Figure 6 plots monthly S&P 500 index and implied volatility spread for both deep-out-of-the-money and out-of-the-money options. I use only options with maturity within 14 - 40 days. DOTM spread exhibits greater fluctuation. Similar to VIX, IV spreads tend to move in the opposite direction of S&P 500.
Figure 7 compares IV spreads and VIX. It seems that they have very similar movements. The correlations between VIX and DOTM spread and VIX and OTM spread are 0.7 and 0.58, respectively. Both are significant at 1% level.