The first is a decline in the natural rate of interest, while the second is a decline in inflation expectations.1,2 The natural rate of interest is the real interest rate that is neutral to economic activity in that it 1  Paul R. Krugman, "It's Baaack: Japan's Slump and the Return of the Liquidity Trap," Brookings Papers on Economic Activity, no. 2 (1998): 137-205.
2 Jess Benhabib, Stephanie Schmitt-Grohé, and Martín Uribe, "The Perils of Taylor Rules," Journal of Economic Theory, vol. 96, no. 1-2 (2001): 40-69. 1  will neither accelerate nor decelerate the economy.