Will the glass be half full or half empty? Brain potentials and emotional expectations.

Brainwave responses to words in context depend on semantic and world-knowledge expectations. Using the N400 component of event-related potentials as an index of word expectation, we explored brain responses to negatively and positively biased sentence frames randomly presented with their emotionally matched highly expected outcome or with violations that included switches to unexpected emotionally opposite outcomes or nonsense. Nonsense elicited large N400 responses regardless of the bias of the preceding sentence frame. Unexpected emotionally opposite outcomes elicited smaller than nonsense N400 responses and subsequent post-N400 frontal positivities, both unaffected by sentence frame bias. Over a midline-posterior scalp region, expected positive outcomes elicited larger N400 responses than negative ones, despite a high and matched word probability. Our study reveals that brains respond to unexpected emotional outcomes regardless of the direction of the emotional switch and hints at the possibility that the strength of positive and negative expectations may be adjusted before experiencing unexpected events.