In this paper, we elaborate on the idea that the difference in implied volatilities estimated from index puts and calls may contain information about market sentiment that is not fully represented in the VIX. The volatility implied from deep-out-of-the money and out-of-the money index puts contains  information about the market demand for portfolio insurance (bearish signal) while the volatility implied from calls may reflect information about market speculators (bullish signal). The volatility spread should be inversely related to market sentiment.
In addition, market sentiment is a variable that is hard to observe. It is usually inferred from other data, often the VIX itself. If the IV spread is highly correlated with II sentiment, and has some predictive power for levels of S&P 500 index (probably short-term), then it could be used as an easily observable proxy for market sentiment.       

Data

VIX (http://www.cboe.com/products/vix-index-volatility/vix-options-and-futures/vix-index/vix-historical-data)
Investor Intelligence Sentiment Data
S&P 500 index
Option metrics

Results

Correlations