Figure 2.

Schematics describing the (a) repeated-background SD and (b) 2AC tasks. In the SD task (a), each trial consisted of two response intervals, R1 and R2. R1 was initiated when the bird pecked the center response port. The reference stimulus (A) played repeatedly until the bird pecked the center response port again, initiating R2. In R2, the reference stimulus repeated a random number of times and then transitioned to the target stimulus (B). Responses to a separate observation port were punished if they occurred before the transition, and rewarded if they occurred after. Failure to respond in R1 and R2 ended the trial without consequence. In the 2AC task (b), trials consisted of a stimulus playback period, initiated by a peck to the center port, followed by a 2 s silent response window. Responses were ignored during presentation of the stimulus, a randomly chosen song comprising a natural sequence of motifs (A–H). Each song was associated with either the left or right port; in the response interval the bird was rewarded or punished if it responded on the correct or incorrect port, and failure to respond ended the trial without consequence.