  We investigate the effect of dynamic electron correlation on high-harmonic
generation in helium atoms using intense visible light (\lambda=390nm). Two
complementary approaches are used which account for correlation in an
approximate manner: time-dependent density-functional theory and a
single-active-electron model. For intensities I~10^{14} W/cm^2, the theories
are in remarkably good agreement for the dynamic polarization and harmonic
spectrum. This is attributed to a low-frequency collective mode together with a
high-frequency single-electron response due to the nuclear singularity, both of
which dominate electron correlation effects. A time-frequency analysis is used
to study the timing and emission spectrum of attosecond bursts of light. For
short pulses, we find a secondary maximum below the classical cut-off. The
imprint of the carrier-envelope-phase, for the time-integrated spectral density
appears at frequencies above the high-frequency drop-off, consistent with
previous studies in the infrared \lambda~800nm.
