In addition, discussions of the effects of the buildup in reserves at the Federal Reserve and other central banks often emphasize the imperfect substitutability of reserves for other bank assets, even when those reserves are remunerated at something like a market interest rate.
More generally, while most of the literature on the effects of monetary policy assumes that the federal funds rate is the single relevant tool for monetary policy, the financial crisis has shown that a wide array of policy measures, acting on the prices of different assets, may be needed in extreme circumstances.