Unequal Contrast Research Articles

«...Удивительный тип, точно Патфайндер Купера: что-то такое в нем непосредственное!»: роман Дж. Ф. Купера «Следопыт» в творческом восприятии И. С. Тургенева

This paper considers Ivan Turgenev’s perception of James Cooper’s works. The so-called sea trilogy should have made an especially deep impression on the Russian writer. The poetics of Cooper’s seascape novels can be clearly seen in Turgenev’s images of the sea element. The Russian writer used the texture or contours of Cooper’s seascapes, creating an expressive image. Turgenev’s impressions of Cooper’s novel “The Pathfinder” were embodied in his story “A Trip to Polesie.” It begins with a picture of a mighty and silent pine forest, with description immersed in the hero’s reflection. The images of nature and man are given in a sharp and unequal contrast. To express the full scale of Polesie’s vastness, a meaningful comparison to the sea is given. This dramatic conversation between nature and man in Turgenev’s work is directly correlated with the reflections of the narrator in Cooper’s exposition of the novel. However, Turgenev contrasts the arising motif of loneliness and despair embracing the self-absorbed consciousness of the narrator with another epic motif in the same woodland slums. These are two extraordinary peasant faces: a simple and sedate hunter-guide Yegor and a dashing and unbridled rogue Ephraim. Both bear a non-coincidental resemblance to the characters of Natty Bumppo and Chingachgook from “The Pathfinder”. Turgenev’s heroes are similar to Cooper’s ones primarily due to being conceptualized in a strong connection with nature, the element of the ancient forest. In both cases, the life of the main characters is in a decisive dependence on the “green ocean.”

A three-response task reveals how attention alters decision criteria but not appearance.

Whether attention alters appearance or just changes decision criteria continues to be controversial. When subjects are forced to choose which of two equal targets, one of which has been pre-cued, has a higher contrast, they tend to choose the cued target. This has been interpreted as attention increasing the apparent contrast of the cued target. However, when subjects must decide whether the two targets have equal or unequal contrast, they respond veridically with no apparent effect of attention. The discrepancy between these comparative and equality judgments is explained by attention altering the decision criteria but not appearance. We supposed that when subjects are forced to choose which of two apparently equal targets has the higher contrast, they tend to proportion their uncertainty in favor of the cued target. To test this hypothesis, we used a three-response task, in which subjects chose which target had the higher contrast but also had the option to report that the targets appeared equal. This task disentangled potential attention effects on appearance from those on the decision criteria. We found that subjects with narrower criteria about what constituted equal contrast were more likely to choose the cued target, supporting the uncertainty stealing hypothesis. Across the population, the effects of the attentional cue are explained as changes in the decision criteria and not changes in appearance.

Determining the Variations of Ca–K Index and Features Using Century-long Equal-contrast Images from Kodaikanal Observatory

Abstract In an earlier analysis of Ca–K spectroheliograms obtained at the Kodaikanal Observatory, the “Good” images were used to investigate variations in the chromosphere. However, the contrast of the images varied on a day-to-day basis. We have developed a new methodology to generate images to form a uniform time series. We adjusted each image’s contrast until the FWHM of the normalized intensity distribution attained a value between 0.10 and 0.11. This “equal-contrast technique” is expected to compensate for the change of emulsion, development, and contrast of the images due to centering of the Ca–K line on the exit slit and sky transparency. In addition, this procedure will correct variations in density-to-intensity conversion for different images. We find that the correlation between sunspot and Ca–K line data improves to a large extent. For example, the correlation coefficient (CC) between monthly averaged sunspots and Ca–K plage areas for the equal-contrast data reaches 0.9 compared to 0.75 for the “Good” data with unequal contrast. The CC for equal-contrast images improves to ∼0.78 from ∼0.46 for the “Okay” data with unequal contrast. Even the CC between the plage area and the daily sunspot number is 0.85 for 100 years of data. This methodology also permits us, for the first time, to study the variations in enhanced, active, and quiet networks with time with high accuracy over about a century. Further, this procedure can be used to combine data from different observatories to make a long time series.

Effects of stimulus contrast and temporal delays in saccadic distraction

In a recent study, we observed that saccadic distraction (i.e., the remote distractor effect, RDE) was reduced when target and distractor were displayed at unequal contrast [Born, S., & Kerzel, D. (2008). Influence of target and distractor contrast on the remote distractor effect. Vision Research, 48(28), 2805–2816]. We hypothesized that arrival times explain the RDE modulation: With equal contrast, target and distractor signals arrive simultaneously in the oculomotor system so that mutual inhibition (and therefore saccadic distraction) is largest. With unequal contrast, high-contrast signals arrive earlier than low-contrast signals, resulting in less mutual inhibition and little saccadic distraction. In the current contribution, we presented target and distractor at different stimulus onset asynchronies (SOA) to re-align arrival times with unequal contrast. Results confirmed that unequal contrast of target and distractor reduced saccadic distraction with simultaneous presentation and that strong distraction could be reestablished by introducing a SOA. However, maximal saccadic distraction also varied strongly with the specific combination of target and distractor contrast. Thus, contrast may not only modulate arrival times of target and distractor signals, but also their strength in the mutual inhibition process. Finally, we found more saccadic distraction when the distractor was presented slightly after the target. A second experiment suggests that alerting effects superimposed on the distraction contribute to this effect, but may not fully explain it. We suggest that distraction may be strongest when the rise-to-threshold of the target-related signal has already advanced.

Perceived direction of plaid motion is not predicted by component speeds

It has been shown that the perceived direction of a plaid with components of unequal contrast is biased towards the direction of the higher-contrast component [Stone, L. S., Watson, A. B., & Mulligan, J. B. (1990). Effect of contrast on the perceived direction of a moving plaid. Vision Research 30, 1049–1067]. It was proposed that this effect is due to the influence of contrast on the perceived speed of the plaid components. This led to the conclusion that perceived plaid direction is computed by the intersection of constraints (IOC) of the perceived speed of the components rather than their physical speeds. We tested this proposal at a wider range of component speeds (2–16 deg/s) than used previously, across which the effect of contrast on perceived speed is seen to reverse. We find that across this range, perceived plaid direction cannot be predicted either by a model which takes the IOC of physical or perceived component speed. Our results are consistent with an explanation of 2D motion perception proposed by [Bowns, L. (1996). Evidence for a feature tracking explanation of why Type II plaids move in the vector sum direction at short durations. Vision Research, 36, 3685–3694.] in which the motion of the zero-crossing edges of the features in the stimulus contribute to the perceived direction of motion.

Cs-corrected HAADF-STEM imaging of silicate minerals

Significant improvement of the resolution for electron microscopy by the development of the spherical aberration corrector (Cs-corrector) brings valuable information to understand local atomic structures in silicate minerals. Cation columns separated by ca. 1.5 A, which originate from the ionic radius of oxygen anion or half of the closest oxygen-oxygen distance, are common in silicates and they are easily resolved by a Cs-corrected transmission electron microscope (TEM) or scanning TEM (STEM) with a resolution close to 1 A. High-angle annular dark-field (HAADF) imaging using Cs-corrected STEM was applied to orthopyroxene (Opx) with augite lamellae, and cronstedtite, an iron-bearing sheet silicate. Noisy contrast in the HAADF images was compensated by advanced noise filtering techniques. All cation columns in the pyroxene structure were resolved in the HAADF images from the c-axis. A sub-angstrom difference in the position of the M2 site between Opx and augite, which is caused by the occupation of the site by Fe (Opx) and Ca (augite), was clearly detected in the HAADF image as well as different contrasts of the cation columns related to occupying elements. A pair of tetrahedral cation columns separated by ca. 1.5 A in cronstedtite observed along [100] directions frequently show unequal contrast, suggesting a difference of the amounts of substituted Fe(3+) between the two columns. Comparison between the experimental contrast with a simple simulation suggests the distribution of Fe(3+) in a tetrahedral sheet avoiding linkage of Fe(3+) coordinating tetrahedrons.

Attraction of flashes to moving dots

Motion is known to distort visual space, producing illusory mislocalizations for flashed objects. Previously, it has been shown that when a stationary bar is flashed in the proximity of a moving stimulus, the position of the flashed bar appears to be shifted in the direction of nearby motion. A model consisting of predictive projections from the sub-system that processes motion information onto the sub-system that processes position information can explain this illusory position shift of a stationary flashed bar in the direction of motion. Based on this model of motion–position interactions, we predict that the perceived position of a flashed stimulus should also be attracted towards a nearby moving stimulus. In the first experiment, observers judged the perceived vertical position of a flash with respect to two horizontally moving dots of unequal contrast. The results of this experiment were in agreement with our prediction of attraction towards the high contrast dot. We obtained similar findings when the moving dots were replaced by drifting gratings of unequal contrast. In control experiments, we found that neither attention nor eye movements can account for this illusion. We propose that the visual system uses predictive influences from the motion processing sub-system on the position processing sub-system to overcome the temporal limitations of the position processing system.

Misperceptions of speed for chromatic and luminance grating stimuli

Errors in the perception of speed of moving visual stimuli can occur when presented stimuli are of unequal contrast and when they appear alongside additional modifier stimuli that move at different speeds. We have examined these misperceptions for chromatic and luminance grating stimuli in order to assess to what extent these different kinds of motion cue might be utilised in the analysis of speed of moving objects. We show that the dependence on contrast of speed matching for luminance and chromatic stimuli is similar over a range of stimulus speeds greater than 4 deg/s. Differences between the contrast dependencies of speed perception for chromatic and luminance stimuli are only evident at slow speeds (< 4 deg/s) and low contrasts. The presence of modifier stimuli can directly influence the perceived speed at both high and low velocities and contrasts. This influence was found to be independent of the modifiers’ chromaticity and was greatest when the modifiers were adjacent to and presented simultaneously with the test and reference stimuli. However, the modifiers were still able to induce measurable changes in perceived speed for increased separations over space and time. Taken together these results indicate that whilst differences do exist in the contrast dependencies of speed perception for chromatic and luminance stimuli, they are evident only for a narrow range of stimulus parameters (i.e. low speed and low contrast). There appears to be ample scope for interactions between chromatic and luminance contrast in speed perception where there is the capacity to pool this information over a relatively broad spatio-temporal extent.

Illusory contrast-induced shifts in binocular visual direction bias saccadic eye movements toward the perceived target position

The perceived binocular visual direction of a fused disparity stimulus with an interocular contrast difference is biased toward the direction signaled by the eye presented with the higher contrast image (J. S. Mansfield & G. E. Legge, 1996). Does the amplitude of binocular saccadic eye movements have a similar bias? We examined saccades to fused disparate Gabor patches with interocular contrast differences. The effect of these contrast differences on saccadic amplitudes was compared to the perceptual biases in binocular direction obtained in a vernier acuity task. Saccades to unequal contrast targets landed between the end points for equal contrast and monocular targets. For three of our eight subjects, the saccadic bias equaled the perceptual effect. For the other subjects, however, saccades were affected to a lesser extent. Three models for binocular combination were used to evaluate these responses: A maximum-likelihood model failed to predict our results, whereas a model with contrast-dependent weighting of direction estimates by two monocular channels and a gain control model of binocular contrast summation gave a better approximation to our data. Both models showed that for the perceptual system, the influence of the eye that was presented with the higher contrast image was more dominant in the binocular combination than expected from the stimulus contrast ratio. The oculomotor system, however, was close to following linear summation.

Perceived direction of plaid motion is not predicted by component speeds

It has been shown that the perceived direction of a plaid with components of unequal contrast is biased towards the direction of the higher-contrast component [Stone, L. S., Watson, A. B., & Mulligan, J. B. (1990). Effect of contrast on the perceived direction of a moving plaid. Vision Research 30, 1049–1067]. It was proposed that this effect is due to the influence of contrast on the perceived speed of the plaid components. This led to the conclusion that perceived plaid direction is computed by the intersection of constraints (IOC) of the perceived speed of the components rather than their physical speeds. We tested this proposal at a wider range of component speeds (2–16 deg/s) than used previously, across which the effect of contrast on perceived speed is seen to reverse. We find that across this range, perceived plaid direction cannot be predicted either by a model which takes the IOC of physical or perceived component speed. Our results are consistent with an explanation of 2D motion perception proposed by [Bowns, L. (1996). Evidence for a feature tracking explanation of why Type II plaids move in the vector sum direction at short durations. Vision Research, 36, 3685–3694.] in which the motion of the zero-crossing edges of the features in the stimulus contribute to the perceived direction of motion.

Unequal contrast drives perceived direction past vector-sum direction in Type 2 plaids

Unequal contrast drives perceived direction past vector-sum direction in Type 2 plaids

Properties of the recombination of one-dimensional motion signals into a pattern motion signal

We have examined the human ability to determine the direction of movement of a variety of plaid patterns. The plaids were composed of two orthogonal sine-wave gratings. When the plaid components are of unequal spatial frequency or sometimes of unequal contrast, observers judge the direction of movement incorrectly. In terms of the two-stage model of Adelson and Movshon (1982), these errors may result from either a misjudgment in the perceived speeds of each of the components or a failure in the combination of one-dimensional component movements into a coherent direction of motion of the two-dimensional plaid pattern, or both. A comparison of the perceived direction of motion of plaids with the relative perceived speeds of the plaid component gratings suggest that both failures occur, but in different circumstances. The relative perceived speed of the plaid components was measured with a spatial and a temporal forced-choice technique, the former leading to larger differences. Our results support the notion that the visual system decomposes a moving plaid into oriented components and subsequently recombines the component motions.

Does contrast disparity alone generate stereopsis?

In two experiments, one using a nulling technique and the other forced-choice testing. observers judged the rotation in depth of dichoptically-viewed, vertical gratings of unequal contrast. In neither experiment was evidence found for a stereoscopic sensation of depth based on contrast disparity alone; spatial frequency disparity, on the other hand, produced a clear rotation of the fused percept about the vertical axis.