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April 7, 2016

The Ability to Detect Color Differences is Not Categorical

Cover of Journal of Experimental Psychology: Human Perception and Performance (small) Categorical perception of color refers to the finding that color perception is influenced by linguistic color category labels, such that colors that fall into different categories (e.g., green and blue) are perceived as more dissimilar than shades that fall within the same category (e.g., two shades of blue).

Although the idea that color perception is categorical is widely accepted, this phenomenon is typically studied using the green–blue boundary. This is problematic because the green–blue boundary coincides with biological determinants of color vision that might produce spurious effects unrelated to linguistic color category. Furthermore, to convincingly demonstrate effects of linguistic category, low-level sensory differences must be equated for between-category and within-category color pairs. This is not trivial because of the biological nature of human color vision: differences in physical determinants of color (wavelength spectra) do not directly map onto perceptual differences.

Witzel and Gegenfurtner (2015) (PDF, 677KB) address these issues in recent work published in the Journal of Experimental Psychology: Human Perception and Performance using the red–brown boundary, which is the least inclined to produce spurious effects based on biological characteristics of color vision.

In Experiment 1, just-noticeable differences (the amount of difference between two stimuli required for difference to be detectable at least half the time) were determined for colors along the red–brown continuum. Four colored disks were briefly presented simultaneously, and participants had to indicate which disk was different from the other three. Accuracy and consistency were emphasized. A staircase procedure was used to determine stimulus pairs for which there was a .72 probability of seeing the difference.

There was no difference in just-noticeable differences for between- versus within-category color pairs. In other words, participants were not better able to detect differences between color pairs that crossed the linguistic category boundary.

In Experiment 2, the same participants were tested in a similar task as Experiment 1. However, in contrast to Experiment 1, only four stimuli were used (two red and two brown) that were chosen based on the just-noticeable differences obtained in Experiment 1, such that perceptual differences between adjacent colors were equated. In addition, the stimuli were displayed until a response was made, and participants were encouraged to respond as quickly as possible.

In this case, a categorical effect was observed: responses were faster and more accurate for between- versus within-category pairs. Thus, linguistic category facilitated performance when low-level sensitivity differences between stimuli were controlled.

Despite the similarities between tasks, category effects were only observed in Experiment 2, where stimuli were presented until response, but not Experiment 1, where stimulus presentation was brief. This suggests that category effects arise at higher, cognitive levels of perceptual processing, perhaps due to attention shifts toward linguistic distinctions, whereas performance when presentation time is brief is driven primarily by sensory information that is not influenced by linguistic category.

Witzel, C., & Gegenfurtner, K. R. (2016). Categorical perception for red and brown. Journal of Experimental Psychology: Human Perception and Performance, 42(4), 540–570. http://dx.doi.org/10.1037/xhp0000154

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Note: This article is in the Basic / Experimental Psychology topic area. View more articles in the Basic / Experimental Psychology topic area.