Showing papers on "Contrast (vision) published in 1970"

The perceived spatial frequency shift: evidence for frequency-selective neurones in the human brain.

Abstract: 1. Prolonged observation of a high-contrast grating pattern causes an apparent shift in the spatial frequency of gratings subsequently viewed with the same retinal region. Gratings of higher and lower frequency than the adapting pattern seem, respectively, higher and lower than in fact they are. 2. There is no significant after-effect at the adapting frequency itself nor at frequencies more than two octaves away. 3. For very low adapting frequencies, the after-effect remains centred at about 3·0 c/deg and declines in strength as the adapting frequency is successively lowered. 4. The magnitude of the after-effect increases with the contrast of the adapting grating and the length of time spent in adaptation. It takes several hours to recover completely from 30 min adaptation. 5. The phenomenon is orientation-specific: a horizontal adapting grating has no effect on vertical test gratings. There is partial interocular transfer of the after-effect. 6. These findings provide further evidence that the visual system of man, like those of the cat and the monkey, contains neurones selectively sensitive to the orientation and dimensions of retinal images, and that these adaptable cells are actually involved in the encoding and perception of the size of simple patterns.

Model of visual adaptation and contrast detection

Abstract: A three-component model of spatial vision is proposed, consisting of (1) a feedback stage, (2) a feedforward stage, (3) a threshold detector. The components correspond to physiological processes; in particular, the feedforward control signal corresponds to the “surround’s” signal in the receptive fields of retinal ganglion cells. The model makes appropriate qualitative predictions of: (l)a square-root law (Δl ∞ l1/2) for detection at low luminances, (2) a Weber law (Δl ∞ l) at high luminances, (3) additivity of threshold masking effects at high background luminances, (4) receptive fields that, in the dark, consist only of an excitatory center and that, in the light, also contain inhibitory surrounds, (5) the variation of spatial characteristics of receptive fields depending on the temporal characteristic of the test stimulus used to measure them, (6) the subjective appearance of Mach bands, (7) sine-wave contrast-threshold transfer functions, (8) the frequent failure of disk-detection experiments to demonstrate inhibitory surrounds, and (9) various second-order threshold effects, such as reduced spatial integration for long-duration stimuli, reduced temporal integration for large-area stimuli, and the increased effect of background luminance on the detection of large-area stimuli. Predictions are improved by assuming there exist various sizes of receptive fields that determine thresholds jointly.

Enhanced contrast in electron microscopy of unstained biological material. II. Defocused contrast of large objects.

Abstract: SUMMARY A contrast minimum is observed when 88 nm diameter polystyrene latex spheres are underfocused, which is related to the wide-angle scattering peaks. Images due to scattered and non-scattered wave components are displaced due to objective-lens spherical aberration and defocus. Maximum overlap of these components produces a contrast minimum at underfocus, related to the spherical aberration of the particular lens used. Similarly, a high-contrast band at carbon-film edges arises from spherical aberration and defocus separation of non-scattered and wide-angle scattered waves. This band increases in contrast with film thickness and in width with lens defocus. These geometrical effects account for the well-known ‘blinking’ of contrast of large biological objects upon swinging through focus without an objective aperture, and for the general contrast increase of defocused large objects. Fresnel fringes account for only a narrow band of enhanced contrast at distinct edges and cannot account for contrast enhancement of large objects lacking distinct edges.

Radiographic image subtraction technique

Abstract: AN X-RAY IMAGE SUBTRACTION TECHNIQUE USEFUL IN ANGIOGRAPHY OR THE LIKE INVOLVES PRODUCING FIRST AND SECOND X-RAY PHOTOGRAPHS OF A PRESELECTED ANATOMICAL SUBJECT AREA, THE SECOND PHOTOGRAPH BEING MADE SUBSEQUENT TO INJECTING AN APPROPRIATE X-RAY CONTRAST MEDIA INTO THE ARTERIES, ETC. THAT HAVE BEEN SELECTED FOR STUDY. THE RESULTANT X-RAY PHOTOGRAPHS ARE THEN PROJECTED THROUGH DIFFERENT COLOR FILTERS TO PROVIDE A COMPOSITE X-RAY IMAGE WHEREIN COMMON PORTIONS OF THE FIRST AND SECOND PHOTOGRAPHS ARE PRESENTED IN THE SUM COLOR OF THE COLOR FILTERS AND PORTIONS PECULIAR TO THE SECOND PHOTOGRAPH ARE PRESENTED IN A COLOR CORRESPONDING TO A PREDETERMINED ONE OF THE COLOR FILTERS. THIS DUAL COLOR COMPOSITE IMAGE IS PROJECTED INTO A COLOR SELECTIVE VIEWING SYSTEM RESPONSIVE TO THE ONE COLOR BUT NONRESPONSIVE TO THE SUM COLOR, THEREBY TO PROVIDE A VIEWABLE X-RAY IMAGE OF ONLY THOSE IMAGE PORTIONS PECULIAR TO THE SECOND X-RAY PHOTOGRAPH.

Color contrast cross-grid displays using undisturbed cholesterics

Abstract: A flat-panel display has been developed that makes use of undisturbed cholesteric liquid crystals at 24°C. The display is x-y addressed. Information is displayed at TV frame rates via color contrast or intensity contrast.

The Reversal of Classical Contrast in Temperature Perception

Abstract: Perceptual assimilation was demonstrated with some sets of thermal stimuli. This result is at variance with previous research, which has consistently yielded contrast effects.

Adaptation of the visual system

Abstract: We recorded the total pulse response of the optic nerve in frogs to varying degrees of increase and decrease of light from the original adapting level. On the basis of these data, we plotted curves of dependence of the magnitude of response on the logarithm of relative value of increase and decrease of light (the amplitude characteristic — AC). The AC is steepest in the zone of adapting background and sloped on either side of it. It follows that under stationary conditions of illumination, the eye is capable of finely differentiating light intensity only within a narrow range (one logarithmic unit). After adaptation to a new level of illumination, the AC shifts along the scale of light intensity in such a way that the steepest portion corresponds to the adapting brightness. Increase in steepness of the AC occurs precisely during the process of adaptation. The contrast sensitivity of the human visual system is greatest near the adapting level and declines on either side of it. It follows that in man steepness of the visual system AC is greatest in the zone of the adapting background. Both increase and decrease of intensity of the adapting background are accompanied by a decline of contrast sensitivity, which rises again during the process of adaptation to a new level. Thanks to adaptive shift of the steep portion of the AC along the scale of light intensity, a visual system having a high contrast sensitivity only within a narrow "working" range is capable of finely differentiating light intensity in significantly changing conditions of illumination.

Effect of Object Contrast on the Images of Straight Edges in Partially Coherent Illumination

Abstract: Images of edges of varying contrast from very low to very high have been calculated for a wide range of partially coherent illumination. Two graphs and four tables are included.

Solid-state luminance channel for color television receiver

Abstract: An improved solid-state luminance channel for a color television receiver having a plurality of interconnected transistor amplifiers wherein contrast and brightness controls are located in respective emitter electrode circuits so that the level of direct current components of the luminance signal remains substantially unaffected with selective settings of such controls A time delay network is further included having both resistive and reactive termination to enhance the frequency response thereof, particularly at high frequencies The resistive termination elements for the time delay means effectively serve as the load impedance for the associated transistor amplifier stage