Altering the primacy bias--how does a prior task affect mismatch negativity?

The role in which two tones are first encountered in an unattended oddball sequence affects how deviance detection, reflected by mismatch negativity, treats them later when the roles reverse: a "primacy bias." We tested whether this effect is modulated by previous behavioral relevance assigned to the two tones. To this end, sequences in which the roles of the two tones alternated were preceded by a go/no-go task in which tones were presented with equal probability. Half of the participants were asked to respond to the short sounds, the other half to long sounds. Primacy bias was initially abolished but returned dependent upon the go-stimulus that the participant was assigned. Results demonstrate a long-term impact of prior learning on deviance detection, and that even when prior importance/equivalence is learned, the bias ultimately returns. Results are discussed in terms of persistent go-stimulus specific changes in responsiveness to sound.