  Many studies have shown that the excitation and inhibition received by
cortical neurons remain roughly balanced across many conditions. A key question
for understanding the dynamical regime of cortex is the nature of this
balancing. Theorists have shown that network dynamics can yield systematic
cancellation of most of a neuron's excitatory input by inhibition. We review a
wide range of evidence pointing to this cancellation occurring in a regime in
which the balance is loose, meaning that the net input remaining after
cancellation of excitation and inhibition is comparable in size to the factors
that cancel, rather than tight, meaning that the net input is very small
relative to the cancelling factors. This choice of regime has important
implications for cortical functional responses, as we describe: loose balance,
but not tight balance, can yield many nonlinear population behaviors seen in
sensory cortical neurons, allow the presence of correlated variability, and
yield decrease of that variability with increasing external stimulus drive as
observed across multiple cortical areas.
