This study examined 'prepulse inhibition' in the context of an auditory two-tone discrimination task performed by 15 healthy subjects. In order to distinguish between masking or excitatory information processes, weak acoustic pulses immediately preceded or succeeded a target or non-target tone in the discrimination. Button-press performance was compared with response in a no-pulse condition. Response bias (beta) became more liberal in the 100 ms prepulse and 200 ms postpulse conditions. Beta correlated with P3b amplitude measures of the event-related potential. The weak pulses increased temporarily the cortical excitability, as measured by the decreased amplitude of the P3b component and thus facilitated a more liberal response bias.