Figure 2.

Measuring tonotopic gradients. A, Stimuli are displayed in spectrograms for PTs, N-BPN (3 frequencies; N-BPN3), and B-BPN; x-axes span 0 to 2.6 s, and y-axes span 0 to 16 Hz (linear scale). B, Group frequency-preference (tonotopic) maps are shown on cortical surfaces; color indicates the center frequency eliciting the greatest BOLD activity. Data for three stimulus types are shown, including PT, N-BPN3, and B-BPN. Note the similarity between N-BPN3 and N-BPN maps in Figure 1D. Isofrequency contours are marked as black (solid and dotted) lines based on frequency reversals determined from gradient maps in C. C, Local frequency gradients are displayed, where color indicates the gradient direction at each point on the tonotopic maps from B, moving from low to high frequencies. Black lines mark gradient reversals between 0/360 and 180° (green to blue, respectively), and black dotted lines mark reversals between 90 and 270° (dark to light colors, respectively). An example of gradient flow is illustrated in a red inset at the right in B and C, and the corresponding points on the rightmost maps are marked. Arrows indicate the direction of tonotopic gradient flow, from low to high frequencies.