The cultural origins of colour categories Tony Belpaeme Artificial Intelligence Lab Vrije Universiteit Brussel

Introduction Colour spectrum is continuous… still we divide it into categories. Why do we divide the spectrum into these categories?

Arbitrary division Until mid 20 th century, colour categories were by many believed to be arbitrary – “… an American describing [the spectrum] will list hues as red, orange, yellow, green, blue, purple … nothing inherent either in the spectrum or the human perception … which would compel its division in this way.” (Gleason, 1961)

Universalism Berlin and Kay (1969) used naming experiments to extract colour categories Subjects marked the focus and extent of colour terms in a colour chart.

Universalism Berlin and Kay noticed a universal pattern in colour naming of different languages. But methodological concerns remained.

Universalism The universal character has been hailed by many and has been reconfirmed by some. (among others Kay and McDaniel, 1978; Durham, 1991; Shepard, 1992; Rosch-Heider, 1972; Kay and Regier, 2003)

Mechanisms Supposing we accept a certain universality of colour categorisation, what mechanisms could underlie this? – Nativism : genetic makeup. – Empiricism : interaction with the environment. – Culturalism : cultural interaction with others.

Nativism What mechanisms could underlie universalism of colour categories accor- ding to nativists ? – Regularities in human early visual perception, especially the opponent character of colour vision. (Kay and McDaniel, 1978) – Regularities in the neural coding of the brain. (Durham, 1991) – Genetic coding of colour categories. (Shepard, 1992)

Empiricism What do the empiricists have to say? – Our ecology contains a certain chromatic structure which is reflected in our colour categories. – We learn our colour categories by interacting with our environment. (e.g. Elman et al., 1996; Yendrikhovskij, 2001) – This all happens without the influence of culture or language.

Culturalism And finally, how do culturalists account for universalism. – Colour categories are culture-specific. – They are learned with a strong causal influence of language and propagate in a cultural process. (e.g. Whorf, 1954; Davidoff et al., 2001; Belpaeme and Steels)

Three stances Three opposing explanations – Nativism. – Empiricism. – Culturalism. – Of course a blend of two or three positions might be possible.

Discussion The discussion has been held on many different fronts – Neurology. – Psychology. – Anthropology. – Linguistics. – Ophthalmology. – Philosophy. We will tackle the discussion from artificial intelligence and computer modelling.

Artificial intelligence AI allows us to create models of natural phenomena, of which we then observe their behaviour. Different premises can be implemented in the models, allowing us to get an insight into the validity of the premises. – E.g. traffic modelling.

Reflections on empiricism Claim: colour categories are extracted from the environment, which contains enough structure to explain universality. Procedure – Collect chromatic data. – Extract colour categories. For this we use a clustering algorithm. – Compare extracted categories with each other and with human colour categories. If the claim is true, we would expect a high correlation between all extracted categories.

Reflections on empiricism The chromatic data – Three data sets: urban, natural and random.

Reflections on empiricism Extracting categories from these data. Categories from natural data: Categories from urban data:

Reflections on empiricism Quantitative comparison – 11 categories extracted from natural and urban data

Reflections on empiricism Comparing with human colour categories – By computing correlation between extracted categories and human categories. – Correlation between extracted categories

Reflections on empiricism Intermediate conclusion – The claim that human colour categories are specified by the distribution of chromatic stimuli in the world is not supported by our data. – However, there does seem to be a twofold influence by The structure of the perceptual colour space. The properties of perceptual categories.

Reflections on nativism Claim: universal colour categories can arrive from natural selection. Procedure: – Take a population of simulated individuals, of which the colour categories are evolved. – Fitness is defined as how well an individual can discriminate colour percepts. If the claim is true, we would expect an evolutionary process to produce a repertoire of colour categories, shared by all individuals.

Reflections on nativism Indeed, all individuals after enough time end up with identical colour categories Evolution keeps track of ecological pressure.

Reflections on nativism Some observations – Colour categories do not seem to be latently present in the brain. (Davidoff et al., 2001) – Technologically advanced cultures use more colour categories than “stone-age” cultures. Question – Is natural evolution fast enough to keep up with technological evolution?

Reflections on nativism In simulation, evolution is not fast enough. – Adapting to ecological changes takes approximately 20 generations (≈ 400 years). Natural evolution is undirected. – Achieving a categorical repertoire can only happen through blind mutations and recombinations of hereditary material. – If evolution would be “rebooted” would humankind arrive at different colour categories?

Reflections on culturalism Claim: language and culture have a causal influence on colour categories, and thus cause universality. If true, we expect linguistic interactions to cause sharing of colour categories. Procedure – Take a population of simulated individuals that learn colour categories and communicate about colour.

Simulating cultural transmission The ingredients – Agent-based simulations An agent is a simulated individual, with perception, categorisation, lexicalisation and communication. Perception maps spectral power distribution onto an internal colour space. Categorisation maps percepts onto categories, categories have prototypical behaviour. Lexicalisation connects categories to words. Communication takes care of uttering word forms. The agents have no way to access the internal state of other agents: no telepathy! – Several agents make a population

Simulating cultural transmission speaker hearer a b L

Reflections on culturalism Colour categories of two agents Sharing of colour categories

Reflections on culturalism Influence of language on categories But as language is culture-specific, cultural evolution cannot explain universalism.

Summary Empiricism is not a good candidate to explain universalism – There is not enough ecological pressure. Nativism can explain universalism, but is to slow to follow ecological pressures. – Also, recent neurophysiological and molecular studies point out many differences in colour perception between individuals. Culturalism can explain the sharing of categories in a culture, but not universalism.

Conclusion A blend of all three positions is needed to explain universalism. But language and culture plays a crucial role as the catalyst which binds the perceptual categories of individuals.