Log Trolands Research Articles

Poster Session: Threshold versus intensity curves measured with a new high-brightness display system.

Classical threshold vs. intensity (tvi) curves were measured using optical systems and were generally limited to increment test stimuli and relatively simple spatial patterns. Modern displays provide more flexibility in terms of stimuli spatial profiles but are usually dim enough that there may be rod intrusion when measuring cone responses. Here we describe a high-brightness display system and present tvi's for increment and decrement achromatic tests. The system consists of a PROPixx three-chip DLP LED color projector (VPixx Technologies, Saint-Bruno, Canada) controlled via a Datapixx display driver, with 12-bit digital to analog conversion per RGB channel. Light from the projector is collected in a large diameter lens and focused on high gain rear projection screen. Retinal illuminance of the background may be varied in three ways: (a) varying the mean current supplied to the LEDs from the controller (adjustable in software); (b) using calibrated neutral density filters mounted near the eye; and (c) changing the midpoint of the RGB channels in software (e.g., making the white background as R=G=B=0.1 instead of 0.5). Method (c) is made easier by the fact that the PROPixx "gamma curve" is linear, which also means that no RGB bits are lost to gamma correction. We will show thresholds for achromatic tests on a white background varying from 0.56 to 4.03 log trolands, with preliminary results suggesting differences in the tvi curves between the increment and decrement tests.

Effect of ocular transverse chromatic aberration on detection acuity for peripheral vision.

We examined the effect of transverse chromatic aberration (TCA) on detection acuity for white-light interference fringes seen in Maxwellian view at various orientations and locations in the visual field. A circular patch (3.5° diameter, 3.2 log Trolands) of nominally high-contrast fringes was produced on the retina by a commercial instrument (the Lotmar Visometer, Haag Streit) mounted on a gimbal for controlled positioning of the stimulus in the visual field from 0° to 35° eccentricity. Detection acuity for white light fringes for all meridians and eccentricities ≥15° was maximum when fringes were oriented parallel to the visual meridian line. This meridional effect disappeared when a narrow-band filter was used to eliminate TCA. The meridional effect also disappeared when the interferometric stimulator was displaced laterally to align the instrument with the eye's local achromatic axis. Modelling confirmed that TCA is the major factor responsible for white-light meridional bias, with minor contribution arising from higher-order monochromatic aberrations and neural factors.

A low-power, LED-based, high-brightness anomaloscope

Color matches made with a Nagel anomaloscope are used in the differentiation of color vision deficiencies. When these color matches are made over a wide range of retinal illuminances, the changes in the color match provide information about the regeneration kinetics and the absorption spectra of the middle- and long-wavelength cone photopigments. These steady-state color matches vary with a variety of conditions, and may have value in screening for eye disease. Recently, high-brightness LEDs have become available that allowed us to construct a LED-based, high-brightness anomaloscope. We used inexpensive, low-energy components to replicate an earlier instrument, getting a maximum retinal illuminance over 5.6 log Trolands.

A luminous efficiency function,V*(λ), for daylight adaptation

We propose a new luminosity function, V*(lambda), that improves upon the original CIE 1924 V(lambda) function and its modification by D. B. Judd (1951) and J. J. Vos (1978), while being consistent with a linear combination of the A. Stockman & L. T. Sharpe (2000) long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cone fundamentals. It is based on experimentally determined 25 Hz, 2 degrees diameter, heterochromatic (minimum) flicker photometric data obtained from 40 observers (35 males, 5 females) of known genotype, 22 with the serine variant L(ser180), 16 with the alanine L(ala180) variant, and 2 with both variants of the L-cone photopigment. The matches, from 425 to 675 nm in 5-nm steps, were made on a 3 log troland xenon white (correlated color temperature of 5586 K but tritanopically metameric with CIE D65 standard daylight for the Stockman and Sharpe L- and M-cone fundamentals in quantal units) adapting field of 16 degrees angular subtense, relative to a 560-nm standard. Both the reference standard and test lights were kept near flicker threshold so that, in the region of the targets, the total retinal illuminance averaged 3.19 log trolands. The advantages of the new function are as follows: it forms a consistent set with the new proposed CIE cone fundamentals (which are the Stockman & Sharpe 2000 cone fundamentals); it is based solely on flicker photometry, which is the standard method for defining luminance; it corresponds to a central 2 degrees viewing field, for which the basic laws of brightness matching are valid for flicker photometry; its composition of the serine/alanine L-cone pigment polymorphism (58:42) closely matches the reported incidence in the normal population (56:44; Stockman & Sharpe, 1999); and it specifies luminance for a reproducible, standard daylight condition. V*(lambda) is defined as 1.55L(lambda) + M(lambda), where L(lambda) and M(lambda) are the Stockman & Sharpe L- & M-cone (quantal) fundamentals. It is extrapolated to wavelengths shorter than 425 nm and longer than 675 nm using the Stockman & Sharpe cone fundamentals.

Effect of stimulus intensity on the sizes of chromatic perceptive fields

The effects of intensity on chromatic perceptive field size were investigated along the horizontal meridian at 10 degrees temporal eccentricity by manipulating stimulus intensity from 0.3 to 3.3 log trolands. Following light adaptation, observers described the hue and saturation of monochromatic stimuli (440-660 nm, in 10 nm steps) for a series of test sizes (0.098-3 degrees) presented along the time period associated with the cone plateau of the dark-adaptation function. Perceptive field sizes of the four elemental hues (red, green, yellow, and blue) and the saturation component were estimated by three observers at each intensity level for each wavelength. In general, perceptive field sizes of blue and red are the smallest, and yellow and green are the largest. Furthermore, perceptive field sizes of all four hues decrease with increasing stimulus intensity, though the absolute change is largest for green and yellow. The decrease in size with increase in intensity cannot be completely explained in terms of saturation or rod signals and is likely, then, attributable to a cone-based mechanism.

What limits detection and resolution of short-wavelength sinusoidal gratings across the retina?

Peripheral resolution acuity for achromatic gratings is known to be limited by the density of the underlying ganglion cell sampling array. After confirming isolation of the short-wavelength sensitive (SWS) system using chromatic adaptation methods, we wished to determine if resolution is limited by blue/yellow ganglion cell sampling (evidenced by a superiority of detection over resolution acuity and the perception of aliasing) and thus directly related to SWS-driven ganglion cell density. We measured detection and resolution acuity between 0° and 35°, using blue sinusoidal gratings superimposed on a yellow adapting background which ranged in intensity from 2.5 to 4.7 log Trolands. At all locations, a break could be observed in the acuity vs. illumination curves followed by a plateau. Detection and resolution acuity were the same for low background intensities, but resolution acuity was lower than detection at higher intensities, accompanied by observations of chromatic aliasing. SWS resolution is sampling-limited across the retina and agrees well with predicted performance based on anatomical estimates of small bistratified ganglion cell density.

Aliasing in peripheral vision for flickering gratings under different levels of illumination

Purpose. To assess how far retinal illumination can be reduced for sine-wave gratings phase reversing at different temporal frequencies in peripheral vision, while maintaining sampling limited resolution acuity performance, as evidenced by an aliasing zone between detection and resolution. Methods. Computer generated sine wave gratings were presented with flicker rates from 0 to 40 Hz under retinal illumination levels of 3.5 to -0.5 log trolands. Resolution and detection thresholds were measured at 30 degrees in the horizontal temporal field using a spatial and temporal 2AFC paradigm respectively. Results. At high illumination levels, detection acuity is higher than resolution acuity between 0 and 40 Hz indicating that resolution is sampling limited. As illumination level decreases the aliasing zone becomes narrower, especially at high temporal frequency until it disappears completely at 0.5 log trolands. Conclusions. Peripheral resolution acuity ceases to be sampling limited below 1.5 log trolands for low temporal frequency gratings and at higher levels for high temporal frequency gratings. Sampling limited acuity was recorded for high frequency gratings under higher illumination levels which could be mediated by the M cells alone, but this is not the case for the lower levels of illumination.

Measuring leukocyte velocity in macular capillaries using a miniaturized blue field simulator: effect of aperture of the pupil

In this paper we present a miniaturized blue field simulator (BFS) which allows the measurement of the velocity (V) pulsatility (P) and number (D) of leukocytes in the macular retinal capillaries. A study on the effect of the aperture size of the blue light source at the subject's pupil on the measured flow parameters was performed. A blue field entoptoscope with a small halogen lamp was used to induce the perception of the "flying corpuscles" and a flat color screen was selected to display a computer simulation of this entoptic phenomenon. The aperture of the blue light source at the pupil was varied by a diaphragm placed at a conjugate pupil plane while the size and retinal irradiance of the blue field stimulus at the fundus was held constant. The results show significant correlations between log pupil area and both V (-18% per log unit) and D (+42% per log unit). When the retinal illuminance is expressed in Log Trolands, V shows no dependency, but D is strongly correlated (+40% per log Troland). When applying the BFS technique, the size of the blue light stimulus at the pupil, if undilated, must be well controlled to minimize instrument-related variations of the blood flow measurements.

Spectral mechanisms of spatially induced blackness: data and quantitative model.

Spectral efficiency functions and tests of additivity were obtained with three observers to identify possible chromatic contributions to spatially induced blackness. Stimuli consisted of a series of monochromatic (400-700 nm; 10-nm steps), 52-arcmin circular test lights surrounded by broadband (x = 0.31, y = 0.37), 63-138-arcmin annuli of fixed retinal illuminance. The stimuli were imaged on the fovea in Maxwellian view as 500-ms flashes with 10-s interstimulus intervals. Observers decreased the intensity of the test center until it was first perceived as completely black. Action spectra determined for two surround levels [2.5 and 3.5 log trolands] had three sensitivity peaks (at approximately 440, 540, and 600 nm), However, when monochromatic surrounds were adjusted to induce blackness in a broadband center, action spectra were unimodal and identical to functions obtained by heterochromatic flicker photometry. Tests of additivity revealed that when blackness is induced by broadband surround into a bichromatic center, there is an additivity failure of the cancellation type. This additivity failure indicates that blackness induction is influenced, in part, by signals from opponent-chromatic pathways. A quantitative model is presented to account for these data. This model assumes that blackness induction is determined by the ratio of responses to the stimulus center and the annulus, and while signals form the annulus are based only on achromatic information, responses from the center are based on both chromatic and achromatic properties of the stimulus.

Studies on the stability of the clinical electro-oculogram.

The electro-oculogram is variable as a clinical test, but the recording technique has not been standardized, and differences in the protocols for adaptation and stimulation among laboratories may contribute to the variability. To analyze some of these factors, we performed more than 100 electro-oculograms on a single subject under different conditions. Both the Arden ratio and the ratio of light peak amplitude to a stable dark-adapted baseline were independent of pupillary dilation but linearly related to retinal illumination measured in trolands. Between 3.0 and 4.0 log trolands, the values began to level off, but they were difficult to interpret because of subject discomfort above 3.5 log trolands. The Arden ratios were influenced by preadaptation light levels and were roughly 20% higher above 90 cd/m2 than below 45 cd/m2. There was a circadian rhythm in dark trough, baseline and light peak values, but the composite Arden ratio and light peak/baseline ratio showed little circadian effect. The light peak/baseline ratios were slightly more stable than the Arden ratios; the variability (defined as [standard deviation x 100]/mean) was 6.6% and 12.5%, respectively. Our results suggest that stimulus intensities for clinical electro-oculographic testing should be between 3.0 and 3.5 log trolands; pupil dilation is optional. This translates into light levels of 141-447 cd/m2 for a 3-mm pupil and 20-63 cd/m2 for an 8-mm pupil. The reduction in variability and independence from preadaptation achieved by using the light peak/baseline ratio instead of the Arden ratio must be weighed against the time required to achieve a stable dark baseline.

Chromatic induction and the locus of unique yellow as a function of stimulus illuminance

AbstractChromatic induction was measured in terms of the wavelength of a central 1°, unique yellow field surrounded by annuli that varied in wavelength, area, contrast, and overall retinal illuminance. In the first experiment, a 1.3° surround was varied from approximately 470 to 670 nm, using an overall illuminance range from 0.5 to 4.0 log trolands. The illuminance contrast between the center and surround was 1.0 (i.e., isoluminant). The results indicated that the wavelength of unique yellow shifted in the direction of the wavelength of the surround at all intensities. The magnitude of the shift, however, decreased with increasing intensity for middle‐ and long‐wavelength surrounds. The latter result was not expected from previous research. Experiment 2 tested the hypothesis that this might be due to a Bezold‐Brücke effect in which the hue of the surround changes with increasing retinal illuminance. The relative influence of Bezold‐Brücke hue shifts on chromatic induction was manipulated by changing surround area and luminance contrast. Results of this experiment were consistent with the hypothesis that changes in color appearance with increases in intensity may influence the magnitude of chromatic induction.

Visual transduction in cones of the monkey Macaca fascicularis.

1. Visual transduction in macaque cones was studied by measuring the membrane current of single outer segments projecting from small pieces of retina. 2. The response to a brief flash of light was diphasic and resembled the output of a bandpass filter with a peak frequency near 5 Hz. After the initial reduction in dark current there was a rebound increase which resulted from an increase in the number of open light-sensitive channels. The response to a step of light consisted of a prominent initial peak followed by a steady phase of smaller amplitude. 3. Responses to dim light were linear and time-invariant, suggesting that responses to single photons were linearly additive. From the flash sensitivity and the effective collecting area the peak amplitude of the single photon response was estimated as about 30 fA. 4. With flashes of increasing strength the photocurrent amplitude usually saturated along a curve that was gentler than an exponential but steeper than a Michaelis relation. The response reached the half-saturating amplitude at roughly 650 photoisomerizations. 5. The response-intensity relation was flatter in the steady state than shortly after a light step was turned on, indicating that bright light desensitized the transduction with a delay. This desensitization was not due to a reduction in pigment content. In the steady state, a background of intensity I lowered the sensitivity to a weak incremental test flash by a factor 1/(1 + I/IO), where IO was about 2.6 x 10(4) photoisomerizations s-1, or about 3.3 log trolands for the red- and green-sensitive cones. 6. Bleaching exposures produced permanent reductions in flash sensitivity but had little effect on the kinetics or saturating amplitude of subsequent flash responses. The sensitivity reductions were consistent with the expected reductions in visual pigment content and gave photosensitivities of about 8 x 10(-9) microns2 (free solution value) for the red- and green-sensitive pigments. During a steady bleaching exposure the final exponential decline of the photocurrent had a rate constant given by the product of the light intensity and the photosensitivity. 7. In some cells it was possible to measure a light-induced increase in current noise. The power spectrum of the noise resembled the spectrum of the dim flash response and the magnitude of the noise was consistent with a single photon response roughly 20 fA in size. 8. The membrane current recorded in darkness was noisy, with a variance near 0.12 pA2 in the band 0-20 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Temporal frequency discrimination above threshold

Temporal frequency discrimination was measured above threshold with a two-alternative spatial forced-choice procedure. Stimuli were two 1 deg homogeneous fields modulated around a mean luminance of 3.7 log trolands. Observers determined which of the two stimuli was modulated at a higher frequency. To avoid differences in apparent modulation depth as a cue for discrimination, all stimuli were matched in apparent modulation depth to an 11 Hz standard that was 0.5 log units above its threshold. Adaptation, caused by repeated presentation of suprathreshold stimuli, was avoided by using a 15 sec inter-trial interval. The relative difference thresholds ( Δff) were a non-monotonic function of frequency. Discrimination was best near 1.5, 4.0 and 30.0 Hz ( Δff= 0.08) and worst near 20.0 Hz ( Δff= 0.50). Control experiments showed that the improvement in discrimination beyond 20.0 Hz was not an artifact of mismatches in apparent modulation depth. These results demonstrate the existence of multiple channels sensitive to different ranges of temporal frequency.

Spatial characteristics of movement detection mechanisms in human vision. II. Chromatic stimuli.

Visual detection of targets moving against structured background fields has been studied with near-monochromatic stimuli, selected so as to isolate the different increment threshold spectral response mechanisms. It is shown that for foveal vision, the red- and green-sensitive mechanisms (pi 5 and pi 4 respectively) yield IMG functions (Barbur and Ruddock, 1980), similar to those found with white light. In contrast, the blue-sensitive (pi 1) mechanism yields a low-frequency IMG response quite unlike that found for the other mechanisms. There is also considerable variation between subjects in this case. Measurements taken 30 degrees off-axis with low (1.4 log trolands) background illumination level, yield a low frequency response IMG function for both rod and cone spectral mechanisms, similar to those found with white light stimuli. At high illumination levels (> 2.2 log trolands) the IMG function for the pi 5-mechanism is shifted to higher spatial frequencies, as is also observed with white light stimuli. A wavelength-selective binocular interaction effect, manifested in the detection of moving targets, is also described, and it is suggested that this may be of value in the study of defective colour vision.

Variability of accommodation during steady fixation at various levels of illuminance.

Using the stigmatoscope, the variability of the dioptric power of the eye during steady fixation was measured for four observers over a range of intensities from 2¯.5 to 3.0 log trolands in one-half logarithmic steps. Two different viewing conditions, (constant and variable size test letters), were studied but no significant difference between them was obtained. There was almost a fourfold decrease in the variability of the settings as the intensity was increased over this range. There was a sharp transition at one troland, suggestive of a discontinuity between scotopic and photopic vision. These changes could be eliminated by cycloplegia of the fixating eye. Essentially the same results were obtained when the refraction of one eye was measured objectively (with a concidence optometer), while the other eye fixated the chart. This latter technique is, however, less valid at the lower intensities because of the tendency to fixate the measuring light of the optometer. The data can be quantitatively described by a theory which postulates that the accommodation is continually fluctuating and that the limits of fluctuation are proportional to threshold ΔI/I.

Chromatic flicker fusion frequency as a function of chromaticity difference.

The critical color fusion frequency (ccff) was measured for each of fifteen pairs of six colors at equal luminance. Measurements were made at five levels of luminance ranging from 9 to 67 trolands. Two subjects were used. The six colors were of high chromatic purity. Their dominant wavelengths covered the range of 445 mμ to 670 mμ in nearly equal steps. The relative chromatic separation between the members of each pair was estimated from Judd’s RUCS and from Wright’s data on the size of equally noticeable color steps in the CIE chromaticity diagram.The results show that log reciprocals of cff and ccff are nearly parallel and linear functions of luminance in log trolands. The product-moment correlation between persistence (1/ccff) and log chromatic separation on the RUCS was 0.84 and 0.91 when log chromatic separation was computed from Wright’s data.

Effect of differential adaptation of the eyes upon threshold sensitivity.

Experimental determination was made of the effect of intense light adaptation (6.20 log Trolands) of one eye upon the dark-adapted threshold of the other eye. A psychophysical method derived from Békésy and analogous to the “up and down” method of Dixon and Mood was used. Three subjects showed a slight increase in sensitivity; one showed no change. The change was not statistically significant for the group. The finding is consistent with the accepted view that the states of adaptation of the eyes are independent and in contradiction to studies which have found an averaging effect.