No study has investigated the
No study has investigated the neural correlates of pro-social preferences in young infants. However, a few studies have demonstrated that the infant ERP component P400 is related to processing of goal directed actions such as grasping (Bakker et al., in press) and pointing (Gredebäck et al., 2010; Melinder et al., in press), to emotional processing (Leppanen et al., 2007), biological motion (Reid et al., 2006) and gaze direction (Senju et al., 2006). In this context it is important to note that P400 amplitudes are larger for functional and goal directed actions (reaching for, or pointing toward objects, looking at interesting sights) than control stimuli that lack these object directed properties.
In this study we examine the neural correlates of infants’ pro-social preferences by measuring EEG and target ERP components hypothesized to be sensitive to pro- and anti-social agents in the hill-climber paradigm (Hamlin et al., 2007). More specifically, 6-month-old infants were presented with two scenarios. In Experiment 1 one agent helps another agent (ball with eyes) to reach the top of a hill, whereas a different agent hinders the circular agent from reaching the top of a hill. In Experiment 2 one agent pushes up an inanimate ball (without eyes) to the top of the hill and another agent pushed the ball down the hill. Following these scenarios infants were presented with repeated images of the two agents that helped or hindered in Experiment 1 and moved up vs down in Experiment 2 (only one image was presented on each trial) and ERP components for these images were analyzed (for similar designs see Kaduk et al., 2013; Parise et al., 2008).
We hypothesize that P400 amplitudes will differ between pro- and anti-social agents (Experiment 1). Furthermore, given that prior studies have demonstrated larger amplitudes of P400 for congruent than incongruent pointing (Gredebäck et al., 2010) and gaze direction (Senju et al., 2006), as well as for upright over inverted biological motion point-light displays (Reid et al., 2006), we predict a larger P400 in response to agents that previously have helped over agents that previously hindered others. The hypothesized common denominator of previously and currently investigated social stimuli is a larger P400 amplitude for functional and typical buy Aminoallyl-dCTP - Cy3 (pointing to objects, looking at interesting sights, and agents that help others). No difference in P400 amplitudes is expected in Experiment 2 where the agent that is being helped and/or hindered is replaced by an inanimate ball.
We also analyzed the Nc component. This mid-latency component occurs approximately 300–700ms after stimulus onset and is most prominent at fronto-central electrodes. It reflects attentional orienting to salient stimuli (Courchesne et al., 1981) and/or a general attentional arousal (Richards, 2003), as it is larger to infrequent than frequent stimuli (e.g. Courchesne et al., 1981) and is larger during periods of sustained attention (Richards, 2003). We examined the Nc in order to preclude attention and other lower level accounts that may provide an alternative explanation for differences found between conditions in the P400.
General discussion Six-month-old infants’ neural activity, as measured by the ERP component P400, differed when observing agents that previously helped and agents that previously hindered others. The P400 amplitude differences did not manifest themselves in Experiment 2, where upward and downward moving agents had no pro- or antisocial connotations. This is the first study that relates detection of prosociality to a specific ERP component and the first to demonstrate that infant P400 amplitudes are mediated by an agent\'s history of helping or hindering others (for prior work on EEG oscillations in 14-month-olds in the context of prosocial behavior see Paulus et al., 2013). The current finding appears consistent with behavioral data at 6 months of age. Prior work has documented overt preferences for helping over hindering agents, measured by both looking times and preferential reaching (Hamlin et al., 2007, 2010) at the same age. We did not measure infants’ behavioral responses to the stimuli, but it should be noted that the current stimuli very closely resemble the stimuli used in previous studies to demonstrate behavioral preferences for pro-social agents and avoidance of antisocial agents in infants of the same age (Hamlin et al., 2007). It is therefore very likely that the neural correlates of social valence processing demonstrated here represent the first stages of the neural process leading to infants’ expression of pro-social preferences.