  We present a multiwavelength analysis of the simultaneous optical and X-ray
light curves of the microquasar V404 Cyg during the June 2015 outburst. We have
performed a comprehensive analysis of all the INTEGRAL/IBIS, JEM-X, and OMC
observations during the brightest epoch of the outburst, along with
complementary NuSTAR, AAVSO, and VSNET data, to examine the timing relationship
between the simultaneous optical and X-ray light curves, in order to understand
the emission mechanisms and physical locations. We have identified all optical
flares which have simultaneous X-ray observations, and performed
cross-correlation analysis to estimate the time delays between the optical and
soft and hard X-ray emission. We have also compared the evolution of the
optical and X-ray emission with the hardness-ratios. We have identified several
types of behaviour during the outburst. On many occasions, the optical flares
occur simultaneously with X-ray flares, but at other times positive and
negative time delays between the optical and X-ray emission are measured. We
conclude that the observed optical variability is driven by different physical
mechanisms, including reprocessing of X-rays in the accretion disc and/or the
companion star, interaction of the jet ejections with surrounding material or
with previously ejected blobs, and synchrotron emission from the jet.
