RMS Contrast Research Articles

Learning letter identification in peripheral vision

Performance for a variety of visual tasks improves with practice. The purpose of this study was to determine the nature of the processes underlying perceptual learning of identifying letters in peripheral vision. To do so, we tracked changes in contrast thresholds for identifying single letters presented at 10° in the inferior visual field, over a period of six consecutive days. The letters (26 lowercase Times-Roman letters, subtending 1.7°) were embedded within static two-dimensional Gaussian luminance noise, with rms contrast ranging from 0% (no noise) to 20%. We also measured the observers’ response consistency using a double-pass method on days 1, 3 and 6, by testing two additional blocks on each of these days at luminance noise of 3% and 20%. These additional blocks were the exact replicates of the corresponding block at the same noise contrast that was tested on the same day. We analyzed our results using both the linear amplifier model (LAM) and the perceptual template model (PTM). Our results showed that following six days of training, the overall reduction (improvement across all noise levels) in contrast threshold for our seven observers averaged 21.6% (range: 17.2–31%). Despite fundamental differences between LAM and PTM, both models show that learning leads to an improvement of the perceptual template (filter) such that the template is more capable of extracting the crucial information from the signal. Results from both the PTM analysis and the double-pass experiment imply that the stimulus-dependent component of the internal noise does not change with learning.

Measurement of Spatial-Frequency and Amplitude Responses of a CCD-Matrix Image-Registration Device

A unique method for studying spatial-frequency and amplitude responses of an image-registration device on the basis of a CCD matrix is developed. A monochromatic light-intensity distribution arising from Fraunhofer diffraction on two equal slits is used as an input signal. A quantitative approach to a spatial resolution of the CCD matrix for registration of a periodic image is developed. The dependence of the rms contrast degree of the output signal on the spatial frequency of the image recorded is studied experimentally. The optical scheme developed and the method proposed for treatment of a digitized signal allow one to determine the amplitude response of the registration device over the whole surface of the CCD matrix and to determine the admissible level of the average intensity of the radiation registered.

A method for measuring the spatial-frequency characteristic of an image-recording device based on a CCD matrix

A method for studying the spatial-frequency characteristic of an image-recording device based on a CCD matrix is described. The foundation of the method consists in the formation of an input signal in the form of interference fringes with a gradually changing period. A quantitative criterion for the spatial resolution of a CCD matrix recording images with periodic structures is developed. The dependence of the rms contrast of the output signal on the spatial frequency of the recorded image is investigated.

Spatial-scale contribution to the detection of mirror symmetry in fractal noise

We investigated how the detection of mirror symmetry depends on the distribution of contrast energy across spatial scales. Stimuli consisted of vertically symmetric noise patterns with fractal power spectra defined by 1/f beta slopes (-2 < or = beta < or = 5). While overall rms contrast remained fixed at 25%, symmetry-detection thresholds were obtained by corrupting the signal with variable amounts of noise with identical spectral characteristics. A first experiment measured thresholds as a function of spectral slope, and performance was found to be substantially facilitated in images with power spectra that characterize natural scenes (1.2 < or = beta < or = 3.2). In a second experiment, symmetry was removed from randomly chosen octave bands and replaced by noise with the same spectral profile. Results revealed that only in images with 1/f2 spectra does performance decrease by constant amounts across all frequency bands. Together, the results imply that symmetry mechanisms extract equal amounts of information from constant-octave frequency bands but lack the ability to whiten stimuli whose spectral slopes differ from those of natural scenes. Results are qualitatively well predicted by a multichannel model that (1) relies on spatial filters with equal-volume point-spread functions and constant-octave frequency bandwidths and (2) restricts the computation of symmetry to spatial regions whose dimensions are proportional to the filters' spatial scale. These findings are also consistent with the notion that mechanisms that mediate the perception of form exploit the ability of early vision to reduce second-order redundancy in natural scenes.

Measurements of Solar Magnetic Element Motion from High‐Resolution Filtergrams

Solar photospheric flowfield properties on sub-05 scales are measured using local correlation tracking (LCT) and object tracking of bright points (MBPs: photospheric bright points associated with elements). The dataset is a 70 minute time series of cospatial and cotemporal G-band 4305 A and wideband 4686 A filtergrams obtained with the 50 cm Swedish Vacuum Solar Telescope on the island of La Palma, Spain. We examine a 29'' × 70'' field of view (FOV) near disk center and compare a 29'' × 29'' network subfield and a 27'' × 27'' apparently nonmagnetic quiet-Sun subfield. The mean time between frames is 23.75 s. Each frame is created by partitioned phase-diverse speckle restoration of three image pairs acquired rapidly in sequence. Angular resolution is ~04 or less in all frames of the restored data set. From LCT on a 04 grid with 083 FWHM apodization, we find the flow speeds to be Rayleigh distributed with a mode of 550 m s-1 and an average of 950 m s-1 in the network FOV; in the quiet FOV the modal speed is 700 m s-1 with a mean of 1100 m s-1. Within the network FOV, a magnetic defined by the loci of tracked MBPs exhibit even greater alteration: rms contrast of the region is 8% higher in the G band compared to areas outside, LCT speeds are reduced by a factor of 1.6, and the convective flow structures are smaller and much more chaotic. Mesogranulation is entirely absent in the region. The modal and mean speeds of 534 tracked MBPs are 300 m s-1 and 1470 m s-1, respectively. MBPs split and merge with mean times of 320 and 404 s respectively. The mean lifetime of MBPs in the data set is 9.33 minutes although approximately 5% of the objects can be tracked for the entire 70 minute duration of the time series.

Temporal Integration and the Masking Effect of Spatiotemporal Noise

In computer-generated spatiotemporal noise every stimulus frame contains a new static noise sample. The spectral density of white spatiotemporal noise is calculated by multiplying the squared rms contrast of noise by the product of the noise check area and the exposure duration of each noise check. When the exposure duration of each noise check is gradually increased, the spectral density of spatiotemporal noise increases, reaching its maximum when noise becomes static. In static spatial noise both stimulus and noise checks have the same duration. The signal-to-noise ratio is known to be constant at detection threshold. Detection thresholds should thus increase in proportion to the spectral density of spatiotemporal noise, which increases with the duration of the noise checks. We measured detection thresholds for stationary cosine gratings embedded in spatiotemporal noise. The exposure duration of the noise checks was increased from one frame duration to the total exposure duration of the stimulus grating. Noise was thus gradually transformed from spatiotemporal to static spatial noise. The contrast energy threshold increased in proportion to the spectral density of spatiotemporal noise up to a noise check duration found to be equal to the integration time for the stimulus grating without noise. After this, energy thresholds remained constant in spite of the increase in the spectral density of spatiotemporal noise. This suggests that the masking effect of spatiotemporal noise increases with the duration of noise checks up to the critical duration marking the saturation of the temporal integration of the signal.

Blur and contrast as pictorial depth cues.

Studies have shown that blur can act as a pictorial cue to depth perception. But blurring a stimulus reduces its contrast, and studies have also shown that contrast can act as a pictorial cue to depth perception. To determine whether blur and contrast have separate influences on depth perception, each variable was independently manipulated in two experiments. Observers reported depth alternations in a simple reversible figure. Both contrast and blur were found to influence depth perception, but blur had its greatest effect at moderate contrasts. When blurred and sharp stimuli were equated on either Michelson or RMS contrast, blur continued to affect depth perception. Hence blur can act as a depth cue independently of contrast. It is speculated that blur is effective as a pictorial cue because of its usual association with other depth cues, particularly in pictures and photographs.

Michelson contrast, RMS contrast and energy of various spatial stimuli at threshold

Contrast sensitivity was measured as a function of median spatial frequency for vertical cosine gratings, narrow-band noise stimuli, and spots with luminance increment or decrement. Contrast sensitivity was expressed in terms of Michelson contrast, RMS contrast, and contrast energy in order to demonstrate the characteristics of various contrast measures. Gratings and noise stimuli had either constant stimulus area or constant number of cycles. Michelson contrast sensitivity was better for gratings than for noise stimuli or spots, whereas RMS contrast sensitivity was almost equal for gratings and noise stimuli but lowest for spots. Contrast energy sensitivity takes into account the stimulus area and was therefore best for spots.

Influence of contrast on foveal and peripheral detection of coherent motion in moving random-dot patterns.

The detection of coherent motion was studied in stroboscopically displayed moving random-dot patterns disturbed by incoherent noise. We determined the threshold signal-to-noise ratio S as a function of velocity V at eccentricities of 0 degrees, 3 degrees, 6 degrees, 12 degrees, 24 degrees, 48 degrees in the temporal visual field of the right eye. At each eccentricity the measurements of S = f(V) were repeated for a range of rms contrast values from 60% (0 dB) in steps of 3 dB down to 1.9% (-30 dB). All stimuli were scaled with eccentricity to keep the ratio of pixel size to acuity constant (about 2). It is shown that the S values in our paradigm are never determined by contrast-threshold effects. They are true correlational thresholds. Bilocal movement detectors are assumed to underlie the detection of coherent motion. The bilocal correlation proves to be rather insensitive to rms contrast down to contrast levels of about 10%. Despite the eccentricity scaling, which is quite effective at high contrast levels, differences between the eccentricities become noticeable at lower contrast levels (below about 30-20%). The fovea is the least, and the far periphery the most, resistent to contrast degradation.

A new measurement of the center-to-limb variation of the rms granular contrast

The center-to-limb variation of the root-mean-square granular contrast at 5520 A is deduced from a set of high-spatial-resolution filtergrams obtained with the Sacramento Peak Observatory Vacuum Tower Telescope. The rms contrast is observed to decrease monotonically between μ = 1.0 and 0.6, and then increase slightly at μ = 0.4. This result is compared with the results of Edmonds (1962) and with the results of Pravdjuk et al. (1974).