Categorical perception (CP) — faster or more accurate discrimination of stimuli from different categories than stimuli separated by an equivalent physical difference within one category — is left-lateralized in adults for several domains: the effect is stronger for stimuli presented to the right visual field (RVF), which projects initially to the language-dominant left hemisphere, and this has been widely read as evidence that language acts online on perception. Holmes and Wolff (2012) challenged this reading by training participants on novel object categories with or without verbal labels: both learning conditions produced statistically comparable left-lateralized CP, suggesting that lateralized CP reflects a general left-hemisphere propensity for categorical processing rather than online language use. This statement asks what the later literature has established about the issues the authors left open: whether language nonetheless drives CP online under particular stimulus domains, category granularities, or task demands; what the boundary conditions of left-lateralized CP are (given, for example, the mirror-image right-hemisphere bias of adult orientation CP for fine distinctions); and how language might shape category representations indirectly through learning, consolidation, and development — all against the background of a literature in which left-lateralized color CP has repeatedly failed to replicate across laboratories. Evidence through 2026 shows real but partial progress — time-course decompositions of visual-search category effects, demonstrations of context-dependent lexical modulation, dissociations of transient and consolidation-dependent linguistic influences, and a meta-analysis documenting large unexplained heterogeneity — while the core question of when, if ever, language plays an online causal role in CP, and under what conditions CP is lateralized, remains open.

Background

Categorical perception (CP) is the influence of learned category knowledge on perceptual discrimination: two stimuli falling on different sides of a category boundary are discriminated faster or more accurately than two stimuli separated by the same physical difference within a category. CP has been documented for speech sounds, colors, objects, faces, and newly learned novel categories, and it is central to two debates in cognitive science: linguistic relativity (whether the categories of one's language shape perception, the so-called Whorfian hypothesis) and the cognitive penetrability of perception by higher-level knowledge.

A key paradigm for linking CP to language is the divided-visual-field (visual half-field) technique. Because visual input from each half of the visual field projects initially to the contralateral cerebral hemisphere, a stimulus flashed briefly (for example, for 200 ms) in the right visual field (RVF) is initially processed by the left hemisphere and one in the left visual field (LVF) by the right hemisphere. Gilbert, Regier, Kay, and Ivry (2006) reported that the color category effect in visual search — faster detection of a target differing from distractors across a color-category boundary than by an equated within-category hue difference — was restricted to the RVF, that is, lateralized to the left hemisphere (a finding replicated by Drivonikou et al., 2007). Because the left hemisphere is dominant for language, this RVF advantage was interpreted as evidence that lexical color codes act online during perception. Convergent support came from studies in which concurrent verbal interference abolished the category effect (Roberson, Pak, and Hanley, 2008), from developmental work showing that color CP is lateralized to the right hemisphere in prelinguistic infants but to the left hemisphere in adults, with the switch occurring when the words that mark the relevant boundary are acquired (Franklin et al., 2008a, 2008b), from fMRI showing left-hemisphere language regions engaged during RVF between-category color discrimination (Siok et al., 2009), from training studies showing that newly learned arbitrary lexical categories produce lateralized color CP (Zhou et al., 2010), and from electroencephalographic evidence that the lateralized effect arises preattentively (Mo et al., 2011).

Holmes and Wolff (2012) asked whether left-lateralized CP actually depends on language. In two experiments, participants learned novel object silhouettes grouped into two categories, either with overt verbal labels or without any labels (category membership learned by feedback only). In a speeded discrimination task with briefly presented bilateral displays, both labeled and unlabeled categories produced CP that was stronger for RVF than LVF targets — left-lateralized — and the lateralized effect was statistically comparable across the label and no-label conditions. A relabeling test found greater interference when participants who had previously learned labels were taught new ones than when label-free learners met labels for the first time, arguing against the possibility that the no-label group had covertly named the categories to an equivalent degree. Participants who received no category training showed no CP at all. Holmes and Wolff concluded that left-lateralized CP does not require online language and may instead reflect a general left-hemisphere propensity for categorical processing — undermining the use of lateralization as a diagnostic of language-driven CP.

The empirical landscape was, however, already conflicted. Large-scale failures to replicate left-lateralized color CP existed by 2011: Witzel and Gegenfurtner (2011), across ten experiments with 230 observers, found visual-search category effects that were strictly bilateral, and Brown, Lindsey, and Guckes (2011) found no category effect at the green–blue boundary and no RVF advantage, with their scaling data better fit by non-categorical models of color difference. Other domains complicate the lateralization story in the opposite direction: adult categorical perception of orientation is lateralized to the right hemisphere (Franklin, Catherwood, Alvarez, and Axelsson, 2010), the mirror image of the adult color pattern, so the direction of hemispheric asymmetry in CP appears to depend on the stimulus domain or on the granularity of the categorical distinction rather than on language alone. This is the setting in which the source paper's open questions arise.

Problem Statement

Following Holmes and Wolff (2012), the problem is to determine the causal role of language in lateralized categorical perception: whether linguistic processing contributes causally to CP at the time of perception — an online role — in any conditions, as opposed to left-lateralized CP reflecting a general left-hemisphere propensity for categorical processing; and to specify the conditions under which CP is lateralized to the left hemisphere. The open issues retained from the source are: (1) whether language nonetheless drives CP online under particular stimulus domains, category granularities, or task demands; and, in particular, whether left-lateralized CP is limited to coarser categorical distinctions, given that adult CP for fine orientation distinctions shows the mirror-image, right-hemisphere bias (Franklin et al., 2010) — a contrast the source itself raises but which, as the authors note, does not by itself address whether such CP depends on language. (2) Whether language's influence is instead, or additionally, indirect: Holmes and Wolff proposed that "language may invite the formation of nonlinguistic categories ... which may in turn give rise to CP," consistent with the developmental finding that left-lateralized color CP emerges with color-term acquisition (Franklin, Drivonikou, Clifford, et al., 2008); discriminating such an indirect route — through category learning, memory consolidation, and development — from an online contribution at the time of perception is part of the same question.

Cross-study variability is central motivating context for this question rather than a separate deliverable: left-lateralized color CP is reliably obtained in some laboratories and has repeatedly failed to replicate in others (Witzel and Gegenfurtner, 2011; Brown, Lindsey, and Guckes, 2011; Suegami, Aminihajibashi, and Laeng, 2014), and differences in stimulus properties, task parameters, participant populations, or analytic choices — together with the large unexplained heterogeneity documented by Andrews et al. (2026) — must be reckoned with in any resolution of the questions above. These questions are empirical, and the evidence through 2026 bears on each of them: the online role of language in CP, the indirect route through learning, consolidation, and development, and the boundary conditions under which CP is lateralized to the left hemisphere.

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