Uniform Test Field Research Articles

The illusory brightening MAE separates low-level motion models.

The brightening MAE provides a challenge to low-level motion models. Introduced by Anstis (1967, Science 155 710-712), uniform fields appear to brighten after adaptation to repeated darkening. When the uniform test field is replaced by a spatial gradient, observers report illusory motion. We first measured the apparent speed of the aftereffect. Observers adapted to a circular pattern of linearly lightening or darkening radial segments, which were later replaced by static luminance gradients. When brightening or darkening, the adjacent segments of the dynamic pattern were set to grey so that any local motion energy at the boundaries was balanced over the adaptation period. At test, the sign of the spatial gradient was chosen appropriately for each segment to deliver uniform rotational motion. We found that the perceived speed of the rotation was precisely inversely proportional to the spatial gradient, as predicted by a spatiotemporal gradient motion computation. Motion energy models employ spatially band-pass front-end filters and therefore will not respond to uniform fields (the even Gabor is an exception). However, if the temporal brightening and spatial luminance gradient were combined before motion analysis, motion at test might be predicted by alternative models. To separate the models we sculpted the profile, introducing non-zero higher-order spatial derivatives. We now observed an apparent slowing of the aftereffect, which increased in turn with the accumulation of higher orders of spatial derivative for both convex and concave luminance ramps. Crucially, computer simulation shows the energy model and the multichannel gradient model predict increased speed from a simple addition of temporal brightening and the test spatial pattern. The data however are consistent with an isolated spatially low-pass first temporal derivative component delivering brightening, divided by multiple spatial derivatives introduced in the test pattern, a previously proposed multichannel gradient architecture, resulting in slowing of perceived speed.Meeting abstract presented at VSS 2014

Boundary Fields in Reverberation Chambers

A previous plane-wave integral representation for fields in a reverberation chamber is extended to analyze fields near the boundaries of a rectangular chamber. The field point can be located near a single wall, a right-angle bend, or a right-angle corner. Multiple image theory is used to ensure exact boundary conditions. Expressions for the ensemble average of the squares of the electric and magnetic field components illustrate the evolution of the fields from the chamber boundary to the center of the chamber where the fields are statistically uniform. The results are relevant to the useful test volume of rectangular reverberation chambers or, equivalently, how far test objects must be located from chamber walls to obtain a uniform test field.

Brightness Induction from Uniform and Complex Surrounds: A General Model

We studied the brightness induced from complex non-figural achromatic surrounds. A spatially uniform test field was surrounded by a random texture composed of two sets of dots. The luminance of each set of dots was modulated sinusoidally at 0.5 Hz. The mean luminance, phase and amplitude of modulation of each set were controlled independently so as to modulate the luminance and/or the contrast of the surround. Brightness induction was measured by a modulation nulling technique. The results were fit by a model in which the total brightness induced by a surround is equal to a weighted spatial summation of the induced effects from each point in the surround. The model incorporates local luminance gain controls in the test and surround fields and assumes that the magnitude of induction from each surround element is gain controlled by the difference between the mean luminance of the test and the individual surround elements. Copyright © 1996 Elsevier Science Ltd.

Color perception within a chromatic context: the effect of short-wavelength light on color appearance

Light at the boundary of a uniform test field ( contrast) has a qualitatively different effect on color perception than light in more remote noncontiguous regions ( context). Basic properties of color perception with contextual short-wavelength light are assessed here with a 1° test field surrounded by either contiguous or noncontiguous 440 or 491 nm light (32 td). Contrasting stimuli are 3 or 5° adapting fields, a thin 1° i.d.–2° o.d. (0.5° wide) contiguous band, or a large 1° i.d.–5° o.d. contiguous surround. Contextual stimuli are a remote 3° i.d.–5° o.d. ring or 0.5° wide noncontiguous bands at various distances from the edge of the 1° test field (2° i.d.–3° o.d., 3° i.d.–4° o.d., or 4° i.d.–5° o.d. bands). Contiguous surrounds have little influence on color appearance, but remote noncontiguous short-wavelength light strong affects the color of the test field, shifting it toward redness. The shift toward redness increases as a thin 440 nm band is moved farther from the test field (up to 5°), unlike the effect of distance on remote middle- and long-wavelength bands. Measurements comparing the effects of 440 nm and luminance-equated 491 nm light indicate a contribution from S cones.

Local and distal factors in visual grating induction

When a uniform test field is surrounded by luminance or chromatic gratings, a grating is induced in the test field. The perceived spatial frequency and orientation of the induced grating can be different from the frequency and orientation of the inducing gratings. Local edge effects are the factors primarily responsible for visual grating induction. Distal parts of the inducing stimulus affect only the amplitude of the induced modulation.

Isolation of opponent-colour mechanisms at increment threshold

An experimental examination was made of some paradigms designed to isolate the opponent-colour system at increment threshold. The effectiveness of a uniform white conditioning field spatially coincident with a 1.05-deg uniform test field was assessed by measuring intensity thresholds for simple detection and for colour discrimination. Values were obtained both by a method of adjustment and by a two-interval forced-choice procedure. For sufficiently high luminances of the conditioning field (3000 td or greater) little or no difference was found between simple-detection and colour-discrimination thresholds over the critical test-flash spectral range 520–620 nm, implying that the paradigm produced almost complete isolation of the opponent-colour system at increment threshold. A control experiment in which thresholds were obtained for a conditioning field larger than the test field gave less satisfactory isolation; near 580 nm the luminance system was found to be at least 0.3 log unit more sensitive than the opponent-colour system. A comparison was also made of the spatially coincident field paradigm with a paradigm in which a modified test stimulus of low temporal and spatial frequency content was presented on a large conditioning field. Test spectral sensitivity curves for simple detection obtained by a method of adjustment showed little difference in effectiveness in opponent-colour isolation.

Psychohysical hallucinations of orientation and spatial frequency.

After inspection of vertical sinusoidal gratings at least three distinct types of subjective or "hallucinated" patterns can be seen on a uniform test field. One type, here called horizontal streaming (H), is already well-known from the work of MacKay. A second type (V) looks like aroughly sinusoidal grating about 1-5 octaves above the adapting spitial frequency. Under optimal conditions a second vertical component appears at about 2 octaves below the adapting frequency. The third category of aftereffect consists of diagonal lines (D) at two orientations (about +/-40 degrees from vertical). The spatial-frequency band at these two orientations appears to be fairly broad, but roughly similar to the adapting frequency. The duration and strength of D increased, while V declined, at higher adapting spatial frequencies. D and V were increasing functions of adapting contrast, while H appeared abruptly only after the highest adapting contrast. H, D, and V are thus all functionally distinct. A schematic model of cortical organization is proposed to account for these phenomena. Pattern channels selective for a given orientation are grouped together with movement channels selective for the orthogonal direction. Antagonism between channels within such "modules" accounts for the streaming effect (H). Inhibition between modules tuned to different orientations and spatial frequencies accounts for the D and V effects: after adaptation of channels in one module, neighbouring module(s) are released from inhibition to produce a spurious response which is seen as a grating-like object in the adapted part of the visual field. During flickering adaptation a "halluncinated" lattice can be seen superimposed on the adapting grating. It apparently consists of Fourier components more remote from the adapting pattern than D and V are. This disinhibitory effect is strong confirmation of the inhibitory model. The regular and highly organized matrix of channels implied by these experiments may constitute a cortical hypercolumn conducting a coarse, piecewise Fourier transformation of the retinal image.

Antagonism between channels for pattern and movement in human vision.

MACKAY1 rediscovered a striking experimentally produced visual hallucination. After inspection of stationary black-and-white stripes, a uniform test field is apparently filled with particles in chaotic streaming motion at right angles to the orientation of the stripes2. A great deal has since been discovered about pattern-analysing channels in the human brain, and much of this information has been gained from the study of visual illusions and after-effects. The streaming hallucination, however, has never been properly explained3, and is now not much more than a curiosity in the textbooks. In this paper an explicit model of the streaming after-effect is proposed, and from it some new subjective after-effects are derived.

Increment thresholds for striped and uniform test fields as a function of background level.

Increment thresholds for a 30° circular striped or uniform test field centered at the fovea were measured against ten different background illuminances, including darkness. At photopic background levels, threshold values for coarse grating stimuli (0.99–0.07 cycle/degree) were lower than the increment threshold for the uniform test field; at scotopic levels, they were higher. The reversal of sequence took place in the zone of transition from photopic to scotopic backgrounds, for which thresholds for the two types of stimuli were similar. Results are attributed to (i) the facilitating effect of contrast during foveal cone vision; (ii) response to the illuminance of the light grating bars during mixed receptor activity; and (iii) sampling of luminous flux from both light and dark bars in peripheral rod-summation pools.