Guided by Damasio's (2003) formulations on background emotions, this study examined the contour of infant affect during interactions with mother and father in relation to the emergence of symbolic expression. One hundred parents and infants were observed in face-to-face interactions and in play sessions at the toddler stage. Parent's and infants' affective states were coded in 1-s frames, and synchrony was assessed. Toddlers' play was microcoded for symbolic level and for reciprocity and intrusiveness. Infant affective contour with mother was rhythmic with 1 episode of positive arousal framed by social gaze. Affective contour with father contained several peaks of positive arousal of shorter duration. Symbolic complexity was comparable and preserved the parent-specific contours, with quicker latencies, higher frequencies, and shorter durations of complex symbolic episodes with father. Sequential relations emerged between parent's and child's symbolic expression, and maternal reciprocity and intrusiveness were sequentially linked to symbolic expansion or constriction, respectively. Parent-infant synchrony and the parent's support of toddler symbolic play predicted symbolic complexity. The need to include time in research on emotions and the dyadic origins of positive emotions are discussed.Keywords: synchrony, arousal, symbolic play, positive emotions, fathersIn his recent book on the neurobiology of emotions, Looking for Spinoza, Antonio Damasio (2003) differentiated three classes of emotions: primary emotions, referring to the basic Darwinian emotions observed across cultural communities and a range of mammalian species; social emotions, those related to the self-inrelationships, such as empathy, shame, pity, or pride; and background emotions. Background emotions, the least researched group of the three, refer to the contour of affect as it is played out in time (e.g., surging, fading, accelerating, exploding, etc.) and mark the organization of arousal and affect into patterned configurations that, although not easily captured by the language of discrete emotions, provide an overall framework for the organization of the self and likely depend on a distinct brain circuitry (Damasio, 1999). Background emotions, therefore, define the ongoing component in emotions, and the central arena for their expression is the social context. During interpersonal communication, individuals perceive and respond to the partner's micro-level behaviors as they cohere into a unified affective message, such as tone of voice, direction of gaze, facial expressions, level of arousal, muscle tone, or body orientation, and the ability to follow second-by-second shifts in such behaviors is essential for the participation in any emotional exchange. Infants are sensitized to the temporal components of emotional communication as soon as they enter the social world, at about 2-3 months of age (D. N. Stern, 1985), through the parent's ongoing synchrony with the infant's micro-level behaviors (Tronick, 1989). Describing the parent-infant affect ...