In the discussion of the relationship between language and thought, the concept of categorical perception (CP) is a key element. A lot of studies have shown that categorical perception of color is affected by language, but lately, researchers have been branching out into other areas.

If you’ve read my previous posts, you might remember that categorical perception is the ability to discriminate between two degrees of something (such as color) more easily when they are distinguished by a linguistic category. For example, it’s easier to distinguish between blue and green than it is to distinguish between two shades of blue, even if they are separated by identical amounts on an objective scale (like the Munsell color scale).

A recent study showed that categorical perception can also be demonstrated in the perception of orientation; in this case, between vertical and oblique (diagonal). What this means is that people are more accurate when distinguishing between two lines when one of them is between 0° and (approximately) 9° from vertical, and the other is more than 9° (this specific degree of tilt was determined using the participants in the study, and does not represent a universal category).

So what does this have to say about the influence of language on thought? The authors of another study tested both adults and pre-verbal infants, and found that both groups exhibited CP of orientation, suggesting a non-linguistic origin of this effect. However, there was also a more interesting result: adults exhibited stronger CP in the left visual field (which is processed by the right side of the brain), while infants showed stronger CP in the right visual field.

These findings contradict the suggestions of two previous studies. The first posited that categorical perception is generally lateralized to the left hemisphere in adulthood — obviously, the right hemisphere bias of the adults in the orientation CP does not support this. The second held that left-hemisphere categorical perception was driven by linguistic factors; however, the fact that pre-verbal infants exhibited categorical orientation perception in the left hemisphere indicates that language is not the underlying factor.

A complicating factor in this interpretation, though, emerges when the results of verbal interference are taken into account. When adults were undergoing verbal interference, a left-hemisphere bias in orientation CP began to re-emerge, suggesting that language does have some effect on the lateralization of categorical orientation perception. While the authors do not provide a solid explanation for this interesting result, they speculate that linguistic knowledge affects the underlying mechanisms that are used for categorical perception (such as within-category compression and between-category expansion).

It’s clear that this study provides a lot of useful information, but also that it raises more questions than it provides answers. What drives orientation CP in pre-verbal infants? What underlying mechanisms support CP? How are they affected by language? Do speakers of languages other than English show different types of orientation CP? All of these questions remain unanswered, but will likely be addressed in the near future, with cross-linguistic, as well as neuroimaging, studies.

References

Franklin, A., Catherwood, D., Alvarez, J., & Axelsson, E. (2010). Hemispheric asymmetries in categorical perception of orientation in infants and adults Neuropsychologia, 48 (9), 2648-2657 DOI: 10.1016/j.neuropsychologia.2010.05.011

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