Space-average Luminance Research Articles

The Effect of Light Level and Small Pupils on Presbyopic Reading Performance.

To examine the impact of small pupils and light levels on reading performance of distance-corrected presbyopes. To determine whether small pupils would enable presbyopes to read at near even at low light levels. To establish the lower range of text luminances, we quantified the space-averaged luminance of text in nine different artificially lit interior environments, and examined the impact of the text characters on space-averaged luminance of electronic and printed displays. Distance and near reading speeds of 20 presbyopes (ages 40-60 years) were measured while viewing through artificial pupils (diameters 1-4.5 mm), natural pupils, or with a multifocal contact lens. Space-averaged text luminance levels varied from 0.14 to 140 cd/m2 (including the range of measured environmental text luminances). Adding black text to a white computer display or paper reduces luminance by approximately 15% to 31%, and the lowest encountered environmental text luminance was approximately 2 to 3 cd/m2. For both distance and near reading performance, the 2- to 3-mm small pupil yielded the best overall reading acuity for space-averaged text light levels ≥ 2 cd/m2. The 2- to 3-mm artificial pupils and the multifocal contact lenses both enabled maximum or near-maximum reading speeds for 0.5 logMAR characters at distance and near, but with natural pupils, reading speeds were significantly reduced at near. Although photon noise at low luminance reduces the visual benefits of small pupils, the benefits of 2- to 3-mm artificial pupils are sufficient to enable >80% of distance-corrected presbyopes to read proficiently at near, even at the lowest text luminances found in interior environments.

Can blindsight be superior to ‘sighted-sight’?

DB, the first blindsight case to be tested extensively ( Weiskrantz, 1986) has demonstrated the ability to detect and discriminate a range of visual stimuli presented within his perimetrically blind visual field defect. In a temporal two alternative forced choice (2AFC) detection experiment we have investigated the limits of DB’s detection ability within his field defect. Blind field performance was compared to his sighted field performance and to an age-matched control group ( n = 6). DB reliably detected the presence of a small (2°), low contrast (7%), 4.6 c/° Gabor patch with the same space-averaged luminance as the background presented within his blind field but performed at chance levels at the same eccentricity (11.3°) within his sighted field. Investigation of detection as a function of stimulus contrast revealed DB’s ability to detect the presence of an 8% contrast stimulus within his blind field, compared to 12% in his sighted field. No significant difference in detection performance between DB’s sighted field and the performance of six age-matched control participants suggests poor sighted field performance does not account for the results. Monocular testing also rules out differences between the eyes as an explanation, suggesting that DB demonstrates superior detection for certain stimuli within his visual field defect compared to normal vision.

Luminance flicker sensitivity in positive- and negative-symptom schizophrenia.

The aim of the present research was to investigate magnocellular and parvocellular channel disorders using luminance-flicker sensitivity in normal observers and a group with schizophrenia. The threshold sensitivity for a sine wave-modulated patch of achromatic flickering light in a gaussian envelope was measured as a function of its temporal frequency (1.0 Hz, 4.0 Hz, 8.0 Hz, 16.0 Hz, 32.0 Hz) and three space average luminance levels (mesopic 3.0 cd/m2, photopic 33.0 cd/m2 and 66.0 cd/m2). The Andreasen scales for the assessment of positive and negative symptoms in schizophrenia were used to classify subjects into subgroups with predominantly positive and negative symptoms. The results showed that there were no significant differences between the control and positive-symptom group in flicker sensitivity as a function of temporal frequency and luminance level, and there were no differences in flicker sensitivity between the three groups at 1.0 Hz at each of the three luminance levels. At 3.0 cd/m2 the negative-symptom group showed significant reductions in flicker sensitivity at 4.0 Hz, 8.0 Hz, 16.0 Hz and 32.0 Hz in comparison with the control and positive-symptom group. At 33.0 cd/m2 the negative-symptom group showed significant reductions in flicker sensitivity at 4.0 Hz and 32.0 Hz, and at 66.0 cd/m2 they showed significant reductions in flicker sensitivity at 4.0 Hz, 8.0 Hz and 32.0 Hz only in comparison with the control. It was concluded that the non-significant differences in flicker sensitivity in the positive-symptom group showed that the processing of temporal information in parvo- and magnocellular channels was unimpaired. Furthermore, the non-significant differences in flicker sensitivity at 1.0 Hz at each of the three luminance levels in the three groups provided evidence that functioning in parvocellular channels was unimpaired in the positive- and negative-symptom group. Finally, it was concluded that the significant reductions in flicker sensitivity at medium and high temporal frequencies in the negative-symptom group provided evidence for an impairment in magnocellular channels.

Pupil responses to stimulus color, structure and light flux increments in the rhesus monkey

Visual stimuli that isolate pupil color and pupil grating responses in human vision have been used to investigate the properties of stimulus-specific pupil responses in the rhesus monkey. We measured and compared pupil responses to light flux increments, isoluminant chromatic stimuli, and gratings of equal and lower space-averaged luminance. The parameters investigated were luminance contrast and chromatic saturation. The results demonstrate clearly the existence of pupil color, pupil grating and pupil light reflex responses in the rhesus monkey. Comparison of pupil color and pupil grating responses of equivalent amplitude reveals similar onset response latencies. However, both are approximately 40 ms longer than the corresponding pupil light reflex latency. In general these pupil responses are qualitatively similar to those observed in humans. However, when compared to equivalent human data, pupil onset response latencies are some 80–100 ms shorter and the pupil shows more rapid recovery from constriction.

Contrast-modulation flicker: dynamics and spatial resolution of the light adaptation process.

We report a perceptual phenomenon that originates from a nonlinear operation during the visual process, and we use these observations to study the functional organization of the responsible nonlinearity; the regulation of visual sensitivity to light. When the contrast of a high frequency grating was modulated while its spatial and temporal average luminance was kept constant, observers saw brightness changes or desaturation in the field. If the contrast was modulated periodically between zero and a peak value, observers saw vivid flicker (contrast-modulation flicker), and this flicker could be seen even when the grating was too fine to be visually resolved as a pattern. This uniform-field flicker can be nulled by a modulation of space-average luminance at the contrast-modulation frequency, with appropriate phase and modulation depth. Contrast-modulation flicker is still measurable with gratings at 100 cycles/deg. The dynamics of contrast-modulation flicker suggest that it results from an early sensitivity-controlling mechanism, acting very rapidly (within about 20 msec). Its dependence on stimulus spatial frequency implies a strictly local luminance nonlinearity, one that either resides within individual photoreceptors or operates on signals from individual receptors.

Shift of equiluminance in congenital color vision deficiencies: pattern-ERG, VEP and psychophysical findings.

We compared electrophysiological responses [pattern-ERG (PERG) and VEP] and psychophysical measures to color stimuli to separate different forms of anomalous color vision. PERG and VEP were recorded from seven normals and 14 subjects with congenital color vision deficiencies. Stimuli were color checkerboards with 0.5 deg check size, phase reversing at 34 rev/sec. The luminances of the red and green parts were varied in opposite direction from 0 to 30 cd/m2, while the hue of individual squares and space-averaged luminance were held constant. This allowed for one equiluminance condition where flicker appeared fused. In the seven normals, the subjective equiluminance was reached at a luminance ratio red/(red + green) = 0.50-0.53. At that point, the PERG amplitude was moderately, and the VEP amplitude sharply reduced. In 14 color anomalous subjects both the PERG and VEP were sharply reduced at equiluminance. These dips were shifted compared to normals and the dip position corresponded to the predicted luminance ratios obtained by calculations from L- and M-cone activation using the Smith-Pokorny transformation. As we found a close correlation of the VEP-dip position and the anomalous quotient, these electrophysiological measures may allow objective assessment of color vision deficiencies.

Lightness, equivalent backgrounds, and anchoring.

Observers compared two center/surround configurations haploscopically. One configuration consisted of a standard surface surrounded by two, three, or four surfaces, each with a different luminance. The other configuration consisted of a comparison surface surrounded by a single annulus that varied in luminance. Center surfaces always had the same luminance but only appeared to have the same lightness with certain annuli (equivalent backgrounds). For most displays, the luminance needed to obtain an equivalent background was close to the highest luminance in the standards surround configuration. Models based on the space-average luminance or the space-average contrast of the standard surround configuration yielded poorer fits. Implications for computational models of lightness and for candidate solutions to the anchoring problem are discussed.

Ignoring Sparse Chromatic Context

The apparent colour of a target depends to some extent on the colour of the background. Jennes and Shevell have recently shown that a red background scattered with white or green dots has a different influence on the apparent colour of a target than a uniform background with the same space-averaged chromaticity and luminance (1995 Vision Research35 797 – 805). This could have important implications for our understanding of colour vision. However, it appears to be a very fragile effect. We were only successful in reproducing it when we used the same long dark-adaptation and pre-adaptation times as those used by Jennes and Shevell. One could argue that with shorter adaptation the effect was masked by gradual changes such as shifts in receptor sensitivity. However, when we interleaved trials with and without white dots for well over 20 min there was still no systematic difference between the two kinds of background. Moreover, when we repeated their experiment with a green rather than a red background, adding white dots had no effect. The long adaptation times that are required to produce the effect and the fact that we only found it for a long-wavelength background suggests that the effect may have to do with differences in the extent of rod adaptation. Although it remains an interesting phenomenon, this would probably make it irrelevant for normal colour vision.

Brightness contrast from inhomogeneous surrounds

The luminance of a test within an inhomogeneous ("checkerboard") surround was adjusted to match the brightness of a comparison patch within a uniform surround. All stimuli were achromatic. Both surrounds had the same space-averaged luminance. With an incremental comparison patch, a test-within-checkerboard at a luminance between the luminances of the brighter and dimmer checks appears dimmer than if viewed within the uniform surround. A decremental comparison patch, however, is matched by a test luminance that is little affected by the inhomogeneity of the surround. In general, the brightness of the test is mediated neither by the space-averaged luminance of an inhomogeneous surround, nor by any equivalent uniform surround, regardless of luminance. We consider alternative models for the brightness of a region that is neither strictly an increment nor decrement with respect to contiguous surrounding surfaces.

Classifying illusory contours by means of orientation discrimination

1. Thresholds for discrimination of orientation were measured for solid and interrupted lines, and for illusory borders defined by offset abutting lines of a variety of configurations. 2. Illusory contours defined by orthogonal lines with a clear gap or overlap manifest excellent orientation discrimination even for very short exposure durations, and are robust to pattern motion and masking. The performance is almost as good as that for real lines. 3. When the contour is made up of abutting lines without gap or overlap, orientation discrimination is not nearly as good, and deteriorates further with short exposure duration, with target motion, and with masking. 4. The introduction of disparity in such illusory borders does not improve the discrimination of its orientation, so that concepts such as surface elaborations do not yet have to be invoked. 5. The results suggest that there is one class of illusory contour whose orientations can be discriminated by virtue of its delivering adequate stimuli to orientation-selective units in primary visual cortex. For contours with a uniform space-averaged luminance, the orientation discrimination is helped by the orthogonality of the generating lines, and this gives support to the concept that orthogonally oriented end-stopped neurons may participate in the orientation discrimination.

Pupillary function in human amblyopia

Quantitative measurements of pupillary function (response amplitude and latency) were made for normal eyes and for normal and fellow amblyopic eyes of groups of strabismic and anisometropic amblyopes. Stimuli consisted of luminance modulation of a large, evenly lit area (pupil light reflex) as well as contrast modulation of sinusoidal gratings (pupil grating response) of fixed, space-averaged luminance. Measurements were made of the direct and the consensual reflex under monocular stimulation. A comparison of the amplitude of the pupil light reflex as a function of luminance modulation showed no significant differences between normal and fellow amblyopic eyes for both the strabismic and anisometropic groups of amblyopes studied. A similar comparison of the associated response latencies showed significant difference between normal and fellow amblyopic eyes for both groups. In general, reductions in response amplitude and latency of the pupil grating response were found in individuals from each group when comparing the good and the affected eyes, although the observed group differences were only significant in the strabismic group. Interestingly, statistically significant reductions in both amplitude and latency for both the pupil light reflex and the pupil grating response were found between the eyes of normal observers and the so-called normal eyes of amblyopes in both groups studied. These results suggest that the type of pupillary deficit in amblyopia is a complicated one, depending not only on the type of amblyopia (strabismic or anisometropic) and the type of stimulus employed (light or pattern), but also on the parameter assessed (amplitude or latency) and whether the amblyopic result is referenced to its fellow normal eye or to the normal eye of a non-amblyopic observer. Since the pupil response to light flux changes is not mediated exclusively via the retinal projection to the midbrain and may also involve the activity of central visual pathways, the results obtained in this study cannot be used to provide definitive evidence for the site of abnormality in amblyopia.

The spatial signal for saccadic eye movements emphasizes visual boundaries

By investigating the visual processing involved when saccades are made to newly appearing targets, we show that this processing is significantly nonlinear and that texture boundary information predominates. We used the global, or center-of-gravity, effect whereby a saccadic eye movement directed to a target consisting of a pair of elements has an amplitude intermediate between that of saccades directed to the individual elements. We measured the effect using target elements with different visual characteristics, including phase-reversal checkerboard targets that had the same space-average luminance as the background. The contribution to the center-of-gravity calculation was used to measure relative salience. We found that positive and negative contrast elements contribute almost equal weightings. Thus, salience, assessed in this way, is a highly nonlinear function of luminance. The salience of checkerboard targets was found to decrease as check size was decreased and increase as the overall size of the target was increased. Checkerboards with an empty center were as effective as were full checkerboards, showing the importance of boundaries in the salience signal.

The effects of luminance on FPL and VEP acuity in human infants

Grating acuity was measured in 16-week-old human infants. Three measurement techniques were used: forced-choice preferential-looking (FPL), and two visual-evoked-potential (VEP) techniques. The stimuli were counterphase flickering sinewave gratings with a space-average luminance of -1.0 or 2.0 log cd/m2. Slightly different luminance-dependent changes occur between FPL and VEP acuities, suggesting that some factor influences the two methods differently as stimulus luminance varies. A comparison between FPL acuities and VEP acuities within infants suggests a quantitative relationship between techniques. Infant's acuity for sinewave gratings with a space-average luminance of -2.0, -1.0, 0.0, 1.0 and 2.0 log cd/m2 was also measured using a single VEP paradigm. The results are compared to the same measurements in adults and to infant and adult ideal observers. VEP acuity in this group of infants improves by about 0.5 log units between -2.0 and 0.0 log cd/m2 and remains asymptotic between 0.0 and 2.0 log cd/m2. This result suggests that luminance-dependent changes in infant acuity cannot be fully accounted for by immaturities in the optics and photoreceptor spacing and efficiency.

A new technique to measure contrast sensitivity in human infants.

We used a new time-efficient procedure to obtain binocular contrast sensitivity functions (CSFs) from 80 infants aged 1-, 3-, 6-, and 12-months-old. The test consisted of 5 sets of large (50 by 28 cm) cards constructed by mounting a 7.2 degrees circular sine wave grating with 1 of 5 spatial frequencies (0.3, 0.6, 1.2, 2.4, and 3.6 cpd at 60 cm) onto light gray matteboard. Each card also contained a second 7.2 degrees unpatterned patch of equal space-average luminance. Depending upon the set, the contrast within the gratings ranged from approximately 25 to 0.5%. To judge the subject's detection of each grating we used a rapid version of preferential looking (PL) similar to that developed for the Teller Acuity Cards. Results showed that compared to previous techniques, the card procedure allows one to estimate an infant's CSF much more simply, rapidly, and inexpensively.

Grating acuity of cats measured with detection and discrimination tasks

In order to help resolve the widely discrepant claims for the grating acuity of the cat, behavioral measurements were made of the acuities of two normal adult cats using two different tasks. In one, a conventional detection task, the cats were required to discriminate between high contrast vertical gratings and a uniform field of the same space-averaged luminance. The second, less commonly-used task, required the cats to discriminate between vertical and horizontal gratings of the same spatial frequency. Both cats obtained thresholds of between 8.5 and 9 cycles/degree, with no difference between tasks. These results suggest that detection of aliased patterns is not a likely factor contributing to the wide range of published acuities, and they provide support for one of two competing models of beta-ganglion cell sampling in the retina.

Spatial-frequency-tuned attenuation and enhancement of the steady-state VEP by grating adaptation

Steady-state visual evoked potentials (VEPs) were recorded from adults using 10%C fast spatial frequency (SF) sweeps of horizontal gratings under two conditions: (a) after exposure to a 40%C grating of 6 or 4 c/deg, and (b) after exposure to a blank screen equalling the adapting gratings in space-averaged luminance. SF adaptation attenuated VEP amplitude near the adapting SF, but maximum attenuation was displaced from the adapting SF for 6 c/deg adaptation. Small displacements in maximum attenuation would be expected if underlying neural subunits are tuned to a small number of different center SFs. In addition, SF adaptation caused amplitude enhancement 1.0–2.0 octaves below the adapting SF, providing electrophysiological evidence in humans for coinhibitory relationships among neural mechanisms that have been postulated on the basis of analogous psychophysical findings. The results are consistent with coinhibition between SF-tuned subunits and between transient and sustained mechanisms.

Visual persistence of spatially filtered images

Periodic stimuli such as sine-wave gratings and checkerboard patterns have been used in many studies of visual perception. It is well known that with such stimuli, visual persistence increases as spatial frequency increases and as contrast decreases. It is not clear, however, that similar relationships obtain for aperiodic stimuli such as natural images. Digitized images of objects (a face and a vase) were submitted to two-dimensional Fourier analysis. Four pairs of spatial frequency band-limited images were created for each image. Each pair consisted of a normal (NP) and a scrambled (SP) phase version, with the magnitude spectrum and space-averaged luminance the same within each pair. Filter bandwidths were one octave wide. Threshold persistence was measured for each spatially filtered image. Visual persistence for SP images increased significantly as spatial frequency increased, whereas no significant differences were found for NP images. This suggests that the temporal processing of complex, aperiodic visual images is influenced by the spatial frequency and contrast of local features within the image and cannot be predicted by space-averaged estimates of contrast and spatial frequency.

The effects of spatial phase on reaction time to spatially filtered images

Many studies of visual perception have used periodic stimuli such as sine-wave gratings and checkerboard patterns. It is well known that reaction time (RT) to such stimuli increases with increasing spatial frequency and decreasing contrast. While this is the case with periodic stimuli it is not clear that these relationships obtain for aperiodic stimuli such as natural scenes. A digitized image of an object (a vase) was submitted to two-dimensional Fourier analysis. Four pairs of spatial frequency band-limited images were created for each image. Each pair consisted of a normal-phase (NP) and a scrambled-phase (SP) version, with the magnitude spectrum and space-averaged luminance the same within each pair. Filter band-widths were 1 octave wide. Manual RT was measured for onset and offset of each spatially filtered image. Mean RT for SP images increased significantly with increasing spatial frequency, while no other significant differences were found with the NP images. This suggests that the temporal processing of complex, aperiodic images is influenced by the spatial frequency and contrast of local regions within the image, rather than by the space-averaged contrast of the entire image, and cannot be predicted by global estimates of contrast and spatial frequency.

Development of contrast sensitivity in the human infant

Contrast sensitivity and grating acuity were measured using the sweep VEP method in a group of 48 infants from 2 to 40 weeks of age and in a group of 10 adults. Sinusoidal gratings were reversed in contrast at 12 alternations per sec at a space-average luminance of 220 cd/m 2. During 10 sec trials, either The contrast or the spatial frequency was increased in a series of 19 steps. Thresholds were estimated by extrapolation of the VEP response functions to zero amplitude. The contrast threshold at low spatial frequencies developed rapidly from 7% contrast at 2–3 weeks to an asymptote of 0.5% at 9 weeks. For adults, maximum sensitivity at low spatial frequencies was 0.32–0.22%. The sweep VEP estimate of grating acuity showed a gradual increase in spatial frequency with age, starting at 5 c/deg during the first month and reaching 16.3 c/deg at 8 months. The mean adult acuity was 31.9 c/deg. There appeared to be two phases in the development of contrast sensitivity and acuity. Between 4 and 9 weeks overall contrast sensitivity increased by a factor of 4–5 at all spatial frequencies. Beyond 9 weeks, contrast sensitivity at low spatial frequencies remained constant, while sensitivity increased systematically at higher spatial frequencies.

Spatial and temporal frequency in figure-ground organization.

Figure-ground organization of an ambiguous pattern can be manipulated by the spatial and temporal frequency content of the two regions of the pattern. Controlling for space-averaged luminance and perceived contrast, we tested patterns in which the two regions of the ambiguous pattern contained sine-wave gratings of 8, 4, 1, or 0.5 cycles per degree (cpd) undergoing on:off flicker at the rates of 0, 3.75, 7.5, or 15 Hz. For a full set of combinations of temporal frequency differences, with each spatial frequency the higher temporal frequency was seen as background for more of the viewing time. For two spatial frequency combinations, 1 and 4 cpd, and 1 and 8 cpd, tested under each of the four temporal frequencies, the lower spatial frequency region was seen as the background for more of the viewing time. When the effects of spatial and temporal frequency were set in opposition, neither was predominant in determining perceptual organization. It is suggested that figure-ground organization may parallel the sustained-transient response characteristics of the visual system.