  A detailed study of the Inelastic Neutron Scattering (INS) spectra of the
high-$T_c$ cuprates based on the Fermi liquid (FL) picture is given. We focus
on the issue of the transformation between the commensurate and incommensurate
(IC) excitation driven by frequency or $temperature$. For
La$_{2-x}$Sr$_x$CuO$_4$ (LSCO), the condition of small $\Delta(0)/v_F a$ (where
$a$ is the lattice constant, and henceforth will be set to 1) can
simultaneously reproduces the always existing IC peaks in the superconducting
(SC) and normal state, and the always fixed location at temperature or
frequency change. For YBa$_2$Cu$_3$O$_{6+x}$ (YBCO), a moderate $\Delta(0)/v_F
a$ and proximity of the van Hove singularity (vHS) at ${\bar M}=(0,\pi)$ to the
Fermi level can reproduce the frequency- and temperature-driven shifting IC
peaks in the SC state, and the vanishing of the IC peak in the normal state.
The commensurate peak is found to be more appropriately described as a random
phase approximation (RPA) effect. We address the conditional peak shifting
behavior to a refined consideration on the nesting effect which is previously
overlook. As a result, both the data on LSCO and the recent data on YBCO (on
YBa$_2$Cu$_3$O$_{6.7}$ by Arai $et$ $al.$ and YBa$_2$Cu$_3$O$_{6.85}$ by
Bourges $et$ $al.$) can be reasonably reconciled within a FL picture. We also
point out that the one-dimensional-like data by Mook $et$ $al.$ on a detwinned
and more underdoped sample YBa$_2$Cu$_3$O$_{6.6}$ could be due to a gap
anisotropy effect discussed by Rendell and Carbotte, and we proceed to suggest
a way of clarifying it.
