S-cone signals invisible to the motion system can improve motion extraction via grouping by color

Division and spreading of attention across color

Biological systems must allocate limited perceptual resources to relevant elements in their environment. This often requires simultaneous selection of multiple elements from the same feature dimension (e.g. color). To establish the determinants of divided attentional selection of color, we conducted an experiment that used multicolored displays with four overlapping random dot kinematograms that differed only in hue. We manipulated (i) requirement to focus attention to a single color or divide it between two colors; (ii) distances of distractor hues from target hues in a perceptual color space. We conducted a behavioral and an electroencephalographic experiment, in which each color was tagged by a specific flicker frequency and driving its own steady-state visual evoked potential. Behavioral and neural indices of attention showed several major consistencies. Concurrent selection halved the neural signature of target enhancement observed for single targets, consistent with an approximately equal division of limited resources between two hue-selective foci. Distractors interfered with behavioral performance in a context-dependent fashion but their effects were asymmetric, indicating that perceptual distance did not adequately capture attentional distance. These asymmetries point towards an important role of higher-level mechanisms such as categorization and grouping-by-color in determining the efficiency of attentional allocation in complex, multicolored scenes.

Mirror symmetry and aging: The role of stimulus figurality and attention to colour

Symmetry perception studies have generally used two stimulus types: figural and dot patterns. Here, we designed a novel figural stimulus-a wedge pattern-made of centrally aligned pseudorandomly positioned wedges. To study the effect of pattern figurality and colour on symmetry perception, we compared symmetry detection in multicoloured wedge patterns with nonfigural dot patterns in younger and older adults. Symmetry signal was either segregated or nonsegregated by colour, and the symmetry detection task was performed under two conditions: with or without colour-based attention. In the first experiment, we compared performance for colour-symmetric patterns that varied in the number of wedges (24 vs. 36) and number of colours (2 vs. 3) and found that symmetry detection was facilitated by attention to colour when symmetry and noise signals were segregated by colour. In the second experiment, we compared performance for wedge and dot patterns on a sample of younger and older participants. Effects of attention to colour in segregated stimuli were magnified for wedge compared with dot patterns, with older and younger adults showing different effects of attention to colour on performance. Older adults significantly underperformed on uncued wedge patterns compared with dot patterns, but their performance improved greatly through colour cueing, reaching performance levels similar to young participants. Thus, while confirming the age-related decline in symmetry detection, we found that this deficit could be alleviated in figural multicoloured patterns by attending to the colour that carries the symmetry signal.

Signals from Single-Opponent Cortical Cells in the Human cVEP

We used the chromatic visual evoked potential (cVEP) to study responses in human visual cortex evoked by equiluminant color stimuli for 6 male and 11 female observers. Large-area, colored squares were used to stimulate Single-Opponent cells preferentially, and fine color-checkerboard stimuli were used to activate Double-Opponent responses preferentially. Stimuli were modulated along the following two directions in color space: (1) the cardinal direction, L-M or M-L of DKL (Derrington, Krauskopf, and Lennie) space; and (2) the line from the white point to the color of the Red LED in the display screen, which was approximately intermediate between the L-M and -S directions in DKL space in cone-contrast coordinates. The amplitudes of cVEPs to large squares were smaller than those to checkerboards, and the latency of the cVEP response to squares was significantly less than the checkerboard latency. The latency of cVEP responses to the squares varied little with cone-contrast unlike the steep reduction of latency with cone-contrast observed in responses to color checkerboard patterns. The dynamic differences between cVEPs to squares and checkerboards support the hypothesis that a distinct neuronal mechanism responded to squares: Single-Opponent cells. Response amplitude, latency, and transientness-and their dependence on cone-contrast-were similar in the responses in the L-M and Red color directions. The similarity supports the hypothesis that the Single-Opponent signals in the cVEP come from a distinct population of cells that receives subtractive inputs from L and M cones, either L-M or M-L.SIGNIFICANCE STATEMENT This article is about characterizing the visual behavior of a distinct population of neurons in the human visual cortex, the Single-Opponent color cells. Based on single-cell results in the visual cortex of macaque monkeys, we used large uniformly colored stimuli to isolate the responses of Single-Opponent cells in the chromatic visual evoked potential (cVEP) recorded on the scalp of human observers. VEP signals recorded under conditions believed to reveal Single-Opponent responses are small and transient. Their time course is relatively unaffected by cone-contrast, and they are relatively insensitive to stimulus modulation of short wavelength-sensitive S cones. Because Single-Opponent cells convey signals that can be used to judge the color of scene illumination, knowing their visual properties is important for understanding color vision.

Language Modulates Categorical Effects of Moving Color Objects

Categorical perception (CP) of color claims that colors from different linguistic categories are discriminated more easily than those from the same category, suggesting that language may interact with visual perception. However, controversy remains regarding whether CP effects derive from language. Recently, CP effects were found in a dynamic paradigm named multiple object tracking (MOT). Here, we investigated whether this kind of CP is dependent on online use of language. We found that the CP effects are reduced by verbal interference when the participants were required to memorize color words during tracking (Experiment 2) but not when the interference stimuli were digits (Experiment 1). Our study suggested that the CP effects in tracking depend partly on online use of language and that the difficulty of verbal interference tasks influences the capability of disrupting CP.

Emergence of crowding: The role of contrast and orientation salience

Crowding causes difficulties in judging attributes of an object surrounded by other objects. We investigated crowding for stimuli that isolated either S-cone or luminance mechanisms or combined them. By targeting different retinogeniculate mechanisms with contrast-matched stimuli, we aim to determine the earliest site at which crowding emerges. Discrimination was measured in an orientation judgment task where Gabor targets were presented parafoveally among flankers. In the first experiment, we assessed flanked and unflanked orientation discrimination thresholds for pure S-cone and achromatic stimuli and their combinations. In the second experiment, to capture individual differences, we measured unflanked detection and orientation sensitivity, along with performance under flanker interference for stimuli containing luminance only or combined with S-cone contrast. We confirmed that orientation sensitivity was lower for unflanked S-cone stimuli. When flanked, the pattern of results for S-cone stimuli was the same as for achromatic stimuli with comparable (i.e. low) contrast levels. We also found that flanker interference exhibited a genuine signature of crowding only when orientation discrimination threshold was reliably surpassed. Crowding, therefore, emerges at a stage that operates on signals representing task-relevant featural (here, orientation) information. Because luminance and S-cone mechanisms have very different spatial tuning properties, it is most parsimonious to conclude that crowding takes place at a neural processing stage after they have been combined.

Motion perception induced by dynamic grouping: a probe for the compositional structure of objects

A new method is described for determining how the visual system resolves ambiguities in the compositional structure of multi-surface objects; i.e., how the surfaces of objects are grouped together to form a hierarchical structure. The method entails dynamic grouping motion, a high level process in which changes in a surface (e.g., increases or decreases in its luminance, hue or texture) transiently perturb its affinity with adjacent surfaces. Affinity is determined by the combined effects of Gestalt and other grouping variables in indicating that a pair of surfaces forms a subunit within an object's compositional structure. Such pre-perturbation surface groupings are indicated by the perception of characteristic motions across the changing surface. When the affinity of adjacent surfaces is increased by a dynamic grouping variable, their grouping is transiently strengthened; the perceived motion is away from their boundary. When the affinity of adjacent surfaces is decreased, their grouping is transiently weakened; the perceived motion is toward the surfaces' boundary. It is shown that the affinity of adjacent surfaces depends on the nonlinear, super-additive combination of affinity values ascribable to individual grouping variables, and the effect of dynamic grouping variables on motion perception depends on the prior, pre-perturbation affinity state of the surfaces. It is proposed that affinity-based grouping of an object's surfaces must be consistent with the activation of primitive three-dimensional object components in order for the object to be recognized. Also discussed is the potential use of dynamic grouping for determining the compositional structure of multi-object scenes.

The integration of local chromatic motion signals is sensitive to contrast polarity

Global motion integration mechanisms can utilize signals defined by purely chromatic information. Is global motion integration sensitive to the polarity of such color signals? To answer this question, we employed isoluminant random dot kinematograms (RDKs) that contain a single chromatic contrast polarity or two different polarities. Single-polarity RDKs consisted of local motion signals with either a positive or a negative S or L-M component, while in the different-polarity RDKs, half the dots had a positive S or L-M component, and the other half had a negative S or L-M component. In all RDKs, the polarity and the motion direction of the local signals were uncorrelated. Observers discriminated between 50% coherent motion and random motion, and contrast thresholds were obtained for 81% correct responses. Contrast thresholds were obtained for three different dot densities (50, 100, and 200 dots). We report two main findings: (1) dependence on dot density is similar for both contrast polarities (+S vs. -S, +LM vs. -LM) but slightly steeper for S in comparison to LM and (2) thresholds for different-polarity RDKs are significantly higher than for single-polarity RDKs, which is inconsistent with a polarity-blind integration mechanism. We conclude that early motion integration mechanisms are sensitive to the polarity of the local motion signals and do not automatically integrate information across different polarities.