  Readiness potential is a widely observed brain activity in several species
including crayfish before the spontaneous behavioral initiation. However, it is
poorly understood how this spontaneous activity is generated. The hypothesis
that some specific, dedicated site is responsible for the spontaneity has been
questioned. Here, by using intracellular recording and staining of the brain
neurons in crayfish and modeling using the sandpile, which is the original
model of self-organized criticality (SOC), we show that readiness potential can
emerge everywhere in the brain because it is a SOC system. Despite the
diversity in neurons and their morphology, brain neurons showed signatures of
criticality and readiness potential. We find that the previously known
readiness potential in a neuron is a consequence of the critical behavior of
the entire network. Indeed, seemingly unrelated membrane potential activity in
neurons in different animals can shape readiness potential when its time series
are averaged after their alignment with respect to the spontaneous behavioral
initiation. We show that the sandpile model not made for the potential, can
form the premovement buildup activity similar to readiness potential. Scaling
properties of the synaptic avalanches are in line with those of vertebrate
species; thus, not only is the critical brain hypothesis supported in crayfish,
but our findings might also provide a unified view of the basis of spontaneity
in animal behavior.
