Beyond Categorical Perception: Gradient Lexical Tone Processing Revealed by Visual Analog Scale

Purpose: While the Visual Analog Scale (VAS) has revealed gradient perception in segmental speech sounds, its application to lexical tones, a critical yet understudied suprasegmental feature, has been absent. This study investigated lexical tone categorization using VAS, directly comparing it with traditional two-alternative forced-choice (2AFC). Method: Eighty-four native speakers categorized an 11-step F0 continuum from Mandarin Tone 1 to Tone 2 in both tasks. Four-parameter logistic functions yielded slope (categorization sharpness) and response variability. Within-category sensitivity (Δ) was quantified from VAS responses.Results: Paired Wilcoxon signed-rank tests showed significantly shallower slopes (p < .001, r = .76) and lower variability (p < .001, r = .87) in VAS versus 2AFC. One-sample t-tests confirmed listeners discriminated fine-grained differences within categories, with Δ reliably exceeding zero (left: M = 0.0335, t(270) = 8.89, p < .001; right: M = 0.0256, t(316) = 8.38, p < .001). Crucially, slope and response variability were weakly correlated in VAS (ρ = .27, p < .05) but strongly negatively correlated in 2AFC (ρ = -.67, p < .001). Moreover, response variability correlated significantly across tasks (ρ = .40, p < .001), while slopes did not. Conclusion: These findings provide the direct evidence for gradient perception at the suprasegmental level, further establishing VAS as a sensitive tool for uncovering the nature of speech categorization. The dissociation between task-dependent gradiency and stable response variability helps reconcile apparent conflicts in the categorical perception literature, suggesting that these conflicts may stem from methodological constraints rather than genuine theoretical disagreements.