Bump Attractors and Waves in Networks of Leaky Integrate-and-Fire Neurons

Bump attractors are wandering localized patterns observed in in vivo experiments of spatially extended neurobiological networks. They are important for the brain's navigational system and specific memory tasks. A bump attractor is characterized by a core in which neurons fire frequently, while those away from the core do not fire. These structures have been found in simulations of spiking neural networks, but we do not yet have a mathematical understanding of their existence because a rigorous analysis of the nonsmooth networks that support them is challenging. We uncover a relationship between bump attractors and traveling waves in a classical network of excitable, leaky integrate-and-fire neurons. This relationship bears strong similarities to the one between complex spatiotemporal patterns and waves at the onset of pipe turbulence. Waves in the spiking network are determined by a firing set, that is, the collection of times at which neurons reach a threshold and fire as the wave propagates. We define and study analytical properties of the voltage mapping, an operator transforming a solution's firing set into its spatiotemporal profile. This operator allows us to construct localized traveling waves with an arbitrary number of spikes at the core, and to study their linear stability. A homogeneous ``laminar"" state exists in the network, and it is linearly stable for all values of the principal control parameter. Sufficiently wide disturbances to the homogeneous state elicit the bump attractor. We show that one can construct waves with a seemingly arbitrary number of spikes at the core; the higher the number of spikes, the slower the wave, and the more its profile resembles a stationary bump. As in the fluid-dynamical analogy, such waves coexist with the homogeneous state, and the solution branches to which they belong are disconnected from the laminar state; we provide evidence that the dynamics of the bump attractor displays echoes of unstable waves, which form its building blocks.