The effect of edge blur on grating induction magnitude

Brightness induction and suprathreshold vision: effects of age and visual field

A variety of visual capacities show significant age-related alterations. We assessed suprathreshold contrast and brightness perception across the lifespan in a large sample of healthy participants (N=155; 142) ranging in age from 16 to 80 years. Experiment 1 used a quadrature-phase motion cancelation technique (Blakeslee & McCourt, 2008) to measure canceling contrast (in central vision) for induced gratings at two temporal frequencies (1 Hz and 4 Hz) at two test field heights (0.5° or 2°×38.7°; 0.052 c/d). There was a significant age-related reduction in canceling contrast at 4 Hz, but not at 1 Hz. We find no age-related change in induction magnitude in the 1 Hz condition. We interpret the age-related decline in grating induction magnitude at 4 Hz to reflect a diminished capacity for inhibitory processing at higher temporal frequencies. In Experiment 2 participants adjusted the contrast of a matching grating (0.5° or 2°×38.7°; 0.052 c/d) to equal that of both real (30% contrast, 0.052 c/d) and induced (McCourt, 1982) standard gratings (100% inducing grating contrast; 0.052 c/d). Matching gratings appeared in the upper visual field (UVF) and test gratings appeared in the lower visual field (LVF), and vice versa, at eccentricities of ±7.5°. Average induction magnitude was invariant with age for both test field heights. There was a significant age-related reduction in perceived contrast of stimuli in the LVF versus UVF for both real and induced gratings.

Comparing the spatial-frequency response of first-order and second-order lateral visual interactions: grating induction and contrast-contrast

The magnitudes of two suprathreshold lateral spatial-interaction effects--grating induction and contrast--contrast--were compared with regard to their dependence upon inducing-grating spatial frequency. Both effects cause the contrast of target stimuli embedded in surrounding patterns to be matched nonveridically. The magnitudes of each effect were measured in a common unit that indexed the degree of nonveridical contrast matching across a large range of target-grating contrasts (+/- 0.80). Grating induction was a low-pass effect with respect to spatial frequency, whereas contrast-contrast was bandpass, peaking at approximately 4.0 cycles deg(-1). The magnitude of grating induction exceeded that of contrast--contrast, both overall and at their optimal frequencies (0.03125 and 4.0 cycles deg(-1), respectively); the two effects are equipotent at an inducing-grating spatial frequency of 1.0 cycle deg(-1). A significant negative correlation between the magnitudes of the two effects suggests a link whereby activation of second-order normalization mechanisms may inhibit first-order mechanisms.

Two competing mechanisms underlying neon color spreading, visual phantoms and grating induction

Neon color spreading is closely related to the photopic visual phantom illusion, since these two completion phenomena are characterized by in-phase lightness induction, and the only difference in the stimulus configuration is the difference in the inducer height. This idea was supported by the present study. Neon color spreading showed almost the same function of critical spatial frequency as photopic visual phantoms (Experiment 1), and the critical spatial frequency was constant as the inducer height was changed (Experiment 2). We also examined the relationship between neon color spreading and grating induction (characterized by counterphase lightness induction) in critical spatial frequency (Experiment 3) and in magnitudes of lightness induction (Experiment 4) as a function of the inducer height. The inducer height at which in-phase (neon color spreading) appearance gave way to counterphase (grating) induction was approximately 0.1 deg. These results suggest that neon color spreading shares a common neural mechanism with the photopic visual phantom illusion and that this mechanism is different from, and competes with, the mechanism of grating induction.

Brightness with and without perceived transparency: when does it make a difference?

Subjects matched the brightness of test patches whose inner (adjacent) surrounds appeared either as transparent overlays on a wider background that included the test patch or as regions differing in reflectance from the test patch and the outer surround. In the above configurations the luminance and spatial extent of the inner surround was identical, thus controlling for the effects of surround luminance. Configuration condition had a significant effect on test-patch brightness. In general, test-patch brightness was significantly elevated under conditions favouring the interpretation of the stimulus as including a transparent overlay. The largest effect occurred for the configuration in which the perception of transparency was supported by stereo depth cues. The brightness effect was mediated by the virtual transmittance of the transparent overlay, increasing in magnitude with decreasing transmittance. Further, the effect of transparency on brightness was greatest for test-patch luminances near to those of their immediate surrounds.

Similar mechanisms underlie simultaneous brightness contrast and grating induction

The experiments explore whether the mechanism(s) underlying grating induction (GI) can also account for simultaneous brightness contrast (SBC). At each of three test field heights (1, 3 and 6 deg), point-by-point brightness matches were obtained from two subjects for test field widths of 32 deg (GI condition), 14, 12, 8, 6, 3 and 1 deg. The point-by-point brightness matches were quantitatively compared, using GI condition matches as a standard, to assess systematic alterations in the structure and average magnitude of brightness and darkness induction within the test fields as a function of changing test field height and width. In the wider test fields induction structure was present and was generally well-accounted for by the GI condition sinewave predictions. As test field width decreased the sinewave amplitude of the induced structure in the test field decreased (i.e., flattened), and eventually became negative (i.e., showed a reverse cusping) at the narrower test field widths. As expected, both subjects showed a decrease in overall levels of brightness and darkness induction with increasing test field height. For any particular test field height, however, relative brightness increased with decreasing test field width. This brightness increase began at larger test field widths as test field height increased. The results are parsimoniously accounted for by the output of a weighted, octave-interval array of seven difference-of-gaussian filters. This array of filters differs from those previously employed to model various aspects of spatial vision in that it includes filters tuned to much lower spatial frequencies. The two-dimensional output of this same array of filters also accounts for the GI demonstrations of Zaidi [(1989) Vision Research, 29, 691-697], Shapley and Reid's [(1985) Proceedings of the National Academy of Sciences USA, 82, 5983-5986] contrast and assimilation demonstration, and the induced spots seen at the street intersections of the Hermann Grid. The physiological plausibility of the filter array explanation of brightness induction is discussed, along with a consideration of its relationship to other models of brightness perception.

Facilitation of luminance grating detection by induced gratings

Grating induction causes a homogeneous test field surrounded by sinewave gratings to possess an induced counterphase grating [McCourt M. E. (1982). Vision Research, 22, 119]. There is currently no consensus about the stage of visual processing at which illusory phenomena such as simultaneous brightness contrast are signaled. We measured the masking efficacy of induced gratings by measuring contrast detection thresholds for targets (sinewave luminance gratings) added in phase to both real and induced gratings which were matched in apparent contrast. At spatial frequencies below c. 0.5 c/deg, target detection and discrimination were comparably facilitated by both real and induced low-contrast pedestals (0.5-2%). At higher spatial frequencies (above 1.0 c/deg) facilitation continued to be observed for targets added in-phase to real grating pedestals, but occurred only for targets added out-of-phase with induced pedestal gratings. Higher inducing frequencies by themselves were not responsible for the observed phase shift of facilitation, however, since both real and induced pedestals produced similar target contrast discrimination functions when inducing frequency was varied by manipulating viewing distance (which holds the ratio of inducing grating period and test field height constant). The results imply the existence of at least two types of lateral interactive processes: one producing in-phase facilitation, and a second producing out-of-phase facilitation. The relative contribution of each process depends upon the ratio of inducing grating period and test field height.

Grating induction: a new explanation for stationary phantom gratings

The magnitude of brightness variations within test fields of grating induction and phantom grating type displays was measured using a pointwise brightness matching paradigm. A range of test field luminances was sampled which encompassed those reported to give rise to both in-phase phantom and counterphase induced gratings. Results from four observers failed to reveal the existence of stationary "in phase" phantom gratings: at all test field luminances the spatial phase of brightness modulations was consistently opposite to that of the inducing grating. At low or high test field luminances, however, test field luminance matches to the bright or dark half-cycles of induced gratings approximate the luminances of the troughs or peaks of the dark or bright half-cycles of the inducing gratings, respectively. Thus, at low test field luminances the dark half-cycle of the inducing grating may appear to continue across the dark test field, and similarly, at high test field luminances the bright half-cycle may appear continuous with the bright test field. Previously misidentified as being "in-phase" with the inducing grating, the appearance of "phantoms" is suggested to arise due to the apparent brightness continuity of these induction-produced half-cycles of the induced grating across the test field.

Contrast-matching analysis of grating induction and suprathreshold contrast perception

The effect that induced gratings [Vision Res. 22, 119 (1982)] exert on the perceived contrast of standard gratings situated within a 0.5 degrees test field was assessed for two observers by a contrast-matching procedure. Five levels of inducing-grating contrast, CI, ranged from 0.0 to 0.75. Functions relating matching contrast, CM, to standard-grating contrast, CS, were obtained at four levels of inducing-grating contrast across a range of standard contrasts, -0.90 < or = CS < or = +0.90, where the sign denotes the spatial phase of the standard relative to the inducing grating. The matching functions possessed three distinct limbs separated by two inflection points; the limb between the inflection points represents a region of high contrast gain. Another measure, canceling contrast, was obtained at the four levels of inducing contrast by variation of CS until the test field appeared spatially homogeneous. Induction magnitude measured in terms of canceling contrast, CC, grew approximately linearly with CI, such that CC = 0.819 (CI). Induction magnitude determined from matching-contrast data obtained for homogeneous test fields (i.e., CM for CS = 0.0) grew as a decelerating function of inducing-grating contrast, such that CM = 0.308(CI]1.8 + 0.096), effectively asymptoting at a contrast of approximately 0.275 for CI > or = 0.50. When the difference between the absolute values of matching and standard contrast, magnitude of CM-magnitude of CS, is plotted against the ratio of standard to inducing-grating contrast, CS/CI, the resulting functions are generally biphasic, revealing regions of both contrast overmatching (i.e., magnitude of CM > magnitude of CS) and contrast undermatching, magnitude of CM < magnitude of CS. A four parameter model is presented that accounts for many features of the raw matching functions and that is mathematically similar to Semmelroth's account of the crispening effect in brightness matching [J. Opt. Soc. Am. 60, 1685 (1970)]. The model describes matching contrast, CM, as the weighted sum of two nonlinear contrast-response functions whose inputs are CS and CS-CI. The results are discussed relative to the crispening effect (the effect of contrast adaptation on perceived contrast) and to similarities and differences in luminance and contrast-domain visual processing.