The Effect of Action on Causal Perception in 3- and 4½-month-old Infants

Research has shown that infants perceive causality beginning at the age of 6 months. However, a recent study demonstrated that 4! -month-old infants perceive causality when they are given the ability to perform causal actions by wearing red mittens covered in Velcro that attach to Velcro on green toy balls. The current experiments examined whether the perceptually similarity between the objects infants interacted with and the stimuli used in the test events accounted for infant’s perception of causality in these events. Results show that the particular objects used in the action task do not constrain 4! -month-old infants’ perception of causality in simple launching events. The data also reveal that 3-month-old infants do not respond to simple launching events in terms of either causality or continuity, following action experience. Results are discussed in terms of the developmental progression of causal perception, as well as possible mechanisms underlying this development. Action and Causality 3 The Effect of Action on Causal Perception in 3and 4! -month-old Infants Perceiving cause and effect is an essential component of humans’ understanding of the world around them. Piaget (1953) proposed that causal understanding emerges within the first year of life through a pattern of activity that he called a secondary circular reaction. According to Piaget, infants engage in a cycle of accidental action accompanied by perception of the effects of that action. This leads to further action – because infants are stimulated by their effect on the world – and ultimately results in cognitive development. Recently, developmental scientists have built upon Piaget’s ideas, investigating both the time course of the development of causal perception in infancy as well as the role that agentive experience may play in facilitating infants’ ability to perceive causality in simple events. Developmental psychologists have long debated the origins of causal perception. Almost half a century ago, Michotte (1963) investigated adults’ perception of causal illusions using a set of direct launching, delayed launching, and no collision stimulus events. In direct launching events, a simple object moved from left to right across a screen until it made contact with a second simple object, which moved immediately following the collision. Adults perceived such direct launching events as causal, and they perceived both delayed launching (in which there was a temporal delay between the collision and the time the second object moved) and no collision (in which the first object stopped moving before making contact with the second object) events as non-causal (Michotte, 1963). Studies that investigate infants’ causal perception often employ habituation and test events similar to those used by Michotte. Based on several studies in which infants at 6! months of age perceived launching events similarly to adults in terms Action and Causality 4 of causality, Leslie (1982, 1984, 1988, 1995; Leslie & Keeble, 1987) proposed that an innate perceptual module, which functions from infancy and is subject to little or no developmental change, is responsible for causal understanding. Several recent empirical studies, however, support an alternative explanation, providing evidence that causal understanding does undergo a developmental progression during the first year of life. By 6 months of age, infants detect the internal structure of Michotte-like launching events in addition to changes on a lower, perceptual level (Leslie 1984). In Leslie’s (1984; Experiment 1A) study, infants were habituated either to a direct launching event or to movement of a single object along the same trajectory. When tested on the reversal of these events, infants in the direct launching condition showed greater recovery of looking time to the reversal event compared to infants in the single movement condition. This provides evidence that infants at 6! months of age are sensitive to changes in the submovements of direct launching events in addition to more global perceptual changes, such as a switch in spatial direction. Leslie (1984) also demonstrated that 6! -month-olds infants detect causal relations in Michotte-like launching events and distinguish a causal event from other events sharing the same spatio-temporal characteristics. Leslie (1984; Experiment 2) habituated two groups of infants to different launching events, then tested the infants on launching events that contrasted with the habituation event in terms of both contact (whether or not the two objects physically touched) and the presence of a delay (whether or not the second object began moving immediately after the first object stopped moving). Thus, the two groups of infants saw two different pairs of habituation and test events that each shared the same spatial and temporal differences. One pair, however, Action and Causality 5 consisted of two non-causal events whereas the other pair consisted of a one causal and one non-causal event. Infants who were shown this second pair of events dishabituated more at test than infants shown two non-causal events, even though both pairs of events differed equally in their physical characteristics, indicating that infants at 6! months of age are sensitive to causal relations. Oakes and Cohen (1990) conducted a similar study to contrast 6and 10-monthold infants’ perception of the physical characteristics of launching events with their perception of causality. Unlike Leslie’s study with 6! -month-olds, Oakes and Cohen (1990) found no evidence of causal perception at 6 months of age. In addition, Oakes and Cohen (1990) found that by 10 months of age, infants responded only to the causality of the events, treating different non-causal events as equivalent and no longer responding to their spatio-temporal differences as infants did in Leslie and Keeble’s (1987) study. A later study by Oakes (1994) revealed that by 7 months of age infants respond to launching events based only on causality and not on differences in the spatial and temporal features of the events. In addition, studies by Cohen and Amsel (1998) with 4-, 5! -, and 6"-month-olds revealed that even before responding to the spatio-temporal properties or causality of launching events, 4-month-olds respond to direct launching, delayed launching, and no collision events based solely on their continuity. By 5! months of age, infants respond on the basis of spatial and temporal perceptual features of the events, as well their continuity, and at 6" months of age infants respond to causality as well as to the spatial and temporal features of the events (Cohen & Amsel, 1998). The clear developmental progression evident in these studies of infants’ response first to the continuity, then to the physical characteristics, and finally to the causality of Action and Causality 6 Michottian launching events, calls for an alternative to Leslie’s innate modular account of infants’ causal perception. Cohen (1988, 1991) proposed an information-processing explanation of infants’ development of causal perception. According to Cohen (1991), systematic developmental changes in how infants process visual information dictate to which aspects of a launching event infants are capable of attending. Cohen (1991) suggested that infants are initially able to process only simple features of objects, such as shape and type of motion, which may explain why 4-month-olds attended to the continuity of movement of the stimuli in Cohen and Amsel’s (1998) study. According to Cohen (1991), infants are later able to process relations among object features at around 5 to 7 months of age, which allows them to begin processing the physical properties of the events as well as the individual objects involved in the events. By 7 to 10 months of age, infants can process relationships among different objects and their actions and integrate them into whole events, allowing them to perceive causality. Cohen’s information-processing explanation aligns well with the time course described by years of research into the developmental progression of infants’ causal perception. Recent empirical findings, however, call for a reworking of Cohen’s theory, suggesting that infants may be capable of perceiving causality at as young as 4! months of age if first given laboratory experience performing causal actions (Rakison, 2009). Rakison proposed that infants are capable of perceiving causality and are sensitive to the causal effects of their own actions before 6 months of age. According to Rakison, infants are unable to generalize from their perception of real-world events to the simple Michottian events used in most laboratory studies because these events are too abstracted from the causal events with which infants are familiar. In Rakison’s study, 4! -month-old Action and Causality 7 infants wore red mittens that were either covered in Velcro, giving infants the ability to perform causal actions by picking up green toy balls also covered in Velcro, or they wore smooth mittens that did not allow infants to engage in causal actions. After habituation to direct launching events involving a red ball moving left to right across the screen and colliding with a green ball, infants were presented with three test events. The familiar test event was a replica of the direct launching habituation event, except the objects now moved from right to left across the screen. The causal switch event changed the position of the red and green balls such that the green ball moved from right to left across the screen, contacting the red ball and causing it to move. The non-causal event was similar to the causal switch test event except that the green ball stopped before contacting the red ball, which then began moving after a 1s delay. Infants in both the Velcro and smooth conditions looked significantly longer at the non-causal test event than the familiar test event, which indicated that they could detect differen