Simple model for spatial-frequency masking and contrast discrimination

1995 ◽  
Author(s):  
Peter G. J. Barten
Keyword(s):  
Simple Model ◽  
Spatial Frequency ◽  
Contrast Discrimination

Linear Acceleration Modifies the Perceived Velocity of a Moving Visual Scene

Perception ◽  
1977 ◽  
Vol 6 (5) ◽  
pp. 529-540 ◽  
Author(s):  
Bernard Pavard ◽  
Alain Berthoz
Keyword(s):  
Simple Model ◽  
Spatial Frequency ◽  
Visual Stimulus ◽  
Linear Acceleration ◽  
Vestibular Stimulation ◽  
Visual Scene ◽  
Image Velocity ◽  
Image Movement ◽  
Velocity Perception ◽  
Perceived Velocity

In the present work, we have shown the effect of a vestibular stimulation on the velocity perception of a moving scene. The intensity of this effect is related to the amplitude of the cart acceleration, image velocity, spatial frequency of the visual stimulus, and the angle between the directions of cart and image movement. A simple model has been developed to determine whether the perception of visual movement is due to the geometric projection of the vestibular evaluation on the visual vector, or the inverse.

Alcohol does not affect visual contrast gain mechanisms

1999 ◽  
Vol 16 (4) ◽  
pp. 675-680 ◽  
Author(s):  
PAULINE PEARSON ◽  
BRIAN TIMNEY
Keyword(s):  
Spatial Frequency ◽  
Detection Threshold ◽  
Discrimination Performance ◽  
Detection Thresholds ◽  
Contrast Gain ◽  
Visual Contrast ◽  
Increment Threshold ◽  
Spatial Frequencies ◽  
Contrast Discrimination ◽  
Induced Changes

It has been suggested that acetylcholine plays a role in contrast discrimination performance and the regulation of visual contrast gain (Smith, 1996). Since alcohol has been shown to reduce levels of acetylcholine and contrast sensitivity, the present study measured the effects of alcohol on contrast discrimination and explored whether the deficits could be explained as a consequence of reduction in contrast gain. Detection thresholds and contrast increment thresholds under placebo and alcohol (0.06% BAC) conditions were measured in six volunteers. Alcohol was found to impair both detection and discrimination of only high spatial frequencies. However, when the base contrasts used in the increment threshold task were equal multiples of detection threshold, no alcohol-induced changes in increment thresholds were obtained at any spatial frequency. We conclude that alcohol impairs contrast discrimination performance but that no change in contrast gain mechanisms need be postulated to account for the data.

Noisy Template Matching: A Model for Detection and Discrimination

Perception ◽  
1997 ◽  
Vol 26 (1_suppl) ◽  
pp. 263-263 ◽  
Author(s):  
W McIlhagga ◽  
A Pääkkönen
Keyword(s):  
Simple Model ◽  
Spatial Frequency ◽  
Template Matching ◽  
Additive Noise ◽  
Frequency Tuning ◽  
Central Place ◽  
Orientation Tuning ◽  
Decision Variable ◽  
Backward Masking ◽  
Spatial Frequency Tuning

The detection and discrimination of simple patterns occupies a central place in visual psychophysics. A wide variety of phenomena have been observed in this paradigm, such as: Weber's law; masking (simultaneous, forward, and backward); masking by noise; spatial frequency tuning; orientation tuning; and area summation. We suggest that many of these phenomena can be explained by a simple model which we call ‘noisy template matching’. In this model, the encoded stimulus is matched to a memorised template. Both stimulus and template are corrupted by additive noise. The template matching operation yields a decision variable, to which more noise is added. This model is very simple, but it has many interesting consequences. It provides qualitative explanations for many of the phenomena mentioned above, and with additional (but we think reasonable) assumptions about lens blur, contrast nonlinearity (Whittle, 1986 Vision Research26 1677 – 1691), uncertainty (Pelli, 1985 Journal of the Optical Society of America2 1508 – 1532), and suboptimal templates, the model also provides good quantitative accounts of these phenomena.

Parallel Independent Encoding of Orientation, Spatial Frequency, and Contrast

Perception ◽  
1995 ◽  
Vol 24 (5) ◽  
pp. 491-499 ◽  
Author(s):  
Alex Vincent ◽  
David Regan
Keyword(s):  
Spatial Frequency ◽  
Random Order ◽  
Frequency Discrimination ◽  
Discrimination Threshold ◽  
Orientation Discrimination ◽  
Stimulus Set ◽  
Response Set ◽  
Opponent Processing ◽  
The Mean ◽  
Contrast Discrimination

Subjects were presented with a set of 216 test gratings in random order. Each had a different combination of orientation, spatial frequency, and contrast. For each test grating, subjects were instructed to judge whether or not orientation was clockwise of the mean of the stimulus set, whether or not spatial frequency was higher than the mean of the stimulus set, and whether or not contrast was higher than the mean of the stimulus set. Each of the three sets of button presses was analyzed with respect to each of the three parameters, giving nine psychometric functions from one response set. It is concluded that, for gratings of high visibility, changes of orientation, spatial frequency, and contrast are encoded independently and in parallel, at least for small changes in these three visual parameters. In another experiment only one of the three parameters was varied at a time. Neither orientation-discrimination threshold, nor spatial-frequency-discrimination threshold, nor contrast-discrimination threshold was appreciably, if at all, lower than when all three parameters were varied simultaneously. It is concluded that interactions between the processing of small changes in orientation, spatial frequency, and contrast are negligible when all three are processed simultaneously. It is proposed that trial-to-trial variations of orientation, spatial frequency, and contrast are unconfounded by opponent processing within a population of neurons, each of which confounds the three variables.

Contrast discrimination: A model and a hypothesis concerning the role of cholinergic modulation in contrast perception

1996 ◽  
Vol 13 (5) ◽  
pp. 873-884 ◽  
Author(s):  
Andrew T. Smith
Keyword(s):  
Spatial Frequency ◽  
Cholinergic Neurons ◽  
Discrimination Performance ◽  
Anticholinergic Drugs ◽  
Cholinergic Modulation ◽  
Contrast Gain ◽  
Neural Noise ◽  
Spatial Frequencies ◽  
Contrast Perception ◽  
Contrast Discrimination

AbstractA model of contrast discrimination performance in human observers is developed and then extended to cover effects on performance of anticholinergic drugs. It is shown that it is necessary to assume that neural noise increases at high spatial frequencies in order to provide a satisfactory model of variations in discrimination performance with spatial frequency. The model results are compared with the results of empirical studies in which the effects of the muscarinic antagonist scopolamine (hyoscine) on contrast discrimination performance in human observers are examined. The purpose of the pharmacological work is to test the hypothesis that the differential contrast gain found psychophysically at different spatial frequencies might reflect differential facilitation by extrinsic cholinergic neurons. Contrast discrimination and contrast increment detection are found to be impaired by scopolamine in a manner that depends on both spatial frequency and base contrast. By comparing the empirical data with the predictions of the model, it is concluded that contrast constancy may reflect differential cholinergic modulation.

Determination of the Best Emulsion for the Microscopy of Crystals

1981 ◽  
Vol 39 ◽  
pp. 32-33
Author(s):  
David A. Grano ◽  
Kenneth H. Downing
Keyword(s):  
Spatial Frequency ◽  
Information Transfer ◽  
Signal To Noise Ratio ◽  
Experimental Situation ◽  
Photographic Emulsion ◽  
Modulation Transfer ◽  
Signal To Noise ◽  
Frequency Space ◽  
Noise Ratio

The retrieval of high-resolution information from images of biological crystals depends, in part, on the use of the correct photographic emulsion. We have been investigating the information transfer properties of twelve emulsions with a view toward 1) characterizing the emulsions by a few, measurable quantities, and 2) identifying the “best” emulsion of those we have studied for use in any given experimental situation. Because our interests lie in the examination of crystalline specimens, we've chosen to evaluate an emulsion's signal-to-noise ratio (SNR) as a function of spatial frequency and use this as our critereon for determining the best emulsion.The signal-to-noise ratio in frequency space depends on several factors. First, the signal depends on the speed of the emulsion and its modulation transfer function (MTF). By procedures outlined in, MTF's have been found for all the emulsions tested and can be fit by an analytic expression 1/(1+(S/S0)2). Figure 1 shows the experimental data and fitted curve for an emulsion with a better than average MTF. A single parameter, the spatial frequency at which the transfer falls to 50% (S0), characterizes this curve.

Density-based discrimination of protein and RNA in the ribosome

1992 ◽  
Vol 50 (1) ◽  
pp. 464-465
Author(s):  
Joachim Frank
Keyword(s):  
Transfer Function ◽  
Spatial Frequency ◽  
Three Dimensional ◽  
Wiener Filter ◽  
Dark Field Image ◽  
Dark Field ◽  
Frequency Range ◽  
Bright Field ◽  
Contrast Transfer Function ◽  
Pass Filter

Cryo-electron microscopy combined with single-particle reconstruction techniques has allowed us to form a three-dimensional image of the Escherichia coli ribosome.In the interior, we observe strong density variations which may be attributed to the difference in scattering density between ribosomal RNA (rRNA) and protein. This identification can only be tentative, and lacks quantitation at this stage, because of the nature of image formation by bright field phase contrast. Apart from limiting the resolution, the contrast transfer function acts as a high-pass filter which produces edge enhancement effects that can explain at least part of the observed variations. As a step toward a more quantitative analysis, it is necessary to correct the transfer function in the low-spatial-frequency range. Unfortunately, it is in that range where Fourier components unrelated to elastic bright-field imaging are found, and a Wiener-filter type restoration would lead to incorrect results. Depending upon the thickness of the ice layer, a varying contribution to the Fourier components in the low-spatial-frequency range originates from an “inelastic dark field” image. The only prospect to obtain quantitatively interpretable images (i.e., which would allow discrimination between rRNA and protein by application of a density threshold set to the average RNA scattering density may therefore lie in the use of energy-filtering microscopes.

Three-fold astigmatism: An important TEM aberration

1993 ◽  
Vol 51 ◽  
pp. 972-973 ◽  
Author(s):  
O.L. Krivanek ◽  
M.L. Leber
Keyword(s):  
High Resolution ◽  
Spatial Frequency ◽  
Field Emission ◽  
Azimuthal Angle ◽  
Hollow Cone ◽  
Small Probe ◽  
Wide Range ◽  
High Resolution Images ◽  
Image Position

Three-fold astigmatism resembles regular astigmatism, but it has 3-fold rather than 2-fold symmetry. Its contribution to the aberration function χ(q) can be written as:where A3 is the coefficient of 3-fold astigmatism, λ is the electron wavelength, q is the spatial frequency, ϕ the azimuthal angle (ϕ = tan-1 (qy/qx)), and ϕ3 the direction of the astigmatism.Three-fold astigmatism is responsible for the “star of Mercedes” aberration figure that one obtains from intermediate lenses once their two-fold astigmatism has been corrected. Its effects have been observed when the beam is tilted in a hollow cone over a wide range of angles, and there is evidence for it in high resolution images of a small probe obtained in a field emission gun TEM/STEM instrument. It was also expected to be a major aberration in sextupole-based Cs correctors, and ways were being developed for dealing with it on Cs-corrected STEMs.

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