Imagine you’re packing for a trip and you have limited suitcase space. You can either pack individual items, like specific shades of red shirts, or you can categorize them into general groups, like all red clothes. Which option requires more mental effort? Researchers investigated this question by measuring pupil size, which is a proxy for mental effort, in a memory task. Participants were asked to memorize ambiguous colors that could be seen as a blend between two categories (like a reddish-orange). They found that when participants had to remember only one color, pupil size was larger when maintaining an ambiguous representation. This suggests that continuously representing the blend required more effort. However, as the memory load increased, pupils size became smaller while maintaining both ambiguous and prototypical colors. Strikingly, memory precision for ambiguous colors decreased significantly with higher memory load, implying that participants relied more on categorical representations for the blends. These findings demonstrate that continuously representing visual information in working memory demands more mental effort than categorizing it.

Abstract
Recent studies on visual working memory (VWM) have shown that visual information can be stored in VWM as continuous (e.g., a specific shade of red) as well as categorical representations (e.g., the general category red). It has been widely assumed, yet never directly tested, that continuous representations require more VWM mental effort than categorical representations; given limited VWM capacity, this would mean that fewer continuous, as compared to categorical, representations can be maintained simultaneously. We tested this assumption by measuring pupil size, as a proxy for mental effort, in a delayed estimation task. Participants memorized one to four ambiguous (boundaries between adjacent color categories) or prototypical colors to encourage continuous or categorical representations, respectively; after a delay, a probe indicated the location of the to-be-reported color. We found that, for memory load 1, pupil size was larger while maintaining ambiguous as compared to prototypical colors, but without any difference in memory precision; this suggests that participants relied on an effortful continuous representation to maintain a single ambiguous color, thus resulting in pupil dilation while preserving precision. Strikingly, this effect gradually inverted, such that for memory load 4, pupil size was smaller while maintaining ambiguous and prototypical colors, but memory precision was now substantially reduced for ambiguous colors; this suggests that with increased memory load participants increasingly relied on categorical representations for ambiguous colors (which are by definition a poor fit to any category). Taken together, our results suggest that continuous representations are more effortful than categorical representations and that very few continuous representations (perhaps only one) can be maintained simultaneously.

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