Lee Lin¹ (), Shunsuke Takano¹, Takato Arima¹, Kenichi Ueno², Ichiro Kuriki¹; ¹Graduate School of Science and Engineering, Saitama University, Saitama, Japan, ²Center for Brain Science, RIKEN, Wako-shi, Saitama, Japan

Humans can perceive millions of distinct color shades, yet they typically describe these shades using just tens of color terms. Understanding how these two distinct representations emerge in the brain remains an open question. Here, we investigated the neural representations underlying color appearance and color categories, a fine-grained representation and a coarser representation that often engages terms, respectively, using functional magnetic resonance imaging (fMRI) technique. We used checkerboard patterns with 12 hues evenly sampled from the isoluminant plane of cone contrast space. Participants viewed this identical set of stimuli while performing one of three tasks. In the color categorization task, participants pressed one of five buttons to report a perceived color category. In the hue-scaling task, participants reported the color appearance of the stimulus by pressing buttons three times to indicate the relative component of the four primary colors: red, green, blue, and yellow. In the baseline task, participants were asked to ignore colors and conduct an attention-demanding task using an RSVP (rapid visual stimulus presentation) letter stream at the stimulus center. Each voxel’s responses across the 12 hues were fit with multiple von Mises functions to increase signal-to-noise ratio. Fitted responses at the 12 hues were then used for representational similarity analysis (RSA) and multidimensional scaling (MDS). Using the categorical clustering index in Brouwer and Heeger (2013) to the 2-dimensional MDS results, we found that early visual areas exhibited higher categorical clustering during the hue-scaling (appearance) task than during the category task. This suggests that appearance-based processing may rely on relatively small number of hue-selective mechanisms than previously assumed. Together with other groups’ reports, our findings indicate that categorical information is not restricted to higher-level visual areas but is evident even in early visual cortex, and the task demands shape how color representation is expressed across the visual areas.

Acknowledgements: The Japanese Society for the Promotion of Sciences (JSPS) KAKENHI No. JP24H00702 to IK.