Past exam of the perception course of the year 2 of the psychology (experimental) course of the University of Oxford / 2022 / 1

Individual differences show that color vision depends on receptor signals, their neural interpretation and the task used to measure them. They provide natural perturbations of the system, but differences in genes, matching, discrimination, naming and subjective appearance must not be treated as interchangeable.

A useful starting point is the principle of univariance. For a cone cell class $j$, a simplified receptor quantum catch is $Q_j=\int N(\lambda )a_j(\lambda )d\lambda$, where $N$ is the incident photon distribution, including optical filtering, and $a_j$ is effective absorptance. One receptor's absorption count does not uniquely identify wavelength: increasing intensity can compensate for a change in spectral sensitivity. Comparing classes supplies spectral information. With three classes, distinct spectra can satisfy three equal-catch constraints and therefore be metamers. This is information loss: if a spectrum is represented by more than three independent wavelength bins, its three-channel sampling matrix has a nontrivial kernel. Around a strictly positive spectrum, sufficiently small changes along that kernel preserve nonnegative stimulus values and all three catches. Thus the observer, not wavelength alone, defines a color match. With luminance fixed, three independent receptor coordinates leave at most two independent chromatic coordinates.

Inherited differences test that account. Dichromacy removes one independent class of cone input; anomalous trichromacy retains three classes but with altered spectral separation, often weakening particular discriminations. L and M are overlapping long- and middle-wave sensitivities, not detectors that individually label light “red” and “green.” Molecular work by Jeremy Nathans and colleagues identified cone pigment genes, then related inherited red–green variation to changes in the corresponding gene arrays.^[ref][ref] The important inference is a relation between biological variation and available spectral signals, not a claim that all observers with one label have identical vision.

A Rayleigh color match makes that inference measurable. An observer adjusts a mixture of two narrowband lights to match a comparison light, with brightness adjusted separately. The accepted mixture and matching range constrain pigment sensitivities and discriminability. A color discrimination task instead measures whether two stimuli can be distinguished, ideally with luminance cues controlled and a fitted psychometric function. Plate tests and everyday color names are less specific: learned labels or a brightness difference can support a correct response without normal chromatic discrimination. Conversely, reduced discrimination does not imply a world without color.

Individual differences also reveal compensation. Heidi Hofer and colleagues used adaptive optics retinal imaging with selective pigment bleaching and retinal densitometry to classify cone cells in living eyes. In their small sample, men passing normal color-vision tests had L:M cone ratios spanning approximately 1.1:1 to 16.5:1; the paper's corrected abstract supplies the lower value. The ratio was not fixed even within the normal category.^[ref] This rules out a simple requirement for one universal cone proportion. For example, opponent color processing could compare gain-adjusted cone signals rather than raw cell counts. It is compatible with post-receptoral gain adjustment and chromatic adaptation, but normal test scores do not prove identical subjective experiences or one uniquely identified compensatory mechanism.

A stronger question is whether more receptor classes create more perceptual dimensions. Heterozygous carriers of certain X-linked pigment variants can, through X-chromosome inactivation, have a retina containing more than three spectrally distinct cone types. Yet four types need not supply four functionally independent signals to behavior. Gabriele Jordan and colleagues combined forced-choice matching, multidimensional scaling and genetic analysis. One of 24 obligate carriers of deuteranomaly showed behavior consistent with tetrachromacy across their tests; most did not.^[ref] This distinction between potential and demonstrated extra dimensionality is crucial. The selected sample cannot establish population prevalence, and additional opsin variants alone cannot establish enhanced discrimination or richer experience.

Finally, optical and experiential differences must be controlled. Lens and macular filtering change the spectrum reaching the cone cells; age, adaptation, illumination and task learning can therefore mimic or modify receptor differences. Repeated measurements should separate within-person variability from stable between-person variation. Matching should be compared with discrimination, naming and appearance judgments rather than assuming that one measures all the others. Color naming can change without changing the receptor information; matching can change without entailing a particular verbal category.

The strongest conclusions arise when genetics, retinal measurements and controlled behavior converge. Variation establishes both the constraints imposed by receptor sampling and the flexibility of later processing; it does not directly disclose another person's color qualia.