Function Of Retinal Illuminance Research Articles

What limits normal visual performance in the dark?

AbstractPurpose Visual performance is affected by changes in the spatial and temporal properties of the retina and/or changes in the quality of the retinal image as a result of increased aberrations and scattered light. The aim of the study was to establish the extent to which retinal and/or optical factors set the limits of visual performance at low light levels.Methods Pupil size, ocular aberrations, scattered light, chromatic sensitivity and contrast acuity were measured as a function of retinal illuminance. Pupil size was measured continuously and display luminance adjusted for constant retinal illuminance.Results Mesopic vision describes the range of light levels over which signals from both rods and cones contribute to the visual response. Visual performance changes with light level: Both the amount of scattered light and the rms wavefront aberration were found to increase rapidly with decreasing light level in the mesopic range. This was paralleled by a massive increase in contrast acuity thresholds and a rapid loss of both red‐green and blue‐yellow chromatic sensitivity.Conclusion The gradual increase in rod signal as the light levels decreases causes changes in the overall spectral sensitivity of the eye with consequences for visual effectiveness. Cone dominated vision is affected most by optical factors; in rod dominated vision the limiting factor becomes the resolving power of the retina. Key aspects of visual performance, such as spatial and temporal contrast sensitivity and acuity, visual delay and colour sensitivity change with light level. Linking such changes allows us to predict visual performance at low light levels where characteristics of the rod and cone system are so different.

On the calculation of optical performance factors from vertebrate spatial contrast sensitivity

A novel technique for calculating the visual optical modulation transfer function (OMTF) is described. The technique involves application of the Rovamo–Barten model of spatial vision to measured contrast sensitivity data. [For details of the basic model see; Rovamo, J., Mustonen, J., & Nasanen, R. (1994). Modelling contrast sensitivity as a function of retinal illuminance and grating area. Vision Research, 34, 1301–1314 and Barten, P. J. G. (1999). Contrast sensitivity of the human eye and its effects on image quality. Washington: SPIE Optical Engineering Press.] In order to obtain OMTF, the model was simplified for use in the high spatial frequency range and also modified to include a transfer function term relating to attenuation by the retinal receptor sampling process. Calculations of OMTF were initially obtained from published contrast sensitivity for the human, cat, rat and chicken. The results were found to correlate well with OMTF values directly obtained through a double-pass optical measuring technique applied to all four species. It was assumed, following this initial test, that the modified Rovamo–Barten model could be used to extract OMTF from vertebrate contrast sensitivity data in general. Using published behavioural contrast sensitivity, further OMTF values were calculated from the model for the pigeon, goldfish, owl monkey, and tree shrew. The results obtained were used to provide a direct inter-species comparison of optical performance for a matched stimulus luminance. This study also confirms that, in many cases, vertebrate optical and receptor sampling processes are well matched in their attenuation properties.

Modelling spatial contrast sensitivity functions for chromatic and luminance-modulated gratings

We extended our detection model of achromatic spatial vision (Rovamo, J., Mustonen, J., & Näsänen, R. (1994a). Modelling contrast sensitivity as a function of retinal illuminance and grating area. Vision Research, 34, 1301–1314) to colour vision by taking into account the fact that due to the spatio-chromatic opponency of retinal ganglion cells and dorsal lateral geniculate nucleus (dLGN) neurons, equiluminous chromatic gratings are not affected by precortical lateral inhibition. We then tested the extended model by using Mullen’s experimental data (Mullen, K. J. (1985). The contrast sensitivity of human color vision to red–green and blue–yellow chromatic gratings. Journal of Physiology, 359, 381–400). The band-pass shape of the spatial contrast sensitivity function for luminance-modulated green and yellow gratings transformed to a low-pass shape, resembling the chromatic spatial contrast sensitivity function for red–green and blue–yellow equiluminous gratings, when the effect of precortical lateral inhibition on grating contrast was computationally removed by dividing luminance contrast sensitivities by spatial frequency (i.e. by af, where a=1°). After the removal of this direct effect of lateral inhibition, there still remained a residual shape difference between the spatial contrast sensitivity functions for chromatic and luminance gratings. It was due to indirect reduction of grating visibility by quantal noise high-pass filtered by precortical lateral inhibition. When this indirect effect of quantal noise was also removed, contrast sensitivity for luminance gratings was about twice the sensitivity for chromatic gratings at all spatial frequencies. This was evidently due to the fact that the chromatic contrast of the equiluminous grating at the opponent stage (Cole, G. R., Hine, T. & McIihagga, W. (1993). Detection mechanisms in L-, M-, and S-cone contrast space. Journal of the Optical Society of America A, 10, 38–51) was about half of the luminance contrast of either of its chromatic component. Thus, if the contrast of the equiluminous chromatic grating were not expressed as the Michelson contrast of one chromatic component grating against its own background (Mullen, K. J. (1985). The contrast sensitivity of human color vision to red–green and blue–yellow chromatic gratings. Journal of Physiology, 359, 381–400) but as chromatic contrast at the opponent stage, contrast sensitivity would be the same for chromatic and luminance gratings.

A Model of Human Contrast Sensitivity as a Function of Retinal Illuminance

We present a mathematical model for signal transformation by the receptor-horizontal-bipolar cell triad, which gives a good prediction of the spatial-contrast modulation transfer of the visual system at different levels of retinal illuminance. We assume that rod-driven and cone-driven bipolar output signals, sr and sc, depend on the mean rates of photoisomerisations in rods and cones, yr and yc, and their derivatives, yr' and yc', as expressed by the equations sr= cr yr'/( dr yr+ dc yc), and sc= cc yc'/( dr yr+ dc yc), where cr, cc, dr, dc are constant coefficients. The total output signal is S= sr+ sc. During scotopic vision sr >> sc and spatial frequency characteristics are determined by the rod system. When illumination increases and rods saturate, contrast sensitivity is determined by the cone system. A formula for calculation of the spatial-contrast modulation transfer function is derived with mean illumination as a parameter. The predictions of the model are in good agreement with experimental data measured at various levels of retinal illumination.

Neural modulation transfer function of the human visual system at various eccentricities

We measured r.m.s. contrast sensitivity as a function of retinal illuminance at various spatial frequencies within 3–37 deg of eccentricity in the nasal visual field. In dim light contrast sensitivity increased in proportion to the square root of retinal illuminance obeying the De Vries-Rose law but in bright light contrast sensitivity was independent of luminance following Weber's law. Critical retinal illuminance ( I c ) marking the transition between the laws was found to be independent of grating area but proportional to the spatial frequency squared at all eccentricities, in agreement with the Van Nes-Bouman law of foveal vision. In addition, the proportionality constant was found to be independent of eccentricity and similar to that of the fovea. According to our contrast detection model of human vision the modulation transfer function ( P MTF ) of the neural visual pathways squared is directly proportional to the critical retinal illuminance. On this basis our result means that P MTF is similar, i.e. equal to spatial frequency across the visual field, thus attenuating low spatial frequencies relatively more than high spatial frequencies. Hence, up to the spatial cut-off frequency determined by the lowest neural sampling density of each retinal location the neural modulation transfer function is independent of visual location.

The cat's pupillary light response under urethane anesthesia.

Pupillary area was measured in urethane-anesthetized cats as a function of retinal illuminance. When appropriate corrections are made for differences in experimental procedures, it was found that the pupillary response of the urethane-anesthetized cat's eyes to light was basically unchanged from that of the alert behaving cat. This preparation may therefore be a very satisfactory one in which to study the pupillary response pathway in a higher mammal.

Modelling contrast sensitivity as a function of retinal illuminance and grating area

We extended the contrast detection model of human vision [Rovamo, Luntinen & Näsänen (1993b) Vision Research, 33, 2773-2788] to low light levels by taking into account the effect of light-dependent quantal noise. The extended model comprises (i) low-pass filtering due to the optical modulation transfer function of the eye, (ii) addition of light-dependent noise at the event of quantal absorption, (iii) high-pass filtering of neural origin (lateral inhibition), (iv) addition of internal neural noise, and (v) detection by a local matched filter whose efficiency decreases with increasing grating area. To test the model we measured foveal contrast sensitivity as a function of retinal illuminance and grating area at spatial frequencies of 0.125-32 c/deg. In agreement with the model, monocular contrast sensitivity at all grating areas increased in proportion to I when retinal illuminance (I) was smaller than critical illuminance. Thereafter the increase saturated and contrast sensitivity became independent of retinal illuminance. Similarly, at all levels of retinal illuminance contrast sensitivity increased in proportion to A when grating area (A) was smaller than critical area. Thereafter the increase saturated and contrast sensitivity became independent of area. Critical level of retinal illuminance increased in proportion to the spatial frequency squared. Critical area marking the saturation of spatial integration was constant at low spatial frequencies but decreased in inverse proportion to spatial frequency squared at medium and high spatial frequencies. The maximum contrast sensitivity obtainable by spatial integration in bright light increased at low spatial frequencies in proportion to spatial frequency, was constant at medium spatial frequencies, and decreased in inverse proportion to spatial frequency cubed at high spatial frequencies. The increase was due to the neural modulation transfer function of the visual pathways whereas the decrease was due to the optical modulation transfer function of the eye. The model explained 91-99% of the total variance of our contrast sensitivity data at various spatial frequencies.

Analysis of visual modulation sensitivity. V. Faster visual response for G- than for R-cone pathway?

To determine the linear, unadapted responses of the cone pathways, we have measured the critical fusion frequency (CFF) for green (555-nm) and red (642-nm) flicker as a function of retinal illuminance. Both functions obeyed the Ferry-Porter law (CFF proportional to log illuminance) to high accuracy over a > or = 5-log-unit range. In both foveola and periphery the CFF/illuminance functions were significantly steeper for green light than for red light. The peripheral 555-nm function had an average slope 1.26 times the average slope of the 642-nm function. An additive model of flicker detection could not account for the observed differences in slope. A threshold independence model, in which detection is based on the most sensitive mechanism, accurately fits the data. Whichever model is assumed, the presence of different slopes for the two wavelength flicker conditions strongly implies that the R- and G-cone pathways have different temporal properties. The occurrence of steeper CFF/illuminance slopes in response to green light implies that the linear (near-CFF) response of the G-cone pathways in inherently faster than that of the R-cone pathways at both retinal loci. These differences in R- and G-cone-mediated temporal properties complicate the fundamental concept of luminance and invalidate it for precise application over the full illuminance range.

Perception of electronic display colours as a function of retinal illuminance

Colour-naming data were objected from four colour-normal observers to determine the nature of the changes in colour appearance that can occur with changes in illuminance for spectral and nonspectral stimuli comparable to those used in electronic displays. Test stimuli were presented as one degree, foveal flashes of one second duration in Maxwellian view. The retinal illuminance of the nine test stimuli varied from −1.0 to 4.0 log trolands (0.01-1 147fL). Shifts in perceived colour were observed with changes in stimulus intensity. At low illuminance levels, the stimuli appeared desaturated, with black and white responses dominating chromatic responses. At higher retinal illuminances, the stimuli were more saturated, although perceived colour was not constant with changing light levels. In general, blue and yellow responses increased at a faster rate relative to red and green responses with increasing illuminance. The data have important implications for the use of colour on electronic displays.

EFFECT OF IMPOSED STEP‐MOVEMENTS AND PULSE‐MOVEMENTS OF THE RETINAL IMAGE ON PERCEPTION OF HUE WITH COLOURED TARGETS

There is strong experimental evidence that colour discrimination depends upon signals originating at colour boundaries. Controlled movements were imposed on a boundary between an illuminated coloured area and a dark area in a previously stabilized image. Red, yellow, green and blue fields were used. Step-movements of amplitude M min arc and pulse-movements of amplitude M min arc and pulse width tau s were studied. The movement M50 to produce 50% positive responses for perception of hue was measured as a function of retinal illuminance, boundary length and speed for step-movements and pulse-width (tau) for pulse movements. Signal/photon-noise ratios were calculated.

Stereoscopic acuity for photometrically matched background wavelengths at scotopic and photopic levels

Two Ss made equidistance settings in a two-rod apparatus using white and four chromatic (red, yellow, Feen, and blue) backgrounds, photometrically matched at each of eight or nine teat levels within a total retinal-illuminance range of 4 log units. The binocular depth setting were analyzed in terms of the angular magnitude of both the Yariable error, ηAD, and the constant error, ηΔR When ηAD is plotted as a function of retinal illuminance, the curves for each of the four chromatic IIIId white backlfound coditions show that at low retinal illuminances, ηAD, is initially large, and with increasing background illumination, ηAD progressively decreases to approach a final low asymptotic value. As predicted by the duplicity theory of vision, each experimental curve shows a dircontinulty at about −1.0 lot td (corresponding to a background luminance value of about 0.0069 fL with the 2.5-mm artificial pupil used) reflecting the transition from rod to cone functioning. The curves representing the different wavelengths essentially overlap throughout the total illumination range, indicating that, for both rod and cone vision, wavelength has no differential effect on the variability of depth settings. The data for ηΔR are less regular than those for ηAD and the rod-cone discontinuities appear less pronounced. The data are analyzed in terms of the relative contribution of the rod and cone mechanisms to performance level.

Effect of striated fields on critical flicker frequency.

Critical flicker frequency measurements were obtained as a function of retinal illuminance for a square centrally regarded field (8.5 degrees on a side) containing various numbers of vertical stripes. The visual field contained under the different experimental conditions 0, 10, 30, 90, and 250 black lines per inch, corresponding to an angular subtense at the eye of 0.63, 0.21, 0.07, and 0.025 degree, respectively. Results indicate that CFF decreases with increases in the number of stripes reaching a minimum at approximately 30 stripes per inch, and then may increase with further increases in the number of stripes. Except at the highest illuminances where the curve for the field containing the largest number of stripes (250 per inch) overtakes that of an unpatterned field, all of the striped fields result in lowered CFF vs log retinal illuminance curves when compared with an unstriped field of the same size.