Color appearance of spatial patterns compared by direct estimation and conjoint measurement.

Perceptual scales of color saturation obtained by direct estimation (DE) and maximum likelihood conjoint measurement (MLCM) were compared for red checkerboard patterns and uniform red squares. For the DE task, observers were asked to rate the saturation level as a percentage, indicating the chromatic sensation for each pattern and contrast. For the MLCM procedure, observers judged on each trial which of two stimuli that varied in chromatic contrast and/or spatial pattern evoked the most salient color. In separate experiments, patterns varying only in luminance contrast were also tested. The MLCM data confirmed previous results reported with DE indicating that the slope of the checkerboard scale with cone contrast levels is steeper than that for the uniform square. Similar results were obtained with patterns modulated only in luminance. DE methods were relatively more variable within an observer, reflecting observer uncertainty, while MLCM scales showed greater relative variability across observers, perhaps reflecting individual differences in the appearance of the stimuli. MLCM provides a reliable scaling method, based only on ordinal judgments between pairs of stimuli and that provides less opportunity for subject-specific biases and strategies to intervene in perceptual judgements.

Optimizing the strength of the Bourdon effect by varying the triangle arrangement.

The Bourdon illusion refers to the perceived bentness of the straight collinear edges when two right-angled triangles are placed apex to apex. We studied this illusion using a cancellation method. In the first of three experiments, we manipulated the apex angle, with six different angles ranging from 4° to 45°. Results indicated that the Bourdon illusion is strongest when the angle is around 12°. In the second experiment, we compared four scalene triangles with a right-angled triangle. The angular shift was most salient when the shape corresponded to a right-angled triangle. In the third experiment, the patterns were created by varying the size of one right-angled triangle while holding the size of the second right-angled triangle constant. Results indicated that the Bourdon illusion was strongest when both right-angled triangles were of equal size. Our data suggest that the Bourdon illusion depends critically upon the specific arrangement of shapes in the display.

Neural circuits for long-range color filling-in.

Surface color appearance depends on both local surface chromaticity and global context. How are these inter-dependencies supported by cortical networks? Combining functional imaging and psychophysics, we examined if color from long-range filling-in engages distinct pathways from responses caused by a field of uniform chromaticity. We find that color from filling-in is best classified and best correlated with appearance by two dorsal areas, V3A and V3B/KO. In contrast, a field of uniform chromaticity is best classified by ventral areas hV4 and LO. Dynamic causal modeling revealed feedback modulation from area V3A to areas V1 and LO for filling-in, contrasting with feedback from LO modulating areas V1 and V3A for a matched uniform chromaticity. These results indicate a dorsal stream role in color filling-in via feedback modulation of area V1 coupled with a cross-stream modulation of ventral areas suggesting that local and contextual influences on color appearance engage distinct neural networks.

Effects of background and contour luminance on the hue and brightness of the Watercolor effect.

Conjoint measurement was used to investigate the joint influences of the luminance of the background and the inner contour on hue- and brightness filling-in for a stimulus configuration generating a water-color effect (WCE), i.e., a wiggly bi-chromatic contour enclosing a region with the lower luminance component on the exterior. Two stimuli with the background and inner contour luminances covarying independently were successively presented, and in separate experiments, the observer judged which member of the pair's interior regions contained a stronger hue or was brighter. Braided-contour control stimuli that generated little or no perceptual filling-in were also used to assess whether observers were judging the interior regions and not the contours themselves. Three nested models of the contributions of the background and inner contour to the judgments were fit to the data by maximum likelihood and evaluated by likelihood ratio tests. Both stimulus components contributed to both the hue and brightness of the interior region with increasing luminance of the inner contour generating an assimilative filling-in for the hue judgments but a contrast effect for the brightness judgments. Control analyses showed negligible effects for the order of the luminance of the background or inner contour on the judgments. An additive contribution of both components was rejected in favor of a saturated model in which the responses depended on the levels of both stimulus components. For the hue judgments, increased background luminance led to greater hue filling-in at higher luminances of the interior contour. For the brightness judgments, the higher background luminance generated less brightness filling-in at higher luminances of the interior contour. The results indicate different effects of the inner contour and background on the induction of the brightness and coloration percepts of the WCE, suggesting that they are mediated by different mechanisms.

Spatial selectivity of the watercolor effect.

The spatial selectivity of the watercolor effect (WCE) was assessed by measuring its strength as a function of the luminance contrast of its inducing contours for different spatial configurations, using a maximum likelihood scaling procedure. The approach has previously been demonstrated to provide an efficient method for investigating the WCE as well as other perceptual dimensions. We show that the strength is narrowly tuned to the width of the contour, that it is optimal when its pair of inducing contours are of equal width, and that the strength can be increased by varying the overall size of the stimulus when the width of the inducing contour is not optimal. The results support a neural substrate that has characteristics not unlike double-opponent, color-luminance cells observed in cortical area V1.

Contributions of contour frequency, amplitude, and luminance to the watercolor effect estimated by conjoint measurement.

The watercolor effect is a long-range, assimilative, filling-in phenomenon induced by a pair of distant, wavy contours of different chromaticities. Here, we measured joint influences of the contour frequency and amplitude and the luminance of the interior contour on the strength of the effect. Contour pairs, each enclosing a circular region, were presented with two of the dimensions varying independently across trials (luminance/frequency, luminance/amplitude, frequency/amplitude) in a conjoint measurement paradigm (Luce & Tukey, 1964). In each trial, observers judged which of the stimuli evoked the strongest fill-in color. Control stimuli were identical except that the contours were intertwined and generated little filling-in. Perceptual scales were estimated by a maximum likelihood method (Ho, Landy, & Maloney, 2008). An additive model accounted for the joint contributions of any pair of dimensions. As shown previously using difference scaling (Devinck & Knoblauch, 2012), the strength increases with luminance of the interior contour. The strength of the phenomenon was nearly independent of the amplitude of modulation of the contour but increased with its frequency up to an asymptotic level. On average, the strength of the effect was similar along a given dimension regardless of the other dimension with which it was paired, demonstrating consistency of the underlying estimated perceptual scales.

Multiple cues add up in defining a figure on a ground.

We studied the contribution of multiple cues to figure-ground segregation. Convexity, symmetry, and top-down polarity (henceforth called wide base) were used as cues. Single-cue displays as well as ambiguous stimulus patterns containing two or three cues were presented. Error rate (defined by responses to uncued stimuli) and reaction time were used to quantify the figural strength of a given cue. In the first experiment, observers were asked to report which of two regions, left or right, appeared as foreground figure. Error rate did not benefit from adding additional cues if convexity was present, suggesting that responses were based on convexity as the predominant figural determinant. However, reaction time became shorter with additional cues even if convexity was present. For example, when symmetry and wide base were added, figure-ground segregation was facilitated. In a second experiment, stimulus patterns were exposed for 150ms to rule out eye movements. Results were similar to those found in the first experiment. Both experiments suggest that under the conditions of our experiment figure-ground segregation is perceived more readily, when several cues cooperate in defining the figure.

A common signal detection model accounts for both perception and discrimination of the watercolor effect.

Establishing the relation between perception and discrimination is a fundamental objective in psychophysics, with the goal of characterizing the neural mechanisms mediating perception. Here, we show that a procedure for estimating a perceptual scale based on a signal detection model also predicts discrimination performance. We use a recently developed procedure, Maximum Likelihood Difference Scaling (MLDS), to measure the perceptual strength of a long-range, color, filling-in phenomenon, the Watercolor Effect (WCE), as a function of the luminance ratio between the two components of its generating contour. MLDS is based on an equal-variance, gaussian, signal detection model and yields a perceptual scale with interval properties. The strength of the fill-in percept increased 10-15 times the estimate of the internal noise level for a 3-fold increase in the luminance ratio. Each observer's estimated scale predicted discrimination performance in a subsequent paired-comparison task. A common signal detection model accounts for both the appearance and discrimination data. Since signal detection theory provides a common metric for relating discrimination performance and neural response, the results have implications for comparing perceptual and neural response functions.

Senescent changes in photopic spatial summation.

Previous studies have demonstrated an inverse relation between the size of the complete spatial summation area and ganglion cell density. We hypothesized that if this relation is dynamic, the spatial summation area at 6° nasal would expand to compensate for age-related losses of retinal ganglion cells but not in the fovea where age-related loss in ganglion cell density is not significant. This hypothesis was tested by measuring contrast thresholds with a series of Gabor patches varying in size. The spatial summation area was defined by the intersection of the segments of a two-branched, piece-wise linear function fitted to the data with slopes of -0.5 and 0 on a plot of log threshold vs. log area. Results demonstrate a 31% increase in the parafoveal spatial summation area in older observers with no significant age-related change in the fovea. The average foveal data show a significant increase in thresholds with age. Contrary to the foveal data, age comparisons of the parafoveal peak contrast thresholds display no significant difference above [corrected] the summation area. Nevertheless, as expected from the increase in summation area, expressing the parafoveal thresholds as contrast energy reveals a significant difference for stimuli that are smaller than the maximal summation area.

The watercolor effect: spacing constraints.

The watercolor effect (WCE) is a long-range color assimilation effect occurring within an area enclosed by a light chromatic contour, which in turn is surrounded by a dark chromatic contour. Here, we studied the effects of chromatic modulation of the WCE for different kinds of spacing between and within the inducing contours, using a hue-cancellation method. When an empty zone or interspace was inserted between the inducing contours (radial spacing), the hue shift required to null the induced coloration rapidly decreased with increasing spacing between the two contours. Similarly, when the continuous contours were replaced by dotted contours (lateral spacing), the shift in chromaticity quickly decreased with increasing distance between the dots. In this case, the decrease was similar for chains of paired dots ("in-phase") and chains of unpaired dots ("out-of-phase"). Results demonstrate that the WCE is strongest when the two inducing contours are spatially contiguous and continuous. The neural implications of these findings are discussed.

Temporal properties of the chromatic and achromatic Craik-O'Brien-Cornsweet effect.

The Craik-O'Brien-Cornsweet effect (COCE) is a visual illusion in which a difference in brightness is observed between two regions of equal luminance separated by a contrast edge with opposite luminance gradients on each side. The COCE also occurs with chromatic contrasts. It has been hypothesized that the COCE is mediated by a cortical filling-in process. According to the filling-in hypothesis, the filling-in or spreading of neural activity proceeds at finite speed, and therefore exhibits some temporal tuning as a function of the width of the area to be filled in. In the present study, observers varied the temporal frequency of a COC grating to determine the maximum temporal frequency at which a temporal modulation of brightness or color was perceived. In the achromatic COCE, contours were modulated along the luminance axis of the DKL color space; in the chromatic COCE, contours were modulated either along the L-M or the S-(L+M) axis. For the achromatic condition, the critical temporal frequency at which the COCE was still perceived had the shape of an inverted U, inconsistent with the filling-in hypothesis: the critical temporal frequency increased with increasing spatial frequency only up to 0.2cycles/deg, but then decreased for higher spatial frequencies. For the chromatic COCE, the critical temporal frequency decreased with increasing spatial frequency, which is also inconsistent with filling-in. Thus, the results of the present study are inconsistent with the idea that the temporal dynamics of the COCE, both achromatic and chromatic, are due to filling-in. Instead, our results are consistent with the spatial filtering properties of the luminance and chromatic systems.

Spatial profile of contours inducing long-range color assimilation.

Color induction was measured using a matching method for two spatial patterns, each composed of double contours. In one pattern (the standard), the contours had sharp edges to induce the Watercolor Effect (WCE); in the other, the two contours had a spatial taper so that the overall profile produced a sawtooth edge, or ramped stimulus. These patterns were chosen based on our previous study demonstrating that the strength of the chromatic WCE depends on a luminance difference between the two contours. Low-pass chromatic mechanisms, unlike bandpass luminance mechanisms, may be expected to be insensitive to the difference between the two spatial profiles. The strength of the watercolor spreading was similar for the two patterns at narrow widths of the contour possibly because of chromatic aberration, but with wider contours, the standard stimulus produced stronger assimilation than the ramped stimulus. This research suggests that luminance-dependent chromatic mechanisms mediate the WCE and that these mechanisms are sensitive to differences in the two spatial profiles of the pattern contours only when they are wide.

Psychophysical assessment of magno- and parvocellular function in schizophrenia.

Recently developed psychophysical techniques permit the biasing of the processing of the stimulus by early visual channels so that responses reflect characteristics of either magno- or parvocellular pathways (Pokorny & Smith, 1997). We used such techniques to test psychophysically whether the global magnocellular dysfunction reported in schizophrenia also affects early processes. Seven schizophrenic patients and 19 normal controls participated. The task was a four-alternative forced-choice luminance discrimination, using a 2 x 2 configuration of four 1-deg squares. Target luminance threshold was determined in three conditions: the stimulus, including the target, was pulsed for 17 ms (pulse paradigm); the target was presented on a steady background of four squares (steady paradigm), or the target was presented alone (no background paradigm). We replicated previous results demonstrating magnocellular and parvocellular signatures in control participants. No evidence for an early magnocellular deficit could be detected as the thresholds of all schizophrenic observers were higher both in the steady paradigm (presumed magnocellular mediation) and in the pulse paradigm (presumed parvocellular mediation). Magnocellular dysfunction, if present in schizophrenia, must concern more integrated processes, possibly at levels at which parvocellular and magnocellular paths interact.

Illusory spreading of watercolor.

The watercolor effect (WCE) is a phenomenon of long-range color assimilation occurring when a dark chromatic contour delineating a figure is flanked on the inside by a brighter chromatic contour; the brighter color spreads into the entire enclosed area. Here, we determined the optimal chromatic parameters and the cone signals supporting the WCE. To that end, we quantified the effect of color assimilation using hue cancellation as a function of hue, colorimetric purity, and cone modulation of inducing contours. When the inner and outer contours had chromaticities that were in opposite directions in color space, a stronger WCE was obtained as compared with other color directions. Additionally, equal colorimetric purity between the outer and inner contours was necessary to obtain a large effect compared with conditions in which the contours differed in colorimetric purity. However, there was no further increase in the magnitude of the effect when the colorimetric purity increased beyond a value corresponding to an equal vector length between the inner and outer contours. Finally, L-M-cone-modulated WCE was perceptually stronger than S-cone-modulated WCE for our conditions. This last result demonstrates that both L-M-cone and S-cone pathways are important for watercolor spreading. Our data suggest that the WCE depends critically upon the particular spatiochromatic arrangement in the display, with the relative chromatic contrast between the inducing contours being particularly important.

Spatial dependence of color assimilation by the watercolor effect.

Color assimilation with bichromatic contours was quantified for spatial extents ranging from von Bezold-type color assimilation to the watercolor effect. The magnitude and direction of assimilative hue change was measured as a function of the width of a rectangular stimulus. Assimilation was quantified by hue cancellation. Large hue shifts were required to null the color of stimuli < or = 9.3 min of arc in width, with an exponential decrease for stimuli increasing up to 7.4 deg. When stimuli were viewed through an achromatizing lens, the magnitude of the assimilation effect was reduced for narrow stimuli, but not for wide ones. These results demonstrate that chromatic aberration may account, in part, for color assimilation over small, but not large, surface areas.

The watercolor effect: quantitative evidence for luminance-dependent mechanisms of long-range color assimilation.

When a dark chromatic contour delineating a figure is flanked on the inside by a brighter chromatic contour, the brighter color will spread into the entire enclosed area. This is known as the watercolor effect (WCE). Here we quantified the effect of color spreading using both color-matching and hue-cancellation tasks. Over a wide range of stimulus chromaticities, there was a reliable shift in color appearance that closely followed the direction of the inducing contour. When the contours were equated in luminance, the WCE was still present, but weak. The magnitude of the color spreading increased with increases in luminance contrast between the two contours. Additionally, as the luminance contrast between the contours increased, the chromaticity of the induced color more closely resembled that of the inside contour. The results support the hypothesis that the WCE is mediated by luminance-dependent mechanisms of long-range color assimilation.