A patchwork quilt where each patch is a recording of activity from a small brain region. One patch was stitched on Monday in London, another on Tuesday in New York, and a third a month ago in Tokyo. This mosaic covers the brain only partially, and there is no common seam to join them. Previously, mapping directed connections, who sends signals to whom, required all regions to be recorded simultaneously with a shared clock.

SPIDER works differently. It takes these scattered recordings and, like a skilled tailor, sews them into a single fabric. First, it estimates activity rhythms (power spectra) at each available location, then mathematically reconstructs the global connectivity matrix. Gaps between regions never recorded together are filled using the nuclear norm. Then, via canonical spectral factorization, directed interactions are computed — who influences whom and at which frequencies.

When tested on real data, a surprising picture emerged. In spontaneous activity of 50 mouse brain regions, connections were mostly recurrent, like echoes bouncing in a loop. But in the theta band (4–8 Hz), a clear hierarchy appeared: signals flowed forward, from the hippocampus to other areas. SPIDER found the same hierarchy in intracranial EEG data from 43 patients — now in humans. So this organizational principle is not random and does not depend on species.

Thus, from a pile of mismatched recordings, a coherent picture of the brain's orchestra emerges. The patchwork quilt becomes a road map along which thoughts travel.