  We use inelastic neutron scattering to study the temperature and energy
dependence of the spin excitation anisotropy in uniaxial-strained
electron-doped iron pnictide BaFe$_{1.9}$Ni$_{0.1}$As$_2$ near optimal
superconductivity ($T_c=20$ K). Our work has been motivated by the observation
of in-plane resistivity anisotropy in the paramagnetic tetragonal phase of
electron-underdoped iron pnictides under uniaxial pressure, which has been
attributed to a spin-driven Ising-nematic state or orbital ordering. Here we
show that the spin excitation anisotropy, a signature of the spin-driven
Ising-nematic phase, exists for energies below $\sim$60 meV in
uniaxial-strained BaFe$_{1.9}$Ni$_{0.1}$As$_2$. Since this energy scale is
considerably larger than the energy splitting of the $d_{xz}$ and $d_{yz}$
bands of uniaxial-strained Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ near optimal
superconductivity, spin Ising-nematic correlations is likely the driving force
for the resistivity anisotropy and associated electronic nematic correlations.
