Infants’ response to moving object occlusion events in which an object passes behind an occluder has a long history as an indicator of infants’ awareness of object persistence or permanence (Bower, Broughton, & Moore, 1971; Goldberg, 1976; Moore, Borton, & Darby, 1978). This early work relied on detecting anticipation of object re-emergence following occlusion. But alternative interpretations of anticipation were identified (Goldberg, 1976; Moore et al., 1978) that led to the conclusion that, on its own, anticipation of re-emergence did not safely indicate that the infant had interpolated the missing segment of the object’s trajectory. Instead, it was possible that infants were anticipating a repeated cycle of discrete events each side of the occluder rather than the emergence of a single persisting object (Goldberg, 1976).
More recently, precise eye-tracker measures of visual tracking (Gredebäck, & von Hofsten, 2004
; Johnson, Amso, & Slemmer, 2003a
; Rosander & von Hofsten, 2004
) and habituation novelty data (Bremner, Johnson, Slater, Mason, Foster, Cheshire, & Spring, 2005
; Bremner, Johnson, Slater, Mason, Cheshire, & Spring, 2007
; Johnson, Bremner, Slater, Mason, Foster, & Cheshire, 2003b
) have produced converging evidence. This work has led to the conclusion that infants’ ability to perceive an object’s trajectory as continuous improves over the early months after birth ( illustrates the habituation novelty displays used). Specifically, 2-month-olds appear unable to perceive trajectory continuity (Johnson et al. 2003b
), whereas 4-month-olds appear to perceive trajectory continuity when the spatial or temporal gap in perception is small (Bremner et al., 2005
; Johnson et al., 2003b
), and 6-month-olds’ ability appears to be well developed (Johnson et al., 2003a
). This has led some investigators to conclude that there are perceptual origins for object identity and permanence (Bremner et al., 2005
, Johnson et al., 2003b
): As infants become increasingly able to detect and retain information specifying perceptual completion, such as collinearity and temporal contiguity, their ability to recognize relations among disparate parts of a visual scene begins to improve. And these perceptual advances form the basis for development of awareness of object identity and permanence.
Figure 1 Schematic depiction of events shown to infants in Johnson et al. 2003b) to gauge perception of trajectory continuity. A: Habituation event. A ball moves behind an occluding screen and re-emerges, then returns on a repetitive cyclic trajectory. B: Discontinuous (more ...)
These studies presented information for the object’s trajectory solely in the visual modality. However, there is evidence indicating modification of very young infants’ response to visual stimuli following auditory stimulation, indicating intersensory matching and interaction of stimulus intensity (Lewkowicz & Turkewitz, 1980
). Also there is strong evidence that redundant presentation of information across modalities recruits infant attention and enhances learning (the intersensory redundancy hypothesis [IRH]: Bahrick, Flom, & Lickliter, 2002
; Bahrick & Lickliter, 2000
; Bahrick, Lickliter, & Flom, 2004
With respect to object movement, it has been demonstrated that 6-month-old infants are sensitive to congruence between timing of a sound and timing of a change in object movement direction, and that this sensitivity is dependent on spatial contiguity between auditory and visual stimuli (Lawson, 1980
). Additionally, there is some work investigating detection of dynamic auditory-visual correspondences for movements in the near-far plane (Pickens, 1994
; Walker-Andrews & Lennon, 1985
). These studies indicate an ability to form correspondences between sound intensity and object distance at 4 to 5 months. And recently it has been demonstrated that infants from 2 to 8 months of age detect auditory-visual congruence and incongruence in events involving laterally moving visual objects and stereophonically simulated moving sounds (Bremner, Slater, Johnson, Mason, Spring, & Bremner, in press
). During habituation trials, infants detected congruence of visual and auditory information for movement and showed recovery of attention when the two sources of information became incongruent, specifically, when the auditory stimulus appeared to move left to right as the visual stimulus moved from right to left, and vice versa.
Evidence of infants’ sensitivity to intersensory information about an object’s trajectory raises the issue of whether providing auditory as well as visual information about an object’s trajectory would enhance young infants’ perception of trajectory continuity across an occlusion. According to the intersensory redundancy hypothesis, optimum conditions for deriving benefit from provision of multisensory information would be those in which both visual and auditory information provide congruent information about an object’s trajectory. Under such conditions, visual and auditory information would specify the object’s trajectory redundantly, and so could be expected to enhance perception of trajectory continuity as the object passed behind an occluder. However, in the present case there is an additional reason why supplementing the visual event with auditory information might enhance perception of trajectory continuity. Specifically, although there is a discontinuity in visual perception of the object as it passes behind an occluder, auditory information arising from a sounding object need not show this discontinuity. And although sound intensity generally reduces when an object passes behind an occluder, sound is not usually eliminated and may not reduce sufficiently to be detected by infants.
In order to benefit from presentation of intersensory information regarding an object’s trajectory, it is of course necessary that infants are capable of localising sound with sufficient accuracy to detect change in location from sound alone. Some investigators have detected a more or less linear increase in localisation ability with age (Morrongiello, 1988
; Morrongiello, Fenwick, & Chance, 1990
), whereas other work suggests that auditory localisation is hard to elicit at around 2 months (Clifton, Morrongiello, Kulig, & Dowd, 1981
; Field, Muir, Pilon, Sinclair, & Dodwell, 1980
; Muir, Clifton, & Clarkson, 1989
). However, although 4-month-olds’ auditory localisation ability falls well short of adult levels, it is well established, with a minimum discriminable auditory angle of around 18 degrees (Morrongiello et al., 1990
). And the fact that for static stimuli auditory-visual spatial co-location occurs in newborns (Morrongiello, Fenwick, & Chance, 1998
) and older infants (Morrongiello, Fenwick, & Nutley, 1998
), and that it occurs from two months upwards in the case of moving stimuli (Bremner et al. in press
), indicates that auditory localisation is sufficient to detect information specifying object location and change in location.
In the series of experiments reported here we investigate different levels at which auditory information may benefit perception of trajectory continuity. In Experiment 1 we investigate the effect of supplementing visual information with continuous dynamic auditory information specifying the object’s trajectory. In Experiment 2 we investigate whether effects of auditory information are limited to the case in which the sound provides dynamic information for movement, or whether such effects also occur when static auditory information specifies continuity over time but does not provide information for movement. In Experiment 3 we investigate two cases in which auditory information may be expected to detract from perception of continuity, one in which the sound disappears and reappears suddenly when the visual object disappears and reappears, and one in which the sound specifies motion in the opposite direction from that specified by vision.