Right away! Early, lateralized color category effect revealed by first-saccade dynamics

In visual search colours from a different category (blue vs. green) are located faster than colours from the same category (aquamarine vs. turquoise). A number of studies have found that this category effect manifests only when targets are presented in the right hemifield. As such the effect has been linked with the lateralization of language and taken as strong support for the Sapir-Whorf hypothesis that language shapes perception. Subsequent studies, however, have often failed to find the left-lateralized category effect and instead found a bilateral category advantage. In the present study we examined the time-course of the (lateralized) category effect, by monitoring the participants’ eye movements in search for equiluminant across- and within- category colour targets. The results showed a bilateral category effect in the mean response times (RT), which comprised of two lateralized category effects: A Whorfian left-hemispheric category effect at an early stage of visual search (i.e., first eye movements) which was followed by a right-hemispheric category effect during later stages of visual search (search time, response). The right-hemispheric category advantage was especially pronounced when participants had initially missed the target (with the first eye movement). To assess whether the results indeed generalize to covert attention shifts, we measured performance in a control task, in which participants were instructed to localise the target while maintaining fixation thus preventing hemifield shifts. An analysis of the RT distribution revealed the same results as in the eye movement task, again showing that the bilateral category effect in the mean RT was composed of an early, left-lateralized and a later, right-lateralized category effect. These findings reconcile previously seemingly discrepant findings in the literature and shed new light on how language-based and non-language based mechanisms facilitate categorical search.