Local Contrast Changes Research Articles

Vernier and contrast discrimination in central and peripheral vision

The present paper asks whether Vernier offset discrimination is limited by the observer’s sensitivity to local contrast change in both central and peripheral vision. To answer this question we compared Vernier discrimination and contrast discrimination thresholds (specified in the same units) for a pair of narrow ribbons of cosine gratings. Because the ribbons are narrow, both the offset information (for Vernier discrimination) and the contrast information (for contrast discrimination) are highly localized. We found that when the stimuli are narrow ribbons, the local contrast cue is the limiting factor in Vernier discrimination. However, our results also show that integration of information along the length of the gratings (the ribbon width) is: (i) different for Vernier and contrast discrimination, and (ii) for Vernier discrimination the integration of information along the length of the gratings differs qualitatively in central and peripheral vision. For narrow ribbons, the peripheral ‘template’ for ribbon Vernier acuity is not as well matched to the stimulus (in two-dimensional spatial frequency space) as the foveal ‘template’.

Topographic Maps and Local Contrast Changes in Natural Images

We call natural image any photograph of an outdoor or indoor scene taken by a standard camera. We discuss the physical generation process of natural images as a combination of occlusions, transpare...

Discrimination of changes in the slopes of the amplitude spectra of natural images: band-limited contrast and psychometric functions.

Thresholds were measured for discriminating changes in the slopes of the amplitude spectra of stimuli derived from photographs of natural scenes and from random-luminance patterns. The variety and magnitudes of the thresholds could be explained by a model based on the discrimination of the changes in band-limited local contrast. Different spatial scales of local contrast (or different spatial-frequency bands of about 1 octave) were implicated for different reference spectral slopes; the model implicated a lower frequency-band for stimuli with shallower amplitude spectra. The implications of the model were tested experimentally by using stimuli in which the spectra were changed within restricted spatial-frequency bands. When the amplitude spectra of the test and reference stimuli differed only within the implicated frequency bands, thresholds were affected little. However, when the test and reference spectra differed at all frequencies except those in the implicated bands, thresholds were elevated markedly. The forms of the psychometric functions for the discrimination task were entirely compatible with the hypothesis that the task relies upon the ability to discriminate changes of contrast. The Weibull functions fitted to the data had slope parameters (beta) in the range 1 to 3, compatible with discrimination of low (but suprathreshold) contrasts.

On the elementary mechanism underlying secondary motion processing.

The movement of luminance-defined targets can be easily extracted by elementary motion detectors (EMDs) of the correlation type which often are referred to as Reichardt-detectors. In contrast to such 'primary motion', in 'secondary motion' the moving target is defined by more complex features, like changes in texture, flicker, or local contrast. Such stimulus attributes have to be extracted from the retinal intensity distribution by some nonlinear preprocessing, before they are fed into motion detectors. An intriguing case is the perception of the movement of the motion signal, as is present in theta motion, where an object moves in a different direction than the texture on its surface. A two-layer model of hierarchically organised EMDs has been postulated to account for such motion extraction. Other than for the first layer, the computational nature of the mechanism underlying motion processing in the second layer so far is a matter of speculation, and is therefore characterized here by means of computer simulations and psychophysical experiments. Random dot kinematograms were generated in which sinusoidally modulated vertical dot motion defined gratings, and coherence thresholds were measured for the direction discrimination of a horizontally travelling modulation function. This was done for a variety of spatial frequencies and speeds of the modulation sinusoid. Thresholds turn out to be lowest not for a particular speed, but for a fixed temporal frequency of the modulation function (about 1 cycle per second), when various combinations of fine and coarse, and fast and slow secondary gratings are tested. This result favours a correlation-type mechanism over a gradient-type scheme which should lead to a speed-optimum independent of spatial frequency.

Discrimination of changes in the second-order statistics of natural and synthetic images

It has been suggested that the second-order statistics of different natural images are all remarkably similar and that neurones and channels in the visual system may exploit this similarity. We have measured the ability of human observers to discriminate changes in these statistics using different natural and synthetic stimulus images and have found that the dependence of their discrimination thresholds upon the reference second-order statistics is similar in form, for both kinds of stimuli. However, there is some variety in the magnitudes of the thresholds for the natural stimulus images; in fact, the second-order statistics of different natural images are more diverse than previously suggested. The discrimination task can be modelled as the discrimination of changes in local contrast within restricted spatial frequency bands and is similar to the discrimination of blur.

Effect of pattern luminance profile on the pattern erg in man and pigeon

Pattern electroretinograms (PERGs) have been recorded in man and pigeon using phase reversing patterns. Studies were made in both species of the effect of grating spatial frequency, grating profile (sine or square) or the size of the squares of a checkerboard on the amplitude of the responses. The results obtained in the two species were similar. In general, the ratio of the amplitudes of the sine wave: square wave responses is that predicted by the Fourier fundamentals of the pattern, and is thus not determined by changes in local luminance or contrast as it is normally defined. Checks however gave larger responses than sine or square wave profile bars matched to the Fourier fundamental on the diagonal. The results are not explicable in terms of any single stimulus parameter and suggest that the PERG generator has a degree of center/surround organisation similar to retinal ganglion cells. It is however argued that, because of the known physiological properties, generators cannot be unequivocally localised by varying the stimulus or comparing retinal, cortical and perceptual response dynamics.

Specimen movement in electron-irradiated paraffin crystals — A model for initial beam damage

The importance of possible causes for blurring due to local specimen movement during high-resolution electron microscope imaging experiments on monolamellar paraffin crystals is evaluated in terms of existing data for the beam damage of such materials. For specimens held at liquid helium temperatures, neither unit cell expansion, nor molecular structural change, nor production of large defects are likely causes for this motion. Rather, electron diffraction results indicate that observed changes in local image contrast are due to specimen flattening during the initial damage process, probably involving a longitudinal molecular slippage similar to the motions responsible for production of screw dislocations.

The human visual evoked potential: Analysis of components due to elementary and complex aspects of form

The visual evoked potential (VEP) elicited by alternation between isodipole visual textures may be separated into an asymmetric and a symmetric part. The asymmetric part reflects processing of complex attributes of form. The spatial and temporal dependence of this response is used to evaluate models for the generation of this response. The symmetric part appears to reflect processing of local luminance and contrast changes. The relation of these components to the VEP elicited by contrast reversal, contrast modulation, and the windmill-dartboard stimulus is discussed.