Summers, the former Treasury secretary, has called on the Securities and Exchange Commission and relevant exchanges to investigate Monday's spike in the VIX. "My understanding is that Monday's volatility in the VIX was somewhat artificial due to the use of illiquid tools in calculating the index," Summers said on a television program on Friday. After Monday's spike in the VIX index, volatility experts said the wild swings in the index could have been caused by several technical factors, including an apparent lack of liquidity, some short covering that failed volatility bets, or simply the way volatility was calculated.