Dynamic coding of temporal luminance variation

The range of variation in environmental stimuli is much larger than the visual system can represent. It is therefore sensible for the system to adjust its responses to the momentary input statistics of the environment, such as when our pupils contract to limit the light entering the eye. Previous evidence indicates that the visual system increasingly centers responses on the mean of the visual input and scales responses to its variation during adaptation. To what degree does adaptation to a stimulus varying in luminance over time result in such adjustment of responses? The first two experiments were designed to test whether sensitivity to changes in the amplitude and the mean of a 9.6° central patch varying sinusoidally in luminance at 0.6 Hz would increase or decrease with adaptation. This was also tested for a dynamic peripheral stimulus (random patches rotating on the screen) to test to what extent the effects uncovered in the first two experiments reflect retinotopic mechanisms. Sensitivity to changes in mean and amplitude of the temporal luminance variation increased sharply the longer the adaptation to the variation, both for the large patch and the peripheral patches. Adaptation to luminance variation leads to increased sensitivity to temporal luminance variation for both central and peripheral presentation, the latter result ruling retinotopic mechanisms out as sole explanations for the adaptation effects.     

Spatial summation in human vision: simple reaction time measurements.

The reaction-time technique was applied to examine spatial summation or area-intensity reciprocity at suprathreshold levels in the fovea. A family of reaction time vs luminance curves was measured in two experiments, and the luminance required to produce a criterion reaction time was computed from these curves to estimate the extent of summation. The first experiment showed that the upper level of spatial summation, the Ricco area, defined for constant reaction time increases with decreasing luminance level and that the upper limit of temporal summation is independent of the change in target size. The second indicated that the Ricco area decreases with increasing pulse duration up to 20-30 ms and then remains constant.     

Time course of brightness under transient glare condition

It was shown that a peripheral glare source reduces the brightness of a foveal stimulus. We hypothesized that this brightness reduction is governed by an inhibitory effect of the glare source on the test. We reported the results of an investigation of the dynamic of brightness reduction of an incremental stimulus immediately after the onset of a glare source in the field of view. A magnitude comparison paradigm using constant stimuli was adopted to determine the luminance that appeared equal in brightness to the standard patch. The luminance of the standard stimulus was in the mesopic range (0.5 cd/m2), and the levels of glare were 15 and 60 lx. Results showed that the time course of brightness reduction followed the typical shape attributed to the Broca-Sulzer effect. Data were fitted with a model that first considers the response of a peripheral ganglion cell to glare and then its inhibitory effect on the test signals. We discussed the plausibility of a postretinal stage of processing.     

Facilitation of shape-from-shading perception by random textures

Most natural objects have a texture on their surface, so the segregation between shading and texture is crucial for the robust perception of three-dimensional structure: The visual system has to decide whether shading or texture evoked the luminance change. We found that the contextual pop-out that results from shading was not suppressed, but was even facilitated, when random texture was added to the luminance of the entire stimulus, indicating the functional segregation and facilitative interaction between shading and texture cues. The local contrast evoked by random texture within a figure or at a boundary was a major factor in the facilitation, suggesting the crucial role of early vision in the interaction between the cues.     

Estimating illuminant color based on luminance balance of surfaces

To accomplish color constancy the illuminant color needs to be discounted from the light reflected from surfaces. Some strategies for discounting the illuminant color use statistics of luminance and chromaticity distribution in natural scenes. In this study we showed whether color constancy exploits the potential cue that was provided by the luminance balance of differently colored surfaces. In our experiments we used six colors: bright and dim red, green, and blue, as surrounding stimulus colors. In most cases, bright colors were set to be optimal colors. They were arranged among 60 hexagonal elements in close-packed structure. The center element served as the test stimulus. The observer adjusted the chromaticity of the test stimulus to obtain a perceptually achromatic surface. We used simulated black body radiations of 3000 (or 4000), 6500, and 20000 K as test illuminants. The results showed that the luminance balance of surfaces with no chromaticity shift had clear effects on the observer's achromatic setting, which was consistent with our hypothesis on estimating the scene illuminant based on optimal colors.     

Impulse response of an S-cone pathway in the aging visual system

Age-related changes in the temporal properties of an S-cone pathway were characterized by the psychophysical impulse-response function (IRF). Participants included 49 color-normal observers ranging in age from 16.8 to 86.3 years. A double-pulse method was used to measure the IRF with S-cone modulation at constant luminance. Stimuli were presented as a Gaussian patch (+/-1SD = 2.3 degrees ) in one of four quadrants around a central fixation cross on a CRT screen. The test stimulus was modulated from the equal-energy white of the background toward the short-wave spectrum locus. Each of the two pulses (6.67 ms) was separated by an interstimulus interval (ISI) from 20 to 720 ms. Chromatic detection thresholds were determined by a four-alternative forced-choice method with staircases for each ISI in one session. IRFs were calculated from the threshold data using a model with four parameters of an exponentially damped sine wave. S-cone IRFs have only an excitatory phase and a much longer time course compared with IRFs for luminance modulation measured with the same apparatus. The results demonstrated significant age-related losses in IRF amplitude, but the latency (time to peak) of the IRF was stable with age.     

Visual adjustments to temporal blur

After observers have adapted to an edge that is spatially blurred or sharpened, a focused edge appears too sharp or blurred, respectively. These adjustments to blur may play an important role in calibrating spatial sensitivity. We examined whether similar adjustments influence the perception of temporal edges, by measuring the appearance of a step change in the luminance of a uniform field after adapting to blurred or sharpened transitions. Stimuli were square-wave alternations (at 1 to 8 Hz) filtered by changing the slope of the amplitude spectrum. A two-alternative-forced-choice task was used to adjust the slope until it appeared as a step change, or until it matched the perceived transitions in a reference stimulus. Observers could accurately set the waveform to a square wave, but only at the slower alternation rates. However, these settings were strongly biased by prior adaptation to filtered stimuli, or when the stimuli were viewed within temporally filtered surrounds. Control experiments suggest that the latter induction effects result directly from the temporal blur and are not simply a consequence of brightness induction in the fields. These results suggest that adaptation and induction adjust visual coding so that images are focused not only in space but also in time.     

Luminace, not brightness, determines temporal brightness enhancement with chromatic stimuli.

Brightness-duration relations for chromatic stimuli were studied using three pulse-to-background luminace relations: chromatic equal-luminance pulses (3.2 cd/m2) were presented as increments of 0.3 or 1.0 log units above a lower luminance achromatic background, or were presented in hue substitution, equated in luminance to the achromatic background, so that no spatio-temporal luminance transients occurred during stimulus presentation. Incremental pulses produced temporal brightness enhancement (the Broca-Sulzer phenomenon), but hue substitution pulses did not. Temporal brightness enhancement thus depends upon the occurrence of luminance transients and cannot be produced by pulsed-to-background brightness differences associated solely with chromaticity differences.     

Three-dimensional spatial grouping affects estimates of the illuminant

The brightnesses (i.e., perceived luminance) of surfaces within a three-dimensional scene are contingent on both the luminances and the spatial arrangement of the surfaces. Observers viewed a CRT through a haploscope that presented simulated achromatic surfaces in three dimensions. They set a test patch to be approximately 33% more intense than a comparison patch to match the comparison patch in brightness, which is consistent with viewing a real scene with a simple lightning interpretation from which to estimate a different level of illumination in each depth plane. Randomly positioning each surface in either depth plane minimized any simple lighting interpretation, concomitantly reducing brightness differences to approximately 8.5%, although the immediate surrounds of the test and comparison patches continued to differ by a 5:1 luminance ratio.     

Frequency and phase contributions to the detection of temporal luminance modulation

Observers detected a temporally modulated luminance pattern embedded in dynamic noise. A Gabor function with a carrier frequency, in separate conditions of 0, 1.56, or 3.12 Hz, modulated signal contrast. Classification images were constructed in the time, temporal frequency, and temporal phase domains. As stimulus frequency increased, amplitudes of the phase images decreased and amplitudes of the frequency images increased, indicating a corresponding shift in the observers' criteria. The reduced use of phase attenuated time-domain images from signal-absent trials, but physical interactions between signal and noise components tended to preserve time-domain images from signal-present trials. The results illustrate a frequency-dependent strategy shift in detection that may reflect a degree of stimulus uncertainty in the time domain.     

Upper-limit luminance for the surface-color mode appearance

A series of experiments were carried out to reveal determinants for the mode of color appearance by measuring the upper-limit luminance of a color chip for the surface-color mode. We used a CRT color monitor to present test and surround stimuli in the surface-color mode. The stimuli were composed of a three-by-three array of color chips on a gray background with a white frame. The observer increased the luminance of a center test color until it just ceased to appear in the surface-color mode. Our results show that this upper-limit luminance was different among test colors, but their brightnesses, calculated from the luminance and brightness/ luminance values, were almost the same and were slightly below the brightness of the white frame. The existence of the surrounding color chips affected the results, but their sizes and spatial arrangements did not. When all of the luminances of the surrounds changed equally, the upper-limit luminances of the test colors for the surface-color mode appearance changed by the same ratio. This result indicates that the brightness of a target was a determinant for selecting the mode of color appearance and that the brightest surround stimulus acted as a cue for determining the judgment.     

Adaptation of spatiotemporal mechanisms by increment and decrement stimuli

Sawtooth modulation has been used in the past to examine visual sensitivity to luminance increments and decrements. The threshold elevation caused by adaptation depends on the spatial profile of the stimulus field and the polarities of the adaptation and test stimuli. We hypothesized that the adaptation effects reflect a change in the sensitivity of the spatiotemporal channels that detect the stimuli. We used a 2-deg disk centered in a larger surround field. Five levels of contrast between the test field and surround were investigated: equiluminant, three intermediate levels, and dark. At each contrast, observers adapted for 5 s to 2-Hz sawtooth modulation (rapid-on or rapid-off). Immediately after adaptation, thresholds were measured for detection of a single cycle of either a rapid-on or a rapid-off waveform. Varying the contrast of the surround affected observers' sensitivity to the polarity of the sawtooth stimulus to the extent that the pattern of sensitivity with the equiluminant surround was the opposite of that with the dark surround. To examine temporal factors, we measured thresholds for slow (500-ms ramps) and fast (8.3-ms pulses) test stimuli. The adaptation effect was preserved with the ramp stimuli but not with the pulse stimuli. Blurring the edge between the test and surround fields in the equiluminant surround condition raised thresholds for all sawtooth test stimuli, suggesting that spatiotemporal channels sensitive to high spatial frequencies and low temporal frequencies facilitate detection in that condition. These findings suggest that adaptation to sawtooth modulation can differentially effect the sensitivity of ON and OFF pathways, but the relative desensitization of each pathway depends on an interaction with the adaptation state of spatiotemporal channels that are involved in detection.     

Processing of second-order motion stimuli in primate middle temporal area and medial superior temporal area

Two rhesus monkeys were subjects in a direction-discrimination task involving moving stimuli defined by either first- or second-order motion. Two different second-order motion stimuli were used: drift-balanced motion consisting of a rectangular field of stationary dots and theta motion consisting of the same rectangular field with dots moving in the direction opposite to that of the object. The two types of stimuli involved different segmentation cues between the moving object and the background: temporal structure of the luminance (flicker) in the case of drift-balanced motion and opposed motion in the case of the theta-motion stimulus. Our monkeys were able to correctly report the direction of each stimulus. Single-unit recordings from the middle temporal (MT) and medial superior temporal (MST) areas revealed that 16 out of 38 neurons (41%) from area MT and 34 out of 68 neurons (50%) from area MST responded in a directionally selective manner to the drift-balanced stimulus. The movement of an object defined by theta motion is not explicitly encoded in the neuronal activity in areas MT or MST. Our results do not support the hypothesis that the neuronal activity in these areas codes for the direction of stimulus movement independent of specific stimulus parameters. Furthermore, our results emphasize the relevance of different segmentation cues between figure and background. Therefore the notion that there are multiple sites responsible for the processing of second-order motion is strongly supported.     

Psychophysical estimation of speed discrimination. II. Aging effects

We studied the effects of aging on a speed discrimination task using a pair of first-order drifting luminance gratings. Two reference speeds of 2 and 8 deg/s were presented at stimulus durations of 500 ms and 1000 ms. The choice of stimulus parameters, etc., was determined in preliminary experiments and described in Part I. Thresholds were estimated using a two-alternative-forced-choice staircase methodology. Data were collected from 16 younger subjects (mean age 24 years) and 17 older subjects (mean age 71 years). Results showed that thresholds for speed discrimination were higher for the older age group. This was especially true at stimulus duration of 500 ms for both slower and faster speeds. This could be attributed to differences in temporal integration of speed with age. Visual acuity and contrast sensitivity were not statistically observed to mediate age differences in the speed discrimination thresholds. Gender differences were observed in the older age group, with older women having higher thresholds.     

Response of visual mechanisms to stimulus onsets and offsets

Transient and sustained visual mechanisms were studied with single, flickering bars of various widths. Wide bars were largely detected on the basis of temporal luminance transients whereas thin bars were detected on the basis of the sustained contrast. A rapidly flickering uniform field selectively masked wide flickering bars, which suggests that different mechanisms detect wide versus thin flickering bars. For coarse spatial patterns, stimulus onsets were slightly more visible than stimulus offsets, and the response to onsets and offsets approximately summated.     

Effects of luminance and stimulus distance on accommodation and visual resolution.

Measurements of accommodation and visual resolution were obtained at a number of luminance levels and stimulus distances. With reductions in luminance the eye approached a "fixed-focus" condition of accommodation for intermediate distances, resulting in successively larger errors in accommodation for both near and far stimuli. The visual resolution values were initially affected by both the luminance and distance of the stimulus. Subsequent measurements of resolution, following the correction of accommodative errors, were found to be independent of the stimulus distance. The findings are discussed with regard to the problems of "night myopia" and variations in visual resolution with stimulus distance.     

直接亮度法测量逆反射亮度系数的研究

采用直接亮度法针对逆反射亮度系数测量展开研究。搭建了测试系统,以逆反射标志为测试对象进行了实验测试,并将直接亮度法与直接发光强度法进行了对比。实验结果表明,直接亮度法在暗室杂散光要求、测试对象、测试距离以及结果的准确度等方面均比直接发光强度法有明显优势,而且测试过程简单,结果不确定度为3.27%(k=1.96)。     

Increment thresholds with various low background intensities at different locations in the peripheral retina

A set of increment threshold data as a function of test-flash diameter, background luminance, and retinal eccentricity is presented. It is shown that for low background intensities the results can readily be described by simple transformations of flash diameter and background luminance: The threshold is independent of eccentricity if the quotient of diameter and eccentricity is constant and if the flash is presented on a background for which the product of background luminance and the square of eccentricity is constant. At an eccentricity of 50 deg, Ricco's law is violated: A small stimulus has a threshold 10 times as high as a large stimulus. On the basis of results found by other investigators for smaller eccentricities, it is concluded that the receptive field size at 50 deg of eccentricity is more than 10 deg (for low background luminances). For eccentricities smaller than 50 deg, a data analysis is given in order to derive an appropriate measure of the size of the sample units. This analysis shows that with increasing background luminance the decrease in the size of the sample unit is steplike rather than gradual.     

Heterochromatic Brightness Match Variability and Transient Adaptation

The discrepancy between the relative brightness and the relative luminance of two lights, one much more highly saturated than the other, is found to decrease markedly over a period of 2-3 min of steady fixation. This finding suggests temporal differences in the adaptive characteristics of chromatic and achromatic visual channels. When the eye has become adapted to the test field, the brightness-luminance discrepancy can be revived to some extent by introducing modest perturbations in the luminance of the relatively unsaturated light. Failure to control the temporal variable could be a major source of the differences, both from subject to subject and within the same subject, often found in heterochromatic brightness matching.     

Luminance spatial scale facilitates stereoscopic depth segmentation

Are differences in luminance spatial frequency between surfaces that overlap in depth useful for surface segmentation? We examined this question, using a novel stimulus termed a dual-surface disparity grating. The dual-surface grating was made from Gabor micropatterns and consisted of two superimposed sinusoidal disparity gratings of identical disparity-modulation spatial frequency and orientation but of opposite spatial phase. Corrugation amplitude thresholds for discrimination of the orientation of the dual-surface grating were obtained as a function of the difference in Gabor (luminance) spatial frequency between the two surfaces. When the Gabor micropatterns on the two surfaces were identical in spatial frequency, thresholds were very high and in some instances impossible to obtain. However, with as little as a 1-octave difference in spatial frequency between the surfaces, thresholds fell sharply to near-asymptotic levels. The fall in thresholds paralleled a change in the appearance of the stimulus from one of irregular depth to stereo transparency. The most parsimonious explanation for this finding is that the introduction of a between-surface luminance spatial-frequency difference reduces the number of spurious cross-surface binocular matches, thus helping to reveal the three-dimensional structure of the stimulus.