Figure 8.

Neural activity projected into sensory-defined subspace reveals sound feature encoding. A, Our approach involves projecting responses of populations of neurons into lower-dimensional condition subspace defined by the stimulus variables (dimensionality reduced from n neurons to p conditions). The eight hypothetical responses in A show idealized population responses that project in corresponding eight points in the AM axis projection versus BW axis projection coordinates. (context dimension not shown.) B, Color-coded stimulus responses (see inset) in the attend AM condition are shown projected into axis-projection subspace. Symbols represent mean responses over simulated trials. Markers representing projections along the AM axis exhibit increasing (red) values, whereas markers representing projections along the BW axis exhibit increasing (blue) values, indicating population-level encoding of sound features. In the upper right quadrant of the inset are purple markers, representing stimuli containing both features, and markers with low color values (including black) are found in the lower left quadrant of the inset. C, Same as B but for the attend BW condition. Axes in B, C are scaled equally. Comparing encoding subspaces across conditions reveals substantial effects of attention on sensory subspace projections. D, Trajectories over time through the sensory subspace for three representative stimuli [AM₀–BW₀ (black), AM₀–BW₃ (blue), AM₃–BW₀ (red)] reveal encoding over the entire course of S2 presentation for the AM attend condition. Trajectories begin at the x and y origin, indicating no sensory evidence for either AM or BW. Directly after stimulus onset, trajectories for each stimulus follow similar paths, then diverge substantially. Trajectories reach a peak of separation, such that AM₀–BW₃ and AM₃–BW₀ projections are in opposite quadrants (upper left and lower right, respectively), both roughly orthogonal to the AM₀–BW₀ projection. Then, during the end of the stimulus response, trajectories approach the origin. E, Same as in D but for the attend BW condition. Trajectories in both conditions exhibit an early, non-selective course wherein each stimulus projects to roughly the same area within the subspace. During the middle of the stimulus, trajectories reach their maximum separation, then trend back toward the origin. These trajectories suggest that feature selection during sound perception evolves over time, beginning with a general detection phase, followed by a discrimination phase, then returning to a non-encoding area of the subspace thereafter. Note that the window size and shift increment here (25 and 5 ms, respectively) in D, E are the same used in Figure 5D–F.