Induced steady color shifts from temporally varying surrounds

Separating monocular and binocular neural mechanisms mediating chromatic contextual interactions

When seen in isolation, a light that varies in chromaticity over time is perceived to oscillate in color. Perception of that same time-varying light may be altered by a surrounding light that is also temporally varying in chromaticity. The neural mechanisms that mediate these contextual interactions are the focus of this article. Observers viewed a central test stimulus that varied in chromaticity over time within a larger surround that also varied in chromaticity at the same temporal frequency. Center and surround were presented either to the same eye (monocular condition) or to opposite eyes (dichoptic condition) at the same frequency (3.125, 6.25, or 9.375 Hz). Relative phase between center and surround modulation was varied. In both the monocular and dichoptic conditions, the perceived modulation depth of the central light depended on the relative phase of the surround. A simple model implementing a linear combination of center and surround modulation fit the measurements well. At the lowest temporal frequency (3.125 Hz), the surround's influence was virtually identical for monocular and dichoptic conditions, suggesting that at this frequency, the surround's influence is mediated primarily by a binocular neural mechanism. At higher frequencies, the surround's influence was greater for the monocular condition than for the dichoptic condition, and this difference increased with temporal frequency. Our findings show that two separate neural mechanisms mediate chromatic contextual interactions: one binocular and dominant at lower temporal frequencies and the other monocular and dominant at higher frequencies (6-10 Hz).

The separation of monocular and binocular contrast

The contrast asynchrony is a stimulus configuration that illustrates the visual system's separable responses to luminance and luminance contrast information (Shapiro, 2008; Shapiro et al., 2004). When two disks, whose luminances modulate in phase with each other, are each surrounded by a disk, one light and one dark, observers can see both the in-phase brightness signals and the antiphase contrast signals and can separate the two. Here we present the results of experiments in which observers viewed a similar stimulus dichoptically. We report that no asynchrony is perceived when one eye is presented with modulating disks and the other eye is presented with the black and white surround rings, nor is an asynchrony perceived in gradient versions of the contrast asynchrony. We also explore the "window shade illusion" (Shapiro, Charles, & Shear-Heyman, 2005) dichoptically and find that when a modulating disk is presented to one eye and a horizontally split black/white annulus is presented to the other, observers perceive a "shading" motion up and down the disk. This shading can be seen in either direction in the binocular condition, but it is almost always seen as moving towards low contrast in the monocular condition. These findings indicate the presence of separable retinal and cortical networks for contrast processing at different temporal and spatial scales.

Perceived segmentation of center from surround by only illusory contours causes chromatic lateral inhibition

When a light and also its surrounding context slowly oscillate in chromaticity over time, the color appearance of the light depends on the relative phase of center and surround. The influence of the surround is generally accounted for by retinotopic center-surround organization, with the surround inhibiting signals from the center. The traditional neural account, however, cannot rule out lateral inhibition due to cortical mechanisms sensitive to object segmentation cues. Experiments here reveal that illusory contours are sufficient to separate a center from its surround. Observers adjusted the Michelson contrast of a matching disk to equal the perceived modulation depth of a central area within a surround. Both the central test and matching disk were maintained at constant luminance and modulated in-phase at 2Hz along one chromatic axis (L/(L+M) or S/(L+M)). The center was perceptually segmented from the surround by either a physical (retinotopic separation) or illusory (cortically represented) triangle contour. Segmentation of center from surround by the illusory contour strongly attenuated the perceived modulation depth for both chromatic axes. Further, the strength of attenuation was consistently greater with the illusory than the physically segmenting triangle. This cannot be accounted for by retinal center-surround antagonism; instead it points to a cortical neural representation of contours, with lateral inhibition following neural mechanisms sensitive to object segmentation cues.

The temporal characteristics of the early and late stages of the L- and M-cone pathways that signal color

Flickering long-wavelength light appears more yellow than steady light of the same average intensity. The hue change is consistent with distortion of the visual signal at some nonlinear site (or sites) that produces temporal components not present in the original stimulus (known as distortion products). We extracted the temporal attenuation characteristics of the early (prenonlinearity) and late (post-nonlinearity) filter stages in the L- and M-cone chromatic pathway by varying the input stimulus to manipulate the distortion products and the measuring of the observers' sensitivity to them. The early, linear, filter stage acts like a band-pass filter peaking at 10-15 Hz with substantial sensitivity losses at both lower and higher frequencies. Its characteristics are consistent with nonlinearity being early in the visual pathway but following surround inhibition. The late stage, in contrast, acts like a low-pass filter with a cutoff frequency around 3 Hz. The response of the early stage speeds up with radiance, but the late stage does not. A plausible site for the nonlinearity, which modelling suggests may be smoothly compressive but with a hard limit at high input levels, is after surround inhibition from the horizontal cells.

Context-dependent judgments of color that might allow color constancy in scenes with multiple regions of illumination

For a color-constant observer, a change in the spectral composition of the illumination is accompanied by a corresponding change in the chromaticity associated with an achromatic percept. However, maintaining color constancy for different regions of illumination within a scene implies the maintenance of multiple perceptual references. We investigated the features of a scene that enable the maintenance of separate perceptual references for two displaced but overlapping chromaticity distributions. The time-averaged, retinotopically localized stimulus was the primary determinant of color appearance judgments. However, spatial separation of test samples additionally served as a symbolic cue that allowed observers to maintain two separate perceptual references.

The Verriest Lecture: color lessons from space, time and motion

The appearance of a chromatic stimulus depends on more than the wavelengths composing it. The scientific literature has countless examples showing that spatial and temporal features of light influence the colors we see. Studying chromatic stimuli that vary over space, time, or direction of motion has a further benefit beyond predicting color appearance: the unveiling of otherwise concealed neural processes of color vision. Spatial or temporal stimulus variation uncovers multiple mechanisms of brightness and color perception at distinct levels of the visual pathway. Spatial variation in chromaticity and luminance can change perceived three-dimensional shape, an example of chromatic signals that affect a percept other than color. Chromatic objects in motion expose the surprisingly weak link between the chromaticity of objects and their physical direction of motion, and the role of color in inducing an illusory motion direction. Space, time, and motion-color's colleagues-reveal the richness of chromatic neural processing.

Changes in perceived temporal variation due to context: contributions from two distinct neural mechanisms

The percept of a time-varying light depends on the temporal properties of light within the surrounding area. The locus of the neural mechanism mediating this lateral interaction is controversial; neural mechanisms have been posited at the LGN (Kremers et al., 2004) or cortical level (D'Antona & Shevell, 2007). To determine the neural locus, changes in perceived temporal variation were compared with ipsilateral versus contralateral surrounding context. In both cases, a temporally varying central field was viewed within a temporally varying surround; relative phase between center and surround was varied. Perceived modulation depth in the central field depended strongly on the relative phase between center and surround, in both the ipsilateral and contralateral conditions. The results revealed lateral interactions arising from both a weak monocular (plausibly LGN) and a stronger central (cortical) mechanism. The monocular contribution was similar over the range of temporal frequencies tested (approx. 3-12Hz), while the central component showed low-pass temporal-frequency selectivity.

The neural pathways mediating color shifts induced by temporally varying light

In natural viewing, an object's background often changes over time. Temporally varying backgrounds were investigated here with a steady test field within a time-varying surrounding chromaticity. With slow surround variation (below approximately 3 Hz), the color appearance of a steady test is also perceived to fluctuate. At somewhat higher temporal frequencies, however, temporal variation of the surround is visible but the test appears steady (R. L. De Valois, M. A. Webster, K. K. De Valois, & B. Lingelbach, 1986); also above approximately 3 Hz, temporal chromatic variation along the l- or s-axis of the MacLeod-Boynton space (symmetric about equal-energy-spectrum "white") shifts the steady appearance of the test field toward redness or yellowness, respectively (A. D. D'Antona & S. K. Shevell, 2006). In the study here, color shifts were measured with temporal surround modulation at 6 Hz or greater along axes intermediate to the l and s directions. Varying the relative phase of simultaneous surround variation in l and s should not change responses within independent l and s pathways but should differentially excite neural representations that combine l and s signals (so-called higher order chromatic mechanisms). Varying the phase of l and s showed that the induced color shifts were accounted for by neural responses both from nearly independent l and s pathways and from higher order chromatic mechanisms.

Induced temporal variation at frequencies not in the stimulus: evidence for a neural nonlinearity

The perceived color of a light depends on surrounding light. When the surround varies slowly over time, a central, physically steady light is perceived to vary also. This perceived temporal variation of the central region, however, is strongly attenuated when the surround varies faster than approximately 3 Hz (R. L. De Valois, M. A. Webster, K. K. De Valois, & B. Lingelbach, 1986). The classical explanation is low-pass temporal filtering at a cortical stage that attenuates the neural representation of temporal frequencies capable of causing induced temporal variation. This theory assumes neural responses are linear, so only temporal frequencies in the stimulus are represented in the neural response. The current experiments revealed that temporal frequencies above 3 Hz are capable of inducing temporal variation. Specifically, with two temporal frequencies superimposed in the surround, the induced temporal variation in the uniform region is at the difference frequency of these two frequencies, even though this frequency is not physically present in the stimulus. The results are accounted for by a nonlinear neural process, which causes temporal variation at the difference frequency, and a following linear temporal filter.