Nature 436, 559–562 (2005)

The panels of Fig. 3 in this Letter are incorrectly cited in the text. In the first full paragraph on page 560, these should be: line 7, ‘Fig. 3b, top’; line 11, ‘Fig. 3b’; line 12, ‘Fig, 3c, top’; and line 13, ‘Fig. 3c, bottom’. Other citations of Fig. 3 are correct. In addition, the vertical voltage scale on the bottom trace in Fig. 3d should read from -70 to 10 mV.