Effect of APB on On and Off ganglion cell activity in the absence of GABAergic and glycinergic inhibition as evaluated with calcium imaging. A, Application of APB (indicated by horizontalbar) abolishes bursting activity in On ganglion cells but not Off ganglion cells in P21 retina. Bursting activity recovers after the washout of APB. B,Quantitative summary of the effect of APB in On and Off ganglion cells in P17–P22 retinae is shown. Burst rates in APB are expressed as a percentage of the rate calculated for the period immediately preceding the application of APB under conditions of GABAergic and glycinergic blockade. APB reduced the burst rate in Off ganglion cells but preferentially abolished bursting altogether in On ganglion cells. These effects were statistically significant (Mann–WhitneyU test, p < 0.001;r = number of recordings; n = number of cells monitored). Bic, Bicuculline;Stry, strychnine.