A training study by Zhou, Mo, Kay, Kwok, Ip and Tan (2010) showed that newly learned, arbitrarily placed lexical categories are by themselves sufficient to produce a lateralized Whorf effect — faster discrimination of colors from different lexical categories, preferentially in the right visual field (left hemisphere). The authors explicitly left two questions open. First, the locus question: does language-driven categorical perception of color arise at the level of perceptual encoding, or at a post-perceptual decision or response stage? The term 'categorical perception' was used descriptively, and the paper takes no position on this issue. Second, the developmental question: the training study demonstrates only that learned categories can produce the effect; it does not establish how learned lexical categories combine or compete with innate or early-developed perceptual biases (infant categorical perception and cross-linguistically universal tendencies in color naming) during natural development to yield adult categorical-perception patterns. Fifteen years of subsequent research has produced conflicting evidence on both questions — preattentive electrophysiological markers favoring an early perceptual locus versus failures to replicate lateralization and signal-detection analyses favoring decisional accounts; and, developmentally, an infant right-hemisphere to adult left-hemisphere lateralization switch that tracks color-term learning, without any quantitative account of how learned and early-developing factors interact. The problem asks for an empirically or computationally grounded resolution of both the processing locus and the learned-versus-early interaction underlying lateralized categorical perception of color.

Background

Categorical perception (CP) is the finding that two stimuli are discriminated faster and more accurately when they fall on opposite sides of a category boundary than when they lie within one category, even when the physical (chromatic) separation is equated. In the color domain, CP boundaries largely follow the color terms of the observer's language: English speakers judge greens and blues straddling the English green–blue boundary as more distinct than do speakers of languages with a single term covering both, and Russian speakers, whose language lexically distinguishes light blue (goluboy) from dark blue (siniy), discriminate cross-category blues faster than English speakers do. This is the empirical core of the 'Whorfian' or linguistic-relativity debate about whether language shapes perception. Gilbert, Regier, Kay and Ivry (2006) reported a further signature: in a speeded visual search task, the category advantage was larger when the target appeared in the right visual field (RVF) than in the left visual field (LVF). Because the RVF projects to the left cerebral hemisphere, where language processing is dominant in most adults, this 'lateralized Whorf effect' was interpreted as evidence that online lexical codes contribute to the category advantage; the effect was reportedly disrupted by concurrent verbal, but not comparable nonverbal, interference. Winawer, Witthoft, Frank, Wu, Wade and Boroditsky (2007) confirmed language-specific boundary effects with the Russian blue categories. Two families of adjacent findings frame the problem. First, prelinguistic infants also show color CP at boundaries that accord well with universal tendencies in color naming across languages, and infant CP is lateralized to the right hemisphere (RH) rather than the left (LH) — the opposite of the adult pattern — suggesting that language-driven CP in adults may not simply build on the infant pattern (Franklin, Drivonikou, Bevis, Davies, Kay and Regier, 2008). Second, several authors had questioned whether color CP is genuinely perceptual at all, noting that category effects can arise from memory or response biases rather than changed sensory encoding; the visual mismatch negativity (vMMN), an event-related potential component indexing automatic, preattentive change detection for stimuli irrelevant to the ongoing task, was introduced to probe this (e.g., Thierry, Athanasopoulos, Wiggett, Dering and Kuipers, 2009). Against this background, Zhou, Mo, Kay, Kwok, Ip and Tan (2010) trained Mandarin speakers for three hours to assign four made-up monosyllabic names to four colors, two greens and two blues, so that pairs that had been within-category (both 'green' or both 'blue') became between-category relative to the newly learned lexical categories. After training, these pairs showed the lateralized pattern — faster discrimination in the RVF than the LVF — while a control group that practiced with the existing names showed no such change. This established that lateralized color CP can be produced by strictly learned categories, even ones violating both universal naming tendencies and the native language's categories. The authors were explicit about two limitations. In a footnote they wrote that they use 'categorical perception' descriptively and 'do not thereby assume a position on whether these effects will ultimately be found to lie on the perception or the response side.' And in the abstract they noted that, despite the demonstrated sufficiency of learning, 'the question remains open whether observed categorical perception for color can be entirely a result of learned categories or may rely to some degree on innate ones,' since linguistic and presumably innate categories tend to coincide in natural development. Later work sharpened both issues. On the locus question, Mo, Xu, Kay and Tan (2011) found the lateralized effect in vMMN with task-irrelevant deviants, arguing for a preattentive stage, and Zhong, Li, Li, Xu and Mo (2015) extended this to short-term trained categories; Kwok, Niu, Kay, Zhou, Mo, Jin, So and Tan (2011) even reported rapid gray-matter increase in left visual cortex V2/3 after a closely similar regime of new color-name learning. But Witzel and Gegenfurtner (2011), across ten experiments with 230 observers, found category effects that were not lateralized at all; Suegami, Aminihajibashi and Laeng (2014) found the category effect in response bias rather than sensitivity and no left-hemisphere lateralization once the spatial-judgment confound was removed; Holmes and Wolff (2012) showed left-lateralized CP for unlabeled novel object categories, so lateralization itself does not imply language; and Witzel and Gegenfurtner (2015) found categorical facilitation only when observers attended to the categorical distinction. Reviews (e.g., Siuda-Krzywicka, Boros, Bartolomeo and Witzel, 2019; Athanasopoulos and Casaponsa, 2020) still characterize the locus as unresolved, and a systematic review of verbal-interference paradigms (Nedergaard, Wallentin and Lupyan, 2023) found mixed evidence for online language involvement. On the developmental question, Franklin, Drivonikou, Clifford, Kay, Regier and Davies (2008) showed that the right-to-left hemispheric switch in color CP tracks whether toddlers have learned the relevant color terms; Skelton, Catchpole, Abbott, Bosten and Franklin (2017) showed that infant hue categories align with the common distinctions of the world's color lexicons and with the cardinal axes of early retinogeniculate color encoding — the cone-opponent channels that carry color signals from the retina to the thalamic relay nucleus of vision — indicating partial biological constraint; and Forbes and Plunkett (2023) showed that infants' perception near category boundaries changes measurably after the acquisition of the corresponding color words. These results jointly indicate that both early-developing biases and later lexical learning contribute, but how they combine or compete — and at what processing stage — remains without a quantitative or mechanistic account.

Problem Statement

The problem is to determine the mechanism and origin of lateralized categorical perception (CP) of color — the faster discrimination of colors from different lexical categories, preferentially in the right visual field (left hemisphere) — along the two linked axes left open by Zhou et al. (2010): (1) Locus. Establish whether language-driven category effects in color discrimination arise at the stage of perceptual encoding — a genuine change in how colors are perceptually represented, for example preattentively as indexed by the visual mismatch negativity — or at a post-perceptual decision or response stage, such as shifts in response criterion, attentional selection, or task strategy. A resolution must discriminate these alternatives with converging evidence and must reconcile the existing conflicting findings, including the vMMN evidence for preattentive lateralized effects on one side and the replication failures, response-bias findings, and non-linguistic lateralized category effects on the other. (2) Developmental origin. In natural development, characterize how learned lexical categories combine or compete with innate or early-developed perceptual biases — the right-hemisphere-lateralized categorical perception of prelinguistic infants and the cross-linguistically universal tendencies in color naming — to produce adult categorical-perception patterns, including the right-to-left hemispheric lateralization shift. A resolution must specify the relative contributions and the mechanism of their interaction. Both parts serve one objective: a mechanistic account of what lateralized color CP is (which processing stage language affects) and where it comes from (how lexical learning reshapes, or is constrained by, early-developing categorical perception).

References

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