Fig. 2.

Illustration of analysis methods. (A) Spike rate variance for each task variable (task cues/rules, motion directions, and colors) was partitioned into three orthogonal contrasts. One contrast (blue) reflected the actual task-relevant grouping of stimulus items (cue shapes, directions, or colors) into categories, and thus captured between-category variance. The other contrasts (gray) reflected the two other possible non–task-relevant paired groupings of items, and, together, captured all within-category variance. An additional term in the analysis (not depicted) partitions out variance related to the behavioral choice (left vs. right saccade). Details are provided in SI Appendix, SI Methods. (B, Top) The sum of variances for all three contrasts (Σ in A) bounds the between-category variance; the total and between-category variances can be equal only for a perfectly categorical neural population with zero within-category variance. (B, Bottom) Purely sensory-driven population would, instead, have equal variance for all three contrasts, and thus between-category variance would equal the average ($\langle \cdot \rangle$ in A) of all three contrasts. (C) The categoricality index measured where actual neural populations fell between these extremes, in terms of the area between the between-category and sensory lower-bound time series, expressed as a fraction of the full area between the upper and lower bounds. Values of 0 and 1 correspond to purely sensory and purely categorical populations, respectively. Details are provided in SI Appendix, SI Methods.