Levels Of Retinal Illuminance Research Articles

The effects of grating area and spatial frequency on contrast sensitivity as a function of light level

Contrast sensitivity was measured as a function of retinal illuminance ( I) for vertical cosine gratings of various circular areas ( A) and spatial frequencies ( f ⩽ 4 c/deg). Spatial frequency and grating diameter varied in inverse proportion to each other in order to keep the relative grating area ( A × f 2) constant at either 3.14,12.6, 50.3, or 201 square cycles. At all grating areas and spatial frequencies contrast sensitivity in dim light first increased in proportion to the square root of retinal illuminance. Then the increase saturated and contrast sensitivity became independent of luminance level in bright light. For gratings with constant relative area contrast sensitivity functions were similar in shape and had the same maximum sensitivity but were shifted horizontally towards lower illuminances with decreasing spatial frequency. However, when replotted as a function of retinal illuminance divided by spatial frequency squared, contrast sensitivity functions fell on a common curve at all levels of relative retinal illuminance ( I/f 2).

Loci of achromatic points throughout the life span

Mixtures of monochromatic lights that appear achromatic were measured for 50 normal, trichromatic observers ranging in age from 11 to 78 years. Stimuli were presented to one eye as a 1 degree-diameter, 1-s flash (10-s interstimulus interval) in Maxwellian view. We found the achromatic locus by varying the intensity ratio of each observer's spectral unique blue and unique yellow while maintaining constant overall retinal illuminance. Measurements were made for three levels of retinal illuminance (10, 100, 1000 trolands). Additional verification of the position of the achromatic locus in color space was obtained for 23 subjects with the use of a mixture composed of 600-nm light and its spectral complement. There were no significant changes in the achromatic loci as a function of age. The mean achromatic locus in CIE chromaticity space was x, y = 0.31, 0.31 or u',v' = 0.21, 0.46. These results suggest that partial compensation for age-related changes in visual mechanisms occurs in a way that preserves constancy of the achromatic locus across the life span.

Effect of ambient illumination on the spatial properties of the center and surround of Y-cell receptive fields.

The primary goal of this study was to expand the description of the filtering properties of the Y-cell receptive field, by quantitatively characterizing the spatial filtering properties of the receptive field's center-and-surround components as a function of adapting light level. A range of more than five orders of magnitude in retinal illuminance were covered, including the vast majority of the cat's functional range of vision. Recordings were taken from optic tract fibers of Y cells in cats under general anesthesia. Sinusoidal gratings and a stimulus designed to selectively probe the properties of the surround mechanism were used. The cells' responses to these stimuli were fit to a Gaussian center-surround receptive-field model, in which six parameters define the properties of the center and surround. Fits were made independently to data collected at each light level and changes in the values of the model's parameters with illuminance are reported. A set of equations that summarize the changes in parameter values is given. From these summary equations, reasonable estimates of the parameters' values can be determined across a wide range of illuminances. Hence, a quantitative model of the spatial properties of the center and surround of the Y-cell receptive field can now be derived from these equations for most of the levels of retinal illuminance experienced by a Y cell. The consistency between the description provided by our equations and results from earlier work is considered.

Visibility, luminance and vernier acuity

To assess the role of quantal limitations and target visibility on vernier acuity, we measured line detection thresholds and vernier thresholds for abutting dark line targets on a uniform background, for a range of retinal illuminance levels and target contrasts. Measurements were made for stimuli presented at the fovea, and at a retinal eccentricity of 2.5 deg. Although the relationship is truly curvilinear, line detection thresholds and vernier thresholds using targets of a fixed contrast, follow a square-root dependence on retinal illuminance over a significant portion of the illuminance range. Once the effect of retinal illuminance on line detection thresholds has been accounted for, there is little further effect of retinal illuminance on vernier thresholds, at least for visibility levels of up to five times the contrast detection threshold. This finding suggests that the spatial mechanisms which mediate changes in the detection threshold and vernier thresholds for abutting targets are similar, and are limited by the same sources of noise. Vernier thresholds for these thin line targets are approximately inversely proportional to target contrast for both retinal loci, and at all retinal illuminance levels tested. However, vernier thresholds for a constant visibility target are always lower for foveal than for eccentric stimuli, even after the effects of increased spatial pooling with increasing eccentricity are accounted for.

Do deviations from radiance-invariance of metameric matches contradict the three pigment theory of foveal trichromacy?

According to the pigment theory of matching, metameric matches result from the equation of the rates of photoisomerizations for each of the three classes of cone pigments excited by the two matched fields. If true, matches are radiance-invariant and additive. Tests of the theory in this paper show small but ubiquitous failures in radiance-invariance due to systematic rather than random errors in matching. A choice between two possible explanations for these systematic errors favors the view that in subjects who deliberately or intuitively search for the middle of the matching range, the errors are due to an asymmetry in the Weber fraction for color (Trezona) at low (but not high) levels of retinal illuminance.

The perception of moving comets at high retinal illuminance levels: a rod-cone interaction effect.

A small circular target of high retinal illuminance level can have a comet-like appearance when presented moving continuously with a speed as low as 0.2 deg/s. This perceived lengthening of the circular target increases with the speed of movement and is only observed for target presentations outside the foveal region. Data on the parametric properties of the "comet effect" are presented together with related results on the time-course of recovery of retinal sensitivity following brief exposure to intense stimuli. Measurement of target spectral irradiance levels which are just sufficient to yield the comet effect suggests that the lengthening of the circular target reflects a rod-cone interaction and therefore it may be due to unsuppressed, saturated rod responses at high retinal illuminance levels. The restriction of the comet effect to areas outside the foveal region is used to produce spatial maps of what appears to be the rod-free area of the retina. A model simulation by means of a computational approach shows that the predicted appearance of the moving target matches very closely the experimental observations on the comet effect. Model predictions based on psychophysical estimates of comet length for the stimulus conditions of these experiments yield an overall response time for the rod system of some 600 ms.

Additivity failures in mesopic brightness matching.

Additivity tests were carried out for a centrally-viewed 10° field using heterochromatic brightness matching at 6 retinal illuminance levels from 0.01 to 1000 photopic troland (Td). The results obtained by the mixture of 487 nm and 660 nm monochromatic stimuli showed two types of additivity failure. At the high levels of the retinal illuminance of 10,100 and 1000 Td, the brightness of the mixture was perceived to be lower than strict validity of the additivity law would predict. This is referred to as additivity failure of the reduction type. At the low levels of the retinal illuminances of 0.01 and 0.1 Td, the mixture appeared to be brighter than the brightness that the additivity law would predict. This phenomenon means additivity failure of the enhancement type. At the intermediate level of 1 Td, both types of additivity failure were seen. The type was determined by the luminance ratio of the two stimulus in the mixture. The results from another pair of mixtures, 463 nm and 567 nm, showed that additivity held at low levels, additivity failure of reduction type at intermediate level, and both types of additivity failure at high levels. The additivity failure observed at the mesopic levels are attributed to interaction between rod responses and responses from the red, green, and blue cones. These interactions are complex functions of luminance level.

Spectral luminous efficiency functions in the mesopic range.

Spectral luminous efficiency functions for mesopic vision were measured extensively to establish a basic data set for the mesopic photometry system. These functions were obtained by the direct brightness matching for a 10 degrees field at nine retinal illuminance levels from 100 to 0.01 Td in 0.5-log-unit steps. Data for 12 subjects were collected. In the mesopic range, the spectral luminous efficiency function, averaged over 12 subjects, gradually changes form with retinal illuminance. This suggests that the mesopic function should be established for all the levels in that range. An attempt was made to find an empirical formula that expresses the mesopic function as a simple combination of the photopic and the scotopic brightness functions.

Displacement thresholds for motion detection under conditions of chromatic adaptation

Abstract— The smallest, continuous target displacement which is sufficient to cause a sensation of movement was measured under conditions of chromatic adaptation. The experiments were carried out for test target and background field wavelengths which selectively isolate the activity of the colour mechanisms of the human eye. The results show that, when target detection is largely mediated by the red or green colour mechanism, a target displacement of 2–3′ in the foveal region is sufficient to cause a reliable sensation of movement. These displacement thresholds show only a small change as a function of target contrast or background retinal illuminance level. Similar results were obtained when the colour of the test target was the same as that of the background field (e.g. green, yellow or red), when no attempt was made to isolate a single colour mechanism. Test target and background wavelengths which isolate the activity of the blue colour mechanism yield displacement thresholds in the range 5–7′ and show a stronger dependence on target contrast and background retinal illuminance level. The experiments were repeated at several eccentricities and the results show that, for a test target and background field of the same colour and under conditions which isolate the green or red spectral response mechanism, the minimum target displacement required for motion detection appears to increase in a manner similar to the centre-width receptive-field size of broad-band, centre-surround transient neurons in the rhesus monkey retina [De Monasterio and Gouras (1975) J. Physiol., Lond.251, 167–195]. In contrast, displacement thresholds obtained under conditions which isolate the blue colour mechanism do not show significant variation for target presentation locations up to 25° of visual angle. These data relate well to other psychophysical results which reveal the spatial properties of motion-detecting mechanisms in human vision [Barbur and Ruddock (1980b) Biol. Cybernetics37, 93–98].

DISPLACEMENT THRESHOLDS FOR MOTION DETECTION UNDER CONDITIONS OF CHROMATIC ADAPTATION

The smallest, continuous target displacement which is sufficient to cause a sensation of movement was measured under conditions of chromatic adaptation. The experiments were carried out for test target and background field wavelengths which selectively isolate the activity of the colour mechanisms of the human eye. The results show that, when target detection is largely mediated by the red or green colour mechanism, a target displacement of 2-3' in the foveal region is sufficient to cause a reliable sensation of movement. These displacement thresholds show only a small change as a function of target contrast or background retinal illuminance level. Similar results were obtained when the colour of the test target was the same as that of the background field (e.g. green, yellow or red), when no attempt was made to isolate a single colour mechanism. Test target and background wavelengths which isolate the activity of the blue colour mechanism yield displacement thresholds in the range 5-7' and show a stronger dependence on target contrast and background retinal illuminance level. The experiments were repeated at several eccentricities and the results show that, for a test target and background field of the same colour and under conditions which isolate the green or red spectral response mechanism, the minimum target displacement required for motion detection appears to increase in a manner similar to the centre-width receptive-field size of broad-band, centre--surround transient neurons in the rhesus monkey retina [De Monasterio and Gouras (1975) J. Physiol., Lond. 251, 167-195].(ABSTRACT TRUNCATED AT 250 WORDS)

Mesopic luminous-efficiency functions for various adapting levels.

The luminous-efficiency functions for a centrally viewed 10 degrees field were measured by heterochromatic brightness matching for various retinal illuminance levels of a reference field at various adapting levels. The subject was always presented with a 45 degrees white adapting light except when the test field for brightness matching was substituted for the adapting field for 500 msec. In order to investigate the contributions of rods and cones to the brightness sensation, the luminous-efficiency curves obtained from two subjects were analyzed with the Ikeda-Shimozono formula. When the subject was presented with an adapting light above about 100 trolands (Td), the luminous-efficiency function became photopic at any luminance level of the test field; similarly, when the test field was at 100 Td, the luminous-efficiency function was photopic at any adapting level.

Spatio-temporal interaction in neurones of the cat's dorsal lateral geniculate nucleus.

Temporal contrast sensitivities in the range 0.33-42 Hz for optimum spatial frequency grating stimuli were measured for large populations of XG and YG neurones. For fewer cells, complete spatial contrast sensitivity profiles were measured at a series of temporal frequencies and, in some cases, at a range of levels of retinal illumination too. Contrast sensitivities were measured from responses of cells reliably different from their maintained discharges. The cells' discharges were recorded extracellularly from the binocular segment of the A laminae of the cat's dorsal lateral geniculate nucleus. At their respective optimum spatial frequencies, YG cell were more sensitive on average than XG cells for most temporal frequencies, though the average temporal contrast sensitivity profiles of both cell classes had similar shapes. The optimum temporal frequency for both cell types was around 5 Hz. XG and YG cells seem to be relatively less sensitive to low temporal frequencies than their ganglion cell counterparts. At a retinal illumination of 230 cd/m2 (pupil, 3 mm2), increasing temporal frequency in the range 0.65-21 Hz produced a relative improvement in low spatial frequency contrast sensitivity in most XG and all YG cells studied. There were some XG cells, though, which showed little or no effect of temporal frequency on their spatial contrast sensitivity curves. At all temporal frequencies, the shapes of spatial contrast sensitivity curves and the cells' temporal contrast sensitivity profiles were not markedly dependent on the criterion level set to measure 'threshold' contrast. Reducing the level of retinal illumination in the range 230-0.007 cd/m2 (pupil, 3 mm2) produced a fall in contrast sensitivities for both XG and YG cells. The loss in sensitivity was more marked at high spatial and high temporal frequencies. The similar shapes of the temporal contrast sensitivity curves of XG and YG cells weakens the suggestion that the human counterparts of these cells would provide a suitable physiological substrate for the psychophysical sustained and transient channels. Although the behaviour of XG and YG cells parallels quite closely changes in cat and human psychophysical spatial contrast sensitivities with temporal frequency and retinal illumination, many problems remain for equating results from the two fields.

Statistics of the maintained discharge of cat retinal ganglion cells.

Action potentials were recorded from single fibres in the optic tracts of anaesthetized cats. Continuous records were obtained at various levels of scotopic and mesopic retinal illumination. In some cases, the light intensity was modulated by a pseudorandom Gaussian white-noise signal. The maintained discharge of on-centre neurones increased while the maintained discharge of off-centre neurons decreased with increased illumination of the receptive field centre. For both cell types, the coefficient of variation declined with increased rate of discharge. There was minimal short-term dependency in the firing patterns, and it was unaffected by the level of retinal illumination. Virtually all of the structure revealed by the normalized autocovariance functions could be attributed to the shape of the interval distributions. The first few coefficients of the serial correlogram were slightly negative. The magnitude of this negativity was not related to illumination. Long-term dependency in the firing pattern was also quite small; the standard deviations of the mean rate in samples of about 1 sec duration were only slightly less than would be predicted from the interval distributions. This dependency tended to increase at higher retinal illuminations. Neural discharges elicited by Gaussian modulation of the light were strikingly different from those elicited by steady light. Modulation caused the first coefficient of the serial correlogram to become more positive, while the next several coefficients became more negative. A corresponding pattern could be seen in the normalized autocovariance functions, and in the differences between the normalized autocovariance and normalized autoconvolution. Long-term dependency also increased dramatically, such that the standard deviations of mean rate were about 60% of what would be expected given the interval distributions observed. These results place a number of constraints upon the ways in which intrinsic noise in the retina may enter the visual processing network. Two alternative models consistent with the data are presented.

Spectral luminous efficiency functions for a ten-degree field in the mesopic range.

Mesopic photometry is one of the current problems of the CIE TC-1.4. The luminous efficiency function for mesopic vision has not been established yet, whereas the functions V(λ) for photopic vision and V'(λ) for scotopic vision have been already standardized. In the present study, we measured the spectral luminous efficiency functions in the mesopic range extensively to offer basic data for making photometry more visually meaningful. A direct brightness matching method of 10° field was employed. White light was used as a reference stimulus, and its retinal illuminance was varied from 0.01 to 100 photopic trolands in 0.5 log unit steps to cover the mesopic range. Spectral luminous efficiency function from 400 to 700 nm in 10 nm steps was measured for each retinal illuminance level. Data of seven subjects (19-23 yr old) were collected. It was shown that the function obtained changed gradually from the photopic function to the scotopic one as retinal illuminance decreased. The average and individual variation of seven subjects’ data were presented as basic data of luminous efficiency for mesopic vision. The change of the function due to the different contributions of photopic and scotopic functions was discussed according to the Ikeda-Shimozono’s formula.

Luminous efficiencies of 2.DEG. and 10.DEG. fields for brightness perception obtained from Japanese subjects.

Luminous efficiencies for brightness are tabulated at every 10 nm step that were obtained for 2° field from 18 Japanese subjects and for 10° field from 9 Japanese subjects. The data were provided by various Japanese laboratories and analyzed in the Research Committee on the luminous efficiency functions in the Illuminating Engineering Institute of Japan. The efficiencies were all determined by the direct comparison method against a reference at some photopic retinal illuminance level. The mean luminous efficiency function of the 18 subjects for the 2° field is fairly close to that presented in a technical report of the CIE Technical Committee TC-1.4, Vision.

Deuteranomalous color matching in the deuteranopic eye.

Two observers were classified as deuteranopes by standard tests including two-degree anomaloscope matches. Color matching similar to the Rayleigh type was then carried out for a 10-degree field size at retinal illuminance ranging from 1 to more than 3000 trolands (td). The results show that at the larger field size and higher levels of retinal illuminance, a third independent color-mediating mechanism with the sensitivity of the deuteranomalous cone is participating in the color match. The results also confirm participation of a different third mechanism with rod sensitivity at levels below about 100 td. There is a range of transition between the two as the level increases above 100 td. Therefore large-field color matching in these deuteranopes is trichromatic at the levels tested, not dichromatic, and a third cone system is found to operate at typical photopic light levels under static viewing conditions in a dichromatic eye.

High-level trichromatic color matching and the pigment-bleaching hypothesis

Maxwell-type color matches with reference to a given “white” stimulus at a retinal illuminance of 100,000 td can be predicted from Maxwell-type color matches made with reference to the same “white” stimulus but at a retinal illuminance of 1000 td by means of a simple hypothesis of bleaching of the visual pigments. The spectral distributions of the fundamentals of color vision, the densities of the visual pigments of the 1000 and 100,000 td levels, as well as the spectral transmit tances of the lens and macula pigments which must be assumed as essential parameters of the visual mechanism emerge with values which appear consistent with those obtained from other investigations. While both the “red” and the “green” fundamentals exhibit strong and predictable bleaching characteristics, the “blue” fundamental appears to bleach only very little under the given experimental conditions. The somewhat unexpected behavior of the “blue” fundamental is not explained, but it is suggested that the pigment associated with the “blue” fundamental may be in a near-diluted state already at moderate levels of retinal illuminance, making it difficult to detect bleaching at higher levels.

Luminous efficiency functions by heterochromatic brightness matching for a wide range of retinal illuminance.

The present photopic photometric system is based on the CIE photopic relative luminous efficiency function V(λ), most of whose data were obtained by the flicker photometry. In recent years, however, it is pointed out that the CIE V(λ) does not necessarily represent the spectral sensitivity for the brightness perception. In the present experiment, the relative luminous efficiency functions were determined by the heterochromatic brightness matching method with a 2° visual field at fovea at the various retinal illuminance levels of 1 troland through 1000 trolands. At illuminance levels higher than 10 trolands, all the relative luminous efficiency curves showed a broader shape than the CIE V(λ). In some observers the relative luminous efficiency curves became broader with increasing the retinal illuminance. The present experimental results sugges that the brightness perception is contributed from the opponent-color channels.

Large-field color naming of dichromats with rods bleached

A color-naming method was used to examine the large-field red/green discriminations of dichromats screened with standard tests. The stimulus was a 12 degrees annular field with the central 4 degrees removed, flashed for 300 ms. Four wavelengths were equated in brightness for each observer at two retinal illuminance levels, approximately 10 and 100 trolands. The stimuli were then presented in random order and the observer was asked to name each, using one of four color terms. The entire experiment was done with the observers dark-adapted and also with the rods bleached. For all four deuteranopes and two of four protanopes, color names were very significantly related to both illuminance and wavelength in both adaptation conditions. The relationship between name and wavelength was similar to that of a normal trichromat, but the performance of a dichromat was very poor by comparison. Performance was generally somewhat better in the dark than with the rods bleached. However, the result in the bleached condition is consistent with recent evidence that at least some observers who are classified as dichromats with standard small-field screening procedures actually have a weak residual third cone mechanism.

The mechanism of brightness enhancement

The paradoxical increase in brightness of a subfusional intermittent light, called brightness enhancement, requires a minimum photopic level of retinal illumination for its initiation. In this study, we have utilized a variable intensity monochromatic bipartite field, half of which was intermittent in order to determine the threshold for brightness enhancement across the visual spectrum. The action spectrum thus obtained in two subjects were identical to independently determined photopic luminosity curves obtained in the same subjects. A neuromechanism accounting for these results is suggested.