Mediofrontal negativity signals unexpected omission of aversive events

Research based on event-related potential (ERP) reported mediofrontal negativities following unexpected negative feedback or performance error. Some authors proposed that these signals reflect reward prediction error for worse than expected outcomes, while others suggested that mediofrontal negativities express medial prefrontal cortex coding for unexpected non-occurrence of a predicted outcome, whether worse or better than expected. Many studies found mediofrontal negativities coding for unexpected negative outcomes; however, few studies found them after unexpected positive outcomes. The present study investigated ERP and skin conductance response (SCR) to the unexpected omission of electric shocks during Pavlovian aversive conditioning. To manipulate expectancies, participants were presented with visual stimuli paired with electric shock on either 80% (CS+1) or 20% (CS+2) of trials. SCR analysis confirmed higher shock-delivery expectancy for CS+1, relative to CS+2. ERP analysis evidenced a stronger negative frontocentral ERP component after unexpected, relative to expected, shock-omission. Methodological and theoretical implications are discussed.