Influence of burst firing on correlated activity. A: illustration of the experimental procedure to reversibly block feedback input from parallel fibers unto pyramidal cells. A pressure pipette is lowered close to the recording site and is used to eject 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX), thereby blocking parallel fiber feedback input unto pyramidal cells. B: CCGs in the absence of stimulation under control (black), block (red), and recovery (green). C: changes in burst fraction BF and cross-correlation coefficient R during the block (red) and after recovery (green). Feedback blockade significantly reduced both BF and R, and both quantities returned to values that were not significantly different from those obtained under control conditions. Statistical significance was ascertained using a pairwise t-test at the P = 0.01 level with n = 21 for block data and n = 18 for recovery data.