visual motion contains a wealth of information about self-motion as well as the three-dimensional structure of the environment. Therefore, it is of utmost importance for any organism with eyes. However, visual motion information is not explicitly represented at the photoreceptor level, but rather has to be computed by the nervous system from the changing retinal images as one of the first processing steps. Two prominent models have been proposed to account for this neural computation: the Reichardt detector and the gradient detector. While the Reichardt detector correlates the luminance levels derived from two adjacent image points, the gradient detector provides an estimate of the local retinal image velocity by dividing the spatial and the temporal luminance gradient. As a consequence of their different internal processing structure, both the models differ in a number of functional aspects such as their dependence on the spatial-pattern structure as well as their sensitivity to photon noise. These different properties lead to the proposal that an ideal motion detector should be of Reichardt type at low luminance levels, but of gradient type at high luminance levels. However, experiments on the fly visual systems provided unambiguous evidence in favour of the Reichardt detector under all luminance conditions. Does this mean that the fly nervous system uses suboptimal computations, or is there a functional aspect missing in the optimality criterion? In the following, I will argue in favour of the latter, showing that Reichardt detectors have an automatic gain control allowing them to dynamically adjust their input–output relationships to the statistical range of velocities presented, while gradient detectors do not have this property. As a consequence, Reichardt detectors, but not gradient detectors, always provide a maximum amount of information about stimulus velocity over a large range of velocities. This important property might explain why Reichardt type of computations have been demonstrated to underlie the extraction of motion information in the fly visual system under all luminance levels.     

Visual transfer characteristics from Mach band measurements

Whenever the eye views a spatially-varying gradient of luminance, the Mach bands arise, causing a subjective distribution different from the luminance distribution of the field of view. This phenomenon has been used for the determination of the contrast-transfer function of the visual system when working under normal viewing conditions, i.e., operating in a region where both luminance and detail size are well above threshold. The Mach bands have been measured by making subjective photometric comparisons and the data have been analyzed under the assumption of linearity of response to input. The hypothesis that the visual system may be treated as an approximately linear link in the imaging chain is satisfied best when the luminances are plotted logarithmically. The possibility of this type of investigation is discussed. The result found is compared with apparent subjective contrast measurements of a sine-wave spatial luminance distribution.     

Vernier acuity predicted from changes in the light distribution of the retinal image

If two thin bars of different luminance are placed side by side, their joint spatial position in a Vernier alignment task is determined simply by their relative luminances. The threshold luminance contrast difference required to produce a just detectable change in spatial position corresponds to a spatial shift of 5-20 arcsec in the centroid of the retinal light distribution, depending upon contrast relative to the background. This technique may be used to measure acuity with a display that has a spatial resolution considerably worse than the Vernier offset threshold. We have also extended the centroid technique to components that differ both in wavelength and luminance. Colour was found to make no essential difference to the task. Taking into account the spread of light in the retinal image, the manifest contrast thresholds are equivalent to threshold intensity increments between adjacent foveal receptors of less than 1% comparable to the values reported by Hecht and Mintz for dark line detection.     

综述: 初级视皮层的亮度信息加工

亮度(luminance)是最基本的视觉信息.与其他视觉特征相比,由于视神经元对亮度刺激的反应较弱,并且许多神经元对均匀亮度无反应,对亮度信息编码的神经机制知之甚少.初级视皮层部分神经元对亮度的反应要慢于对比度反应,被认为是由边界对比度诱导的亮度知觉(brightness)的神经基础.我们的研究表明,初级视皮层许多神经元的亮度反应要快于对比度反应,并且这些神经元偏好低的空间频率、高的时间频率和高的运动速度,提示皮层下具有低空间频率和高运动速度通路的信息输入对产生初级视皮层神经元的亮度反应有贡献.已经知道初级视皮层神经元对空间频率反应的时间过程是从低空间频率到高空间频率,我们发现的早期亮度反应是对极低空间频率的反应,与这一时间过程是一致的,是这一从粗到细的视觉信息加工过程的第一步,揭示了处理最早的粗的视觉信息的神经基础.另外,初级视皮层含有偏好亮度下降和高运动速度的神经元,这群神经元的活动有助于在光照差的环境中检测高速运动的低亮度物体.     

Neural adjustments to chromatic blur

The perception of blur in images can be strongly affected by prior adaptation to blurry images or by spatial induction from blurred surrounds. These contextual effects may play a role in calibrating visual responses for the spatial structure of luminance variations in images. We asked whether similar adjustments might also calibrate the visual system for spatial variations in color. Observers adjusted the amplitude spectra of luminance or chromatic images until they appeared correctly focused, and repeated these measurements either before or after adaptation to blurred or sharpened images or in the presence of blurred or sharpened surrounds. Prior adaptation induced large and distinct changes in perceived focus for both luminance and chromatic patterns, suggesting that luminance and chromatic mechanisms are both able to adjust to changes in the level of blur. However, judgments of focus were more variable for color, and unlike luminance there was little effect of surrounding spatial context on perceived blur. In additional measurements we explored the effects of adaptation on threshold contrast sensitivity for luminance and color. Adaptation to filtered noise with a 1/f spectrum characteristic of natural images strongly and selectively elevated thresholds at low spatial frequencies for both luminance and color, thus transforming the chromatic contrast sensitivity function from lowpass to nearly bandpass. These threshold changes were found to reflect interactions between different spatial scales that bias sensitivity against the lowest spatial grain in the image, and may reflect adaptation to different stimulus attributes than the attributes underlying judgments of image focus. Our results suggest that spatial sensitivity for variations in color can be strongly shaped by adaptation to the spatial structure of the stimulus, but point to dissociations in these visual adjustments both between luminance and color and different measures of spatial sensitivity.     

Effects of plumbous ion on some functions of transfer RNA

Plumbous ion has been shown to catalyze the degradation of RNA, and the effects of lead-catalyzed depolymerization on some of the functions of tRNA has been investigated. Exposure to Pb²⁺ brings about a rapid decrease but not a complete loss of amino acid acceptance capacity. The tRNA molecules responsible for this residual activity have not been degraded and represent a class of molecules that are resistant to Pb²⁺. The ratio of susceptible to resistant tRNA increases when the temperature is raised. The effect of varying the Pb²⁺ concentration was studied, and there is a threshold Pb²⁺ concentration below which the tRNA is not inactivated.     

Implementation of an elaborated neuromorphic model of a biological photoreceptor

We describe here an elaborated neuromorphic model based on the photoreceptors of flies and realised in both software simulation and hardware using discrete circuit components. The design of the model is based on optimisations and further elaborations to the mathematical model initially developed by van Hateren and Snippe that has been shown to accurately simulate biological responses in simulations under both steady-state and limited dynamic conditions. The model includes an adaptive time constant, nonlinear adaptive gain control, logarithmic saturation and a nonlinear adaptive frequency response mechanism. It consists of a linear phototransduction stage, a dynamic filter stage, two divisive feedback loops and a static nonlinearity. In order to test the biological accuracy of the model, impulses and step responses were used to test and evaluate the steady-state characteristics of both the biological (fly) and artificial (new neuromorphic model) photoreceptors. These tests showed that the model has faithfully captured most of the essential characteristics of the insect photoreceptor cells. The model showed a decreasing response to impulsive stimuli when the background intensity was increased, indicating that the circuit adapted to background luminance in order to improve the overall operating range and better encode the contrast of the stimulus rather than luminance. The model also showed the same change in its frequency response characteristics as the biological photoreceptors over a luminance range of 70,000 cd/m², with the corner frequency of the circuit ranging from 10 to 90 Hz depending on the current state of adaptation. Complex naturalistic experiments have also further proven the robustness of the model to perform in real-world scenario. The model showed great correlation to the biological photoreceptors with an r ² value exceeding 0.83. Our model could act as an excellent platform for future experiments that could be carried out in scenarios where in vivo intracellular recording from biological photoreceptors would be impractical or impossible, or as a front-end for an artificial imaging system.     

Regular occurance of simultaneous brightness contrast in the mesopic region

Clear evidence of electrophysiological effects causing simultaneous contrast phenomena in the visual system of animals has lately been presented. These findings incite to subjective perceptual psychophysiological experiments and give some directions concerning the interpretation and generalizations of the results. Psychophysical measurements have been made of the visual response to sinusoidal, spatially varying stimuli in the mesopic region well above contrast threshold. The results are presented with the characteristic properties, object contrast, spatial frequency, and average luminance of the physical luminance distribution as parameters. Some regular features of the fundamental input-output relations of the visual system are elucidated. Non-linear as well as quasi-linear processes are predicted to be prevalent.     

The contribution of blue-sensitive cones to spatial responses of post-receptoral visual channels in man

Psychophysical methods developed for the investigation of spatial and temporal pathways in human vision have been applied in combination with the two-colour increment threshold technique of W. S. Stiles to study the way in which signals from blue-sensitive cones are transmitted along the visual pathways. A flicker sensitive spatio-temporal filter, designated 'ST2', has been examined by background modulation methods, and spatial filters sensitive to bars of a specific width by grating adaptation methods employing dichoptic presentation of stimuli. It is shown that the blue-sensitive (pi 3) spectral mechanism contributes to both classes of filter response, in a manner similar to that observed for the red-sensitive spectral mechanism. The binocularly driven, bar-sensitive filters have broad-band spectral response characteristics, thus the data demonstrate that signals arising in blue-sensitive cones converge onto a luminance channel. The results of this investigation, together with those previously published for a second (ST1) spatio-temporal filter, describe a variety of post-receptoral responses involving the pi 3 spectral mechanism.     

Space-variant visual processing: spatially limited visual channels

A multichannel model incorporating visual inhomogeneity is presented in this paper. The parameters that describe inhomogeneity have been experimentally obtained both at threshold and in several suprathreshold conditions. At threshold, probability summation is taken into account in order to determine the spatial extent of visual channels from experimental data showing an asymptotic increase in sensitivity with increasing grating area. At suprathreshold contrast, the region where luminance variations at several scales are visible has also been found. The results support a spatially limited multichannel model of early visual processing and set out a basis for studying perceptual phenomena from the viewpoint of linear space-variant visual processing.     

Image Quality Characteristics of Handheld Display Devices for Medical Imaging

Handheld devices such as mobile phones and tablet computers have become widespread with thousands of available software applications. Recently, handhelds are being proposed as part of medical imaging solutions, especially in emergency medicine, where immediate consultation is required. However, handheld devices differ significantly from medical workstation displays in terms of display characteristics. Moreover, the characteristics vary significantly among device types. We investigate the image quality characteristics of various handheld devices with respect to luminance response, spatial resolution, spatial noise, and reflectance. We show that the luminance characteristics of the handheld displays are different from those of workstation displays complying with grayscale standard target response suggesting that luminance calibration might be needed. Our results also demonstrate that the spatial characteristics of handhelds can surpass those of medical workstation displays particularly for recent generation devices. While a 5 mega-pixel monochrome workstation display has horizontal and vertical modulation transfer factors of 0.52 and 0.47 at the Nyquist frequency, the handheld displays released after 2011 can have values higher than 0.63 at the respective Nyquist frequencies. The noise power spectra for workstation displays are higher than 1.2×10⁻⁵ mm² at 1 mm⁻¹, while handheld displays have values lower than 3.7×10⁻⁶ mm². Reflectance measurements on some of the handheld displays are consistent with measurements for workstation displays with, in some cases, low specular and diffuse reflectance coefficients. The variability of the characterization results among devices due to the different technological features indicates that image quality varies greatly among handheld display devices.     

Visual sensitivity of ground squirrels to spatial and temporal luminance variations

Summary We have investigated the visual sensitivity of the California ground squirrel (Speromphilus beecheyi) to spatial and temporal luminance patterns. Spatial contrast sensitivity functions were determined in behavioral discrimination experiments in which the stimuli were sinusoidally-modulated luminance gratings. These squirrels were found to be maximally sensitive to spatial frequencies of about 0.7 cycles/ degree (c/d), and they are unable to discriminate gratings whose frequencies exceed 4 c/d. Similar results were obtained in electrophysiological experiments when the visually evoked cortical potential (VECP) was recorded from anesthetized squirrels. A third experiment involved tests of the ability of ground squirrels to discriminate square-wave gratings of much higher luminance (340 cd/m²). The finest gratings which were discriminable at this luminance level did not exceed 3.9–4.3 c/d and, thus, we conclude that the maximal spatial resolution of the California ground squirrel is about 4 c/d (corresponding to a bar separation of 7.5). In another behavioral experiment the abilities of ground squirrels to discriminate sinusoidally flickering lights (mean luminance = 3.4 cd/m²) was measured. The results show that ground squirrels are maximally sensitive to lights flickering at a rate of about 18 Hz, and that the highest rates that are still discriminable are slightly above 60 Hz.Abbreviations c/d cycles/degree - CFF critical flicker frequency - VECP visually evoked cortical potential This research was supported by Grant EY 00105 from the National Eye Institute. We thank David Birch who participated in some preliminary behavioral experiments and Kenneth Long who provided the histological material from which measurements of receptor spacing were made.     

Light adaptation in Drosophila photoreceptors: II. Rising temperature increases the bandwidth of reliable signaling

It is known that an increase in both the mean light intensity and temperature can speed up photoreceptor signals, but it is not known whether a simultaneous increase of these physical factors enhances information capacity or leads to coding errors. We studied the voltage responses of light-adapted Drosophila photoreceptors in vivo from 15 to 30 degrees C, and found that an increase in temperature accelerated both the phototransduction cascade and photoreceptor membrane dynamics, broadening the bandwidth of reliable signaling with an effective Q(10) for information capacity of 6.5. The increased fidelity and reliability of the voltage responses was a result of four factors: (1) an increased rate of elementary response, i.e., quantum bump production; (2) a temperature-dependent acceleration of the early phototransduction reactions causing a quicker and narrower dispersion of bump latencies; (3) a relatively temperature-insensitive light-adapted bump waveform; and (4) a decrease in the time constant of the light-adapted photoreceptor membrane, whose filtering matched the dynamic properties of the phototransduction noise. Because faster neural processing allows faster behavioral responses, this improved performance of Drosophila photoreceptors suggests that a suitably high body temperature offers significant advantages in visual performance.     

Light Enhances the Turnover of Phosphatidylinositol in Rat Retinas

Light stimulation of isolated rat retinas is shown to enhance the turnover of phosphatidylinositol (PI) as demonstrated by a light-dependent increase in [³H]inositol incorporation and concurrent hydrolysis of existing PI. Studies with rat retinas incubated with [³H]inositol and then microdissected at the level of the outer plexiform layer into photoreceptor cell and inner retina layers indicated that the light-enhanced incorporation of [³H]inositol was associated with the photoreceptor cell layer. The rate of PI hydrolysis in retinas prelabeled in vivo with [³H]inositol was higher in light than in dark incubations and was higher in the photoreceptor cell layer than within the inner retina. Within the photoreceptor cell layer, PI turnover involved 2%/min of the total PI contentin dark and 6–8%/min in light. In contrast to what has been reported for stimulus-enhanced turnover of PI in some tissues, this light-enhanced turnover of PI in the retina was not associated with detectable reductions in PI content. Parallel studies of sodium (²²Na) uptake demonstrated that the photoreceptor cells remained functional during these incubations as they retained the capacity to restrict the entry of ²²Na in light but not in dark.     

Blue flicker modifies the subfoveal choroidal blood flow in the human eye

The objective of the present study was to reveal an interaction between choroidal blood flow (ChBF) and light-induced photoreceptor activity, a physiological coupling that has been already demonstrated for retinal blood flow but rejected for ChBF. Ten healthy adults volunteered for this study. A real-time recording near-infrared laser-Doppler flowmeter was used to quantify the subfoveal ChBF while the luminance of blue flicker between 1 and 64 Hz was first increased and then decreased by 4.0 log units in 1.0-log unit steps between 0.0375 and 375 cd/m2. In separate testing, flash electroretinograms (ERGs) provided electrophysiological indexes of the relative response of short-wave cones (s-cones) and rods to blue light stimulation. Group-averaged, normalized ChBF measurements revealed that it was modulated by approximately 9% by flicker frequency. Increasing the blue flicker luminance from low to high attenuated the subfoveal ChBF, volume, and velocity by approximately 32%, approximately 30%, and approximately 5%, respectively. Decreasing the luminance from high to low over the same range had no effect on the subfoveal choroidal hemodynamics. The markedly different effects of reversed directions of change in blue flicker luminance on the subfoveal ChBF were linked to transitions between rod-dominated and s-cone-dominated retinal responses. Collectively, these findings indicate that the blue light-induced photoreceptor response is associated with a differential distribution of the ChBF across the ocular fundus according to the degree and type of retinal photoreceptor stimulated.     

Functional EF-Hands in Neuronal Calcium Sensor GCAP2 Determine Its Phosphorylation State and Subcellular Distribution In Vivo,and Are Essential for Photoreceptor Cell Integrity

The neuronal calcium sensor proteins GCAPs (guanylate cyclase activating proteins) switch between Ca²⁺-free and Ca²⁺-bound conformational states and confer calcium sensitivity to guanylate cyclase at retinal photoreceptor cells. They play a fundamental role in light adaptation by coupling the rate of cGMP synthesis to the intracellular concentration of calcium. Mutations in GCAPs lead to blindness. The importance of functional EF-hands in GCAP1 for photoreceptor cell integrity has been well established. Mutations in GCAP1 that diminish its Ca²⁺ binding affinity lead to cell damage by causing unabated cGMP synthesis and accumulation of toxic levels of free cGMP and Ca²⁺. We here investigate the relevance of GCAP2 functional EF-hands for photoreceptor cell integrity. By characterizing transgenic mice expressing a mutant form of GCAP2 with all EF-hands inactivated (EF⁻GCAP2), we show that GCAP2 locked in its Ca²⁺-free conformation leads to a rapid retinal degeneration that is not due to unabated cGMP synthesis. We unveil that when locked in its Ca²⁺-free conformation in vivo, GCAP2 is phosphorylated at Ser201 and results in phospho-dependent binding to the chaperone 14-3-3 and retention at the inner segment and proximal cell compartments. Accumulation of phosphorylated EF⁻GCAP2 at the inner segment results in severe toxicity. We show that in wildtype mice under physiological conditions, 50% of GCAP2 is phosphorylated correlating with the 50% of the protein being retained at the inner segment. Raising mice under constant light exposure, however, drastically increases the retention of GCAP2 in its Ca²⁺-free form at the inner segment. This study identifies a new mechanism governing GCAP2 subcellular distribution in vivo, closely related to disease. It also identifies a pathway by which a sustained reduction in intracellular free Ca²⁺ could result in photoreceptor damage, relevant for light damage and for those genetic disorders resulting in “equivalent-light” scenarios.     

Detection of light intensity in the frog retina: evidence from dark and light adaptation

The frog retina was stimulated with light flashes homogeneous in space but not time. The time heterogeneity of stimulation was created by abrupt change of a referent stimulus for a stimulus with different luminance. Such changes form a time pattern, as well as sharp borders of luminance between the neighbor areas of the visual field form a spatial pattern. The electroretinogram recorded in response to presentation of a triad of stimuli: the onset of a short flash of homogeneous light after long dark (or light) adaptation of a retina, brief sequence of the referent and test light flashes varied in luminance, and the offset, with returning to the initial level of adaptation. It was shown that responses of the retina under conditions of time heterogeneity of stimulation could be divided in two types as well as under conditions of spatial heterogeneity. Such a dual change in amplitude confirms our earlier hypothesis on the existence of two mechanisms of luminance coding in the frog retina. The first mechanism encodes power characteristics of light, it forms the information on the absolute level of the environmental luminance. Its activity is connected basically with receptors and cells of the external plexiform layer of the frog's retina. It is responsible for the b-wave of the electroretinogram. The other mechanism associated with RERG is based on a vector code of stimuli. This mechanism forms the information on spatial and time differentiation of the light flow in the visual field and is connected basically with cells of the internal plexiform layer. The results suggest that the frog retina has the individual mechanism for time pattern detection, distinguishing it from the homogeneous light flow in a similar way as in case of spatial light pattern detection. It is possible that the first mechanism is responsible for the detection of any new stimulus in general, irrespective of its specificity, whereas the second mechanism serves for the measurement of suprathreshold differences between stimuli.     

The use of fractal image statistics in the estimation of lateral spatial extent

It has been argued that the characteristics of many commonly occurring surface textures are such that the resulting luminance distributions have the statistical properties of fractals, over a wide range of spatial scales. We show that, when fractal luminance distributions are spatially filtered, the spatial density of zero-crossings obtained is inversely proportional to the scale of filtering, and is not strongly dependent on the fractal dimension of the pattern used. We propose that this predictable property of natural images could provide a basis for the estimation of lateral spatial extent by counting zero-crossings within an interval at a variety of spatial scales, and averaging over spatial scale. We carried out experiments to compare the relative apparent lateral extents of fractal patterns and patterns of equally spaced bars, as a function of the number of bars. The results are in good agreement with theory.