Summary Although spiral waves are ubiquitous features of nature, and have been observed in many biological systems, their existence and potential function in mammalian cerebral cortex remains uncertain. Using voltage-sensitive dye imaging, we found that spiral waves occur frequently in the neocortex in vivo, both during pharmacologically induced oscillations and during sleep-like states. While their lifespan is limited, spiral waves can modify ongoing cortical activity by influencing oscillation frequencies and spatial coherence, and by reducing amplitude in the area surrounding the spiral phase singularity. During sleep-like states, the rate of occurrence of spiral waves varies greatly depending on brain states. These results support the hypothesis that spiral waves, as an emergent activity pattern, can organize and modulate cortical population activity on the mesoscopic scale and may contribute to both normal cortical processing and to pathological patterns of activity such as those found in epilepsy.

Earth's sand seas (dune fields) experience winds that blow with different strengths and from different directions in line with the seasons. In response, dune fields show a rich variety of shapes, from crescentic barchans to star and linear dunes. These dunes commonly exhibit complex and compound patterns with a range of length scales and various orientations, which up to now have remained difficult to relate to wind cycles. Here, we develop a model for dune orientation that explains the coexistence of bedforms with different alignments in multidirectional wind regimes. This model derives from subaqueous experiments, which show that a single bidirectional flow regime can lead to two different dune orientations depending on sediment availability, i.e., the erodibility of the bed. Sediment availability selects the overriding mechanism for the formation of dunes: increasing in height from the destabilization of a sand bed (with no restriction in sediment availability) or elongating in a finger on a nonerodible surface from a localized sand source. These mechanisms drive the dune orientation. Therefore, dune alignment maximizes dune orthogonality to sand fluxes in the bed instability mode, while dunes are aligned with the mean sand transport direction in the fingering mode. Applied to Earth's deserts, the model quantitatively predicts the orientation of rectilinear dunes and their superimposed patterns. This field study suggests that many linear dunes on Earth elongate from sources, and are simply aligned with the mean sand transport direction.