Extended Data Fig. 9. Across-probes mean unipolar (left), bipolar (middle), and CSD-derived (right) relative power maps.

Left: Relative power maps were computed on a unipolar LFP referenced to an outer guide tube (as in Figs. 1–3). Middle: Data was first locally referenced to a bipolar montage by subtracting one electrode from its immediate neighbor along the probe. Subsequently, power was computed, and then the relative power across channels was calculated. Right: We first computed the CSD, then power, and then relative power across channels. The laminar depth of all maps is shown with respect to the alpha-beta/gamma cross-over channel from the unipolar data. We computed the spectrolaminar pattern based on the CSD (and, for completeness, the bipolar signal, see Methods) to determine whether in the spectral domain, CSD could provide better spatial estimates of the position of layer 4 than in the time domain. In contrast to the unipolar spectrolaminar maps, the bipolar and CSD spectrolaminar maps contained less features and peak relative power occurred in the superficial layers across frequencies. To determine whether the CSD/bipolar spectrolaminar maps contain more anatomical information compared to the unipolar spectrolaminar maps, we measured the distance between ‘power drop-off’ and layer 4. Power drop-off is the laminar depth at which CSD/bipolar relative power is equal to 0.6 (this often results in multiple intercepts, the value closest to layer 4 was used, see Methods). The mean distance from the CSD power drop-off to layer 4 across all areas was 96 μm (±126 μm SEM) and the distance from bipolar drop-off to layer 4 across all areas was 125 μm (±122 μm SEM). In contrast to the distance from the unipolar alpha-beta/gamma crossover to layer 4 across all areas (46 μm mean ±51 μm SEM). Both the CSD and bipolar drop-off to layer 4 distance metrics were more variable than the unipolar alpha-beta/gamma crossover (Ansari-Bradley test, P < 0.01 for unipolar vs. CSD, P < 0.01 for unipolar vs. bipolar). In general, visualization of the bipolar or CSD spectrolaminar pattern are complimentary to unipolar pattern, especially as they appear to demarcate the borders of superficial cortical layers. However, unipolar spectrolaminar pattern offers a more complex profile, including directionality. That is, if a probe is inserted into a deep region, far from the cortical surface, unipolar spectrolaminar power would provide more information, such as whether the gray matter is upright or inverted (L1-to-L6 or L6-to-L1; for example, area MST is inverted in our recordings, see Fig. 1b). Bipolar and CSD gamma drop-off were less informative since they do not provide directionality. Manual curation and comparison with unipolar crossover are required to determine the location of L4 in reference to bipolar or CSD relative power. Overall, bipolar and CSD spectrolaminar patterns are useful complements that could be plotted in addition to unipolar maps and are especially useful in identifying superficial layers. However, unipolar spectrolaminar patterns provide more information in identifying layer 4.