title: The second-agent effect: communicative gestures increase the likelihood of perceiving a second agent.
abstract: BACKGROUND
  Beyond providing cues about an agent's intention, communicative actions convey information about the presence of a second agent towards whom the action is directed (second-agent information). In two psychophysical studies we investigated whether the perceptual system makes use of this information to infer the presence of a second agent when dealing with impoverished and/or noisy sensory input.
  
  
  RESULTS
  Participants observed point-light displays of two agents (A and B) performing separate actions. In the Communicative condition, agent B's action was performed in response to a communicative gesture by agent A. In the Individual condition, agent A's communicative action was replaced with a non-communicative action. Participants performed a simultaneous masking yes-no task, in which they were asked to detect the presence of agent B. In Experiment 1, we investigated whether criterion c was lowered in the Communicative condition compared to the Individual condition, thus reflecting a variation in perceptual expectations. In Experiment 2, we manipulated the congruence between A's communicative gesture and B's response, to ascertain whether the lowering of c in the Communicative condition reflected a truly perceptual effect. Results demonstrate that information extracted from communicative gestures influences the concurrent processing of biological motion by prompting perception of a second agent (second-agent effect).
  
  
  CONCLUSIONS
  We propose that this finding is best explained within a Bayesian framework, which gives a powerful rationale for the pervasive role of prior expectations in visual perception.
authors.affiliation: Center for Cognitive Science, Department of Psychology, University of Torino, Torino, Italy.
authors.forename: Valeria
authors.initials: V
authors.lastname: Manera
authors.forename: Marco
authors.initials: M
authors.lastname: Del Giudice
authors.forename: Bruno G
authors.initials: BG
authors.lastname: Bara
authors.forename: Karl
authors.initials: K
authors.lastname: Verfaillie
authors.forename: Cristina
authors.initials: C
authors.lastname: Becchio
mesh_terms: D000293:Adolescent; D000328:Adult; D001499:Bayes Theorem; D003142:Communication; D005260:Female; D005868:Gestures; D006801:Humans; D008297:Male; D010465:Perception / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D055815:Young Adult