1,721,091 research outputs found
Color perception: Objects, constancy, and categories
No full textColor has been scientifically investigated by linking color appearance to colorimetric measurements of the light that enters the eye. However, the main purpose of color perception is not to determine the properties of incident light, but to aid the visual perception of objects and materials in our environment. We review the state of the art on object colors, color constancy, and color categories to gain insight into the functional aspects of color perception. The common ground between these areas of research is that color appearance is tightly linked to the identification of objects and materials and the communication across observers. In conclusion, we argue that research should focus on how color processing is adapted to the surface properties of objects in the natural environment in order to bridge the gap between the known early stages of color perception and the subjective appearance of color
Categorical sensitivity to color differences
Full text linkCategorical perception provides a potential link between color perception and the linguistic categories that correspond to the basic color terms. We examined whether the sensory information of the second-stage chromatic mechanisms is further processed so that sensitivity for color differences yields categorical perception. In this case, sensitivity for color differences should be higher across than within category boundaries. We measured discrimination thresholds (JNDs) and color categories around an isoluminant hue circle in Derrington-Krauskopf-Lennie (DKL) color space at three levels of lightness. At isoluminant lightness, the global pattern of JNDs coarsely followed an ellipse. Deviations from the ellipse coincided with the orange-pink and the blue-green category borders, but these minima were also aligned with the second-stage cone-opponent mechanisms. No evidence for categorical perception of color was found for any other category borders. At lower lightness, categories changed substantially, but JNDs did not change accordingly. Our results point to a loose relationship between color categorization and discrimination. However, the coincidence of some boundaries with JND minima is not a general property of color categorical boundaries. Hence, our basic ability to discriminate colors cannot fully explain why we use the particular set of categories to communicate about colors. Moreover, these findings seriously challenge the idea that color naming forms the basis for the categorical perception of colors. With respect to previous studies that concentrated on the green-blue boundary, our results highlight the importance of controlling perceptual distances and examining the full set of categories when investigating category effects on color perception
Categorical facilitation with equally discriminable colors
Full text linkThis study investigates the impact of language on color perception. By categorical facilitation, we refer to an aspect of categorical perception, in which the linguistic distinction between categories affects color discrimination beyond the low-level, sensory sensitivity to color differences. According to this idea, discrimination performance for colors that cross a category border should be better than for colors that belong to the same category when controlling for low-level sensitivity. We controlled for sensitivity by using colors that were equally discriminable according to empirically measured discrimination thresholds. To test for categorical facilitation, we measured response times and error rates in a speeded discrimination task for suprathreshold stimuli. Robust categorical facilitation occurred for five out of six categories with a group of inexperienced observers, namely for pink, orange, yellow, green, and purple. Categorical facilitation was robust against individual variations of categories or the laterality of target presentation. However, contradictory effects occurred in the blue category, most probably reflecting the difficulty to control effects of sensory mechanisms at the green–blue boundary. Moreover, a group of observers who were highly familiar with the discrimination task did not show consistent categorical facilitation in the other five categories. This trained group had much faster response times than the inexperienced group without any speed–accuracy trade-off. Additional analyses suggest that categorical facilitation occurs when observers pay attention to the categorical distinction but not when they respond automatically based on sensory feed-forward information
Memory color
No full textA memory color is the color a beholder considers to be characteristic for an object based on their experience with that object. For example, the memory color of a banana is yellow for most people because they associate a banana with yellow in their memory
Are red, yellow, green, and blue perceptual categories?
No full textThis study investigated categorical perception for unique hues in order to establish a relationship between color appearance, color discrimination, and low-level (second-stage) mechanisms. We tested whether pure red, yellow, green, and blue, (unique hues) coincide with troughs, and their transitions (binary hues) with peaks of sensitivity in DKL-space. Results partially confirmed this idea: JNDs demarcated perceptual categories at the binary hues around green, blue and less clearly around yellow, when colors were isoluminant with the background and when accounting for the overall variation of sensitivity by fitting an ellipse. The categorical JND pattern for those three categories was in line with the effect of the second-stage mechanisms. In contrast, the results for unique red, binary red-yellow, and the JNDs for dark colors clearly contradicted categorical perception. There was a JND maximum around the center of red and JNDs strongly decreased away from the center. Although this observation alone would also be in line with categorical perception; unique red was shifted away from the center towards yellow so that unique red was close to the minimum instead of the maximum JND, hence contradicting categorical perception. In addition, we also showed that observers do not adjust unique hues more consistently than binary hues, confirming a previous study. Taken together, our findings suggest that some of the unique hues could be inherent in the early stages of color processing. At the same time, they also raise questions about complex effects of lightness, chroma and instructions on the measurements of JNDs and unique hues
The time course of chromatic adaptation in human early visual cortex revealed by SSVEPs
Full text linkPrevious studies have identified at least two components of chromatic adaptation: a rapid component with a time scale between tens of milliseconds to a few seconds, and a slow component with a half-life of about 10 to 30 seconds. The basis of the rapid adaptation probably lies in receptor adaptation at the retina. The neural substrate for the slow adaptation remains unclear, although previous psychophysical results hint at the early visual cortex. A promising approach to investigate adaptation effects in the visual cortex is to analyze steady-state visual evoked potentials (SSVEPs) elicited by chromatic stimuli, which typically use long durations of stimulation. Here, we re-analyzed the data from two previous pattern-reversal SSVEP studies. In these experiments (N = 49 observers in total), SSVEPs were elicited by counter-phase flickering color- or luminance-defined grating stimuli for 150 seconds in each trial. By analyzing SSVEPs with short time windows, we found that chromatic SSVEP responses decreased with increasing stimulation duration and reached a lower asymptote within a minute of stimulation. The luminance SSVEPs did not show any systematic adaptation. The time course of chromatic SSVEPs can be well described by an exponential decay function with a half-life of about 20 seconds, which is very close to previous psychophysical reports. Despite the difference in stimuli between the current and previous studies, the coherent time course may indicate a more general adaptation mechanism in the early visual cortex. In addition, the current result also provides a guide for future color SSVEP studies in terms of either avoiding or exploiting this adaptation effect
Categorical perception for red and brown
No full text<p>This data supplements the article:</p>
<p>Witzel, C., & Gegenfurtner, K. R. (2016). Categorical perception for red and brown. Journal of Experimental Psychology: Human Perception & Performance, 42(4), 540-570. doi:10.1037/xhp0000154</p>
<p>The Excell-file with the data includes 3 sheets:</p>
<p><strong>Sheet 1 (jnd): </strong>JND data from Figure 4.a of the above article.</p>
<p>- columns = 20 test colours.</p>
<p>- rows = 14 observers.</p>
<p><strong>Sheet 2 (rt): </strong>Response time data from Figure 6.a of the above article.</p>
<p>- columns = three kinds of colour pairs (AB, BC, & CD) and the location of the target (left vs. right).</p>
<p>- rows = 15 observers.</p>
<p><strong>Sheet 3 (er): </strong>Error rates from Figure 6.b of the above article.</p>
<p>- columns and rows as in sheet 2.</p>This research was funded by the Deutsche Forschungsgemeinschaft (#SFB TRR 135), and CW was financially supported by a German Academic Exchange Service (DAAD) postdoctoral fellowship and by J. Kevin O'Regan's ERC Advanced Grant "FEEL" (#323674)
The execution of saccadic eye movements suppresses visual processing of both color and luminance in the early visual cortex of humans
No full textOur eyes execute rapid, directional movements known as saccades, occurring several times per second, to focus on objects of interest in our environment. During these movements, visual sensitivity is temporarily reduced. Despite numerous studies on this topic, the underlying mechanism remains elusive, including a lingering debate on whether saccadic suppression affects the parvocellular visual pathway. To address this issue, we conducted a study employing steady-state visual evoked potentials (SSVEPs) elicited by chromatic and luminance stimuli, while observers performed saccadic eye movements. We also employed an innovative analysis pipeline to enhance the signal-to-noise ratio, yielding superior results compared to the previous method. Our findings revealed a clear suppression effect on SSVEP signals during saccades when compared to fixation periods. Notably, this suppression effect was comparable for both chromatic and luminance stimuli. We went further to measure the suppression effect across various contrast levels, which enabled us to model SSVEP responses using contrast response functions. The results suggest that saccades primarily reduce response gain without significantly affecting contrast gain, and that this reduction applies uniformly to both chromatic and luminance pathways. In summary, our study provides robust evidence that saccades similarly suppress visual processing in both the parvocellular and magnocellular pathways within the human early visual cortex, as indicated by SSVEP responses. The observation that saccadic eye movements impact response gain rather than contrast gain implies that they influence visual processing through a multiplicative mechanism
Determinants of colour constancy and the blue bias
No full textWe investigated several sensory and cognitive determinants of colour constancy across 40 illumination hues. In the first experiment, we measured colour naming for the illumination and for the colour induced by the illumination on the colorimetric grey. Results confirmed that the induced colours are approximately complementary to the colour of the illumination. In the second experiment, we measured colour constancy using achromatic adjustments. Average colour constancy was perfect under the blue daylight illumination and decreased in colour directions away from the blue daylight illumination due to undershooting and a strong blue bias. Apart from this blue bias, colour constancy was not related to illumination discrimination and to chromatic detection measured previously with the same setup and stimuli. We also observed a strong negative relationship between the degree of colour constancy and the consensus of naming the illumination colour. Constancy coincided with a low naming consensus, in particular because bluish illumination colours were sometimes seen as achromatic. Blue bias and category consensus alone explained >68%, and all determinants together explained >94% of the variance of achromatic adjustments. These findings suggest that colour constancy is optimised for blue daylight
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