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Sensitivity adjustments occur at several stages and over several timescales. They reveal adaptive gain control and flexible reference points, but no single aftereffect uniquely identifies one anatomical site. Changes in threshold, apparent hue, an achromatic setting and a receptor current must be distinguished before explaining them with one adaptation mechanism.

At the receptor level, a finite photocurrent range has to represent environments with very different mean light levels. Julie L. Schnapf and colleagues measured membrane currents from individual macaque cone outer segments. Responses to a maintained light step had an initial component followed by a smaller steady response, and a steady background reduced sensitivity to an added weak flash. This desensitization was distinguishable from loss of pigment through bleaching.^[ref] Direct recording establishes that some gain adjustment occurs before any color name, decision or cortical object representation. The preparation is macaque tissue, and a measured cone current is not identical to a human appearance judgment.

An idealized normalization model illustrates the benefit. If a mechanism's incremental gain is proportional to $1/(I+I_0)$, maintaining a fixed response increment requires $\Delta I\propto I+I_0$. At high backgrounds this gives approximately constant fractional contrast $\Delta I/I$, a Weber-like regime. It should not be confused with a universal law across all intensities. Under a known-background Poisson counting model with mean $N$, the noise standard deviation is $\sqrt{N}$; a small increment has signal-to-noise ratio approximately $\Delta N/\sqrt{N}$. A fixed signal-to-noise criterion then gives fractional threshold proportional to $N^{-1/2}$ instead. The two predictions derive from different assumptions about what limits performance.

Mean adaptation and contrast adaptation also differ. A diagonal cone-gain model replaces $Q_j$ by $g_j{Q}_j$, with gains related to recent stimulation. It can reduce the effect of a persistent chromatic shift in the illuminant. If the signal changes approximately as $Q_j^{\prime }=k_j{Q}_j$, choosing $g_j^{\prime }=g_j/k_j$ cancels that change. This is exact only for the assumed channelwise scaling; real spectral illuminant changes need not transform every surface's catches by the same three factors.

To test adjustments beyond mean cone excitation, experiments can modulate color around an unchanged time average. John Krauskopf and colleagues found direction-selective threshold elevation after temporal chromatic or luminance modulation, supporting separable adaptable mechanisms for particular directions near threshold.^[ref] Michael A. Webster and colleagues then measured appearance after adaptation to modulation along different color-space directions. A color monitor, a modulated adapting field and asymmetric appearance matches allowed testing effects beyond detection thresholds. Appearance shifted away from the adapting direction, including directions intermediate to the simplest cardinal color directions.^[ref] Thus a three-independent-channel account sufficient for some threshold results was not sufficient for all suprathreshold appearance changes. Constant mean catches constrain a simple mean-receptor explanation, but do not prove that all receptor nonlinearities have been removed.

Longer experience changes reference points too. Jay Neitz and colleagues altered chromatic experience with colored filters over days and measured unique yellow settings. Shifts persisted for one to two weeks after filter use ended.^[ref] Persistence after removing the immediate adapting stimulus demonstrates more than an instantaneous receptor response. It supports experience-dependent normalization without showing that a color category or verbal label has been rewritten. Any interocular transfer must also be measured explicitly; duration alone does not identify cortex.

A natural perturbation extends the range to months. Peter B. Delahunt and colleagues followed achromatic settings before and after removal of cataractous lenses. Removal greatly changed short-wavelength input; settings continued to readjust over roughly three months, whereas chromatic discrimination recovered much faster.^[ref] An achromatic setting compensates for a perceived tint, so its physical shift should not be mistaken for a same-direction tint in appearance. The findings indicate distinct adjustment timescales. They are observational evidence with a changing optical input, not a randomized isolation of one neural site or a general treatment recommendation.

These studies support a multilevel experimental strategy. Measure receptor responses, detection thresholds and appearance separately; manipulate mean and contrast independently; test spatial specificity, interocular transfer and recovery. Control exposure, eye position, stimulus calibration, response criterion and learning. A loss of sensitivity can reflect adaptation that preserves dynamic range rather than damage, and stable appearance can coexist with changed receptor inputs. Conversely, perfect compensation is not guaranteed.

L, M and S describe the relative spectral sensitivities of cone classes. Calling them red, green and blue confuses receptor sampling with the color appearance constructed from comparisons and context. The shorthand also wrongly suggests three narrow, nonoverlapping detectors or the primaries of a display.

First, each class has a broad sensitivity curve. A long-wavelength-sensitive cone responds to many wavelengths, and its sensitivity maximum is not a unique red stimulus. L- and M-cone sensitivities overlap extensively, while S cone cells are relatively more sensitive at shorter wavelengths. Light at one wavelength can therefore excite more than one class. Andrew Stockman and colleagues derived L- and M-cone sensitivity estimates using color-matching data and measurements from observers of known pigment genotype. Their analysis emphasizes broad overlap and the need for isolation procedures to measure one class without contamination from others.^[ref] Exact curves depend on pigment variant and optical filtering, so the letters denote relative classes rather than three invariant color labels.

Second, the principle of univariance prevents one cone from identifying hue. For a monochromatic stimulus, let mean absorption be $Q=Ia(\lambda )$. Whenever $a({\lambda }_1)$ and $a({\lambda }_2)$ are positive, intensities satisfying $I_2=I_{1}a({\lambda }_1)/a({\lambda }_2)$ give the same $Q$. The single receptor cannot distinguish those two inputs on absorption count alone. Calling that receptor a red detector would assign a color judgment to a signal that cannot even uniquely distinguish the two wavelengths. It remains true that spectral sensitivity constrains the comparison; univariance does not mean all receptor types are identical.

Third, perceptual color requires combinations. Comparing L and M signals and comparing S with L+M supplies chromatic information; summing appropriately weighted signals contributes to luminance-related processing. An L-cone increment can be part of several stimulus changes depending on what happens to M and S and on the adapted background. Neither an L response nor a raw L:M ratio by itself specifies a context-independent color word. The retinal and cortical opponent circuitry demonstrated by Dennis M. Dacey and Bevil R. Conway provides mechanisms for comparisons, rather than a relabelling of three cone outputs as three sensations.^[ref][ref]

Fourth, “blue” or “red” need not identify a single wavelength. Spectrally different lights can be metamers, and a purple appearance can arise from a mixture rather than a monochromatic wavelength. Conversely, the same physical spectrum can look different against another surround or after adaptation. Cone labels cannot replace the distinction between emitted or reflected spectrum, receptor catch, opponent representation and reported hue.

Fifth, the display analogy is backwards. An RGB screen emits mixtures from three selected primary spectra. The cone cells receive and integrate the resulting spectrum; their response curves are not identical to those primary spectra. If primary intensities are $p_k$ and cone sensitivities to each primary form matrix $B$, then $\mathbf{Q}=B\mathbf{p}$. Off-diagonal terms are generally nonzero because each primary can excite several cone classes. If $B$ is nonsingular, an intended catch change can be computed through $B^{-1}$ within the display's feasible gamut. It does not follow that every requested catch vector is realizable with nonnegative bounded primary intensities. Calibration and observer differences therefore matter in cone-isolating experiments.

Finally, color terms themselves vary across languages and tasks, whereas receptor class is a biological classification. Receptor genes, color matches, discrimination and linguistic categorization are related but distinct measurements. The labels L, M and S prevent an unwarranted assumption that one anatomical cell class directly instantiates one learned word or unique subjective hue.

Language is not necessary for the basic capacity to discriminate or organize colors, but it can influence color judgments, attention and memory in particular tasks. The evidence supports neither the claim that words create all color experience nor the claim that language is irrelevant whenever the stimuli are visible.

The starting distinctions are essential. A receptor sensitivity, a color-matching relation, a discrimination threshold, a category boundary and a color name are different measurements. Molecular differences can change available cone signals, while linguistic experience can change how available information is grouped or used. Finding different words for two samples does not show that one group cannot discriminate them. Finding the same matching relation does not establish identical subjective appearance or identical naming.

The biological basis imposes constraints independent of a person's vocabulary. Three cone classes provide overlapping spectral measurements, and later circuits compare those measurements. Altered receptor signals or adaptation can change performance without requiring a new word. Conversely, changing a response strategy can change performance while leaving receptor input intact. The central empirical question is therefore which stage and task language influences, not whether one can name a color without knowing its name.

Prelinguistic evidence rules out a strict requirement for color words. Alice E. Skelton and colleagues used infants' recognition-memory responses to sample relations among colors across the hue circle. Infants' categorical distinctions showed organization related to common divisions in adult color lexicons and to early visual mechanisms.^[ref] Infants did not need to produce the relevant color terms to show those distinctions. The inference is not that newborns possess an immutable adult lexicon: the infants already had visual experience, and looking-time measures require controls for discriminability, novelty and attention. The title's biological interpretation should not be strengthened into proof that experience plays no role.

Cross-language studies also challenge a rigid universal partition. Debi Roberson and colleagues compared English speakers with Berinmo speakers using naming, recognition and category-learning tasks. Effects of linguistic category boundaries appeared for the categories used by each group, and response-bias controls weakened a simple claim that universally focal colors always enjoy a memory advantage.^[ref] The article's historical title is retained as a citation, not adopted as a description of people. The comparison shows that category use and memory cannot be reduced to one vocabulary-independent set of preferred labels. Because language group covaries with cultural and visual experience, it does not isolate vocabulary as the sole cause.

A more direct manipulation was used by Jonathan Winawer and colleagues. Russian has conventional basic terms distinguishing lighter and darker blues that English usually groups under “blue.” Participants saw a target square and two alternatives together and selected the matching alternative. Russian speakers showed a speed advantage for comparisons crossing their linguistic boundary. A concurrent verbal task removed that advantage, while a spatial interference task did not produce the same pattern.^[ref] The simultaneous display reduces the long-term color-memory explanation, and the interference manipulation implicates access to language-related processes during performance. It does not establish a new receptor limit, a complete inability to discriminate within a category, or a necessary change in private color qualia. Concurrent tasks can affect more than one processing stage.

These results motivate separating sensitivity from response facilitation. A faster match can reflect attentional selection, category-assisted comparison or decision formation. To claim changed sensory resolution, an experiment should measure psychometric functions over controlled stimulus separations, fit location, slope and lapse parameters, and test whether any effect persists with nonverbal responses and reversed response mappings. Equal physical distance in a standard color space does not automatically ensure equal discriminability for every participant; individual thresholds and adaptation should be measured where they matter.

A stronger causal program can combine within-person interference, language-learning interventions and longitudinal or bilingual comparisons, while matching exposure and task difficulty. Testing transfer to new samples distinguishes memorized stimulus labels from a changed category rule. Comparing immediate discrimination, delayed recognition, naming and appearance matches identifies whether an effect generalizes across measurement levels. These are proposed strategies rather than invented experimental outcomes. No test based only on verbal descriptions can directly establish equality or inequality of another observer's subjective experience.

The seemingly opposing findings can therefore coexist. Biology and early experience can produce color organization before fluent naming, while learned labels subsequently guide the use of that organization. Color vision does not depend on language for its existence; linguistic resources can nevertheless alter how people attend to, classify, remember and rapidly compare colors under specified conditions.

A hue judged to contain no perceptual mixture of a specified opponent pair, such as a yellow judged neither reddish nor greenish. Its measured setting depends on observer and conditions and should not simply be identified with one retinal cardinal axis.