Academic journal article Perception and Psychophysics

Infants' Perception of Depth from Cast Shadows

Academic journal article Perception and Psychophysics

Infants' Perception of Depth from Cast Shadows

Article excerpt

Five- and 7-month-old infants viewed displays in which cast shadows provided information that two objects were at different distances. The 7-month-olds reached preferentially for the apparently nearer object under monocular-viewing conditions but exhibited no reaching preference under binocular-viewing conditions. These results indicate that 7-month-old infants perceive depth on the basis of cast shadows. The 5-month-olds did not reach preferentially for the apparently nearer object and, therefore, exhibited no evidence of sensitivity to cast shadows as depth information. In a second experiment, 5-month-olds reached preferentially for the nearer of two objects that were similar to those used in the first experiment but were positioned at different distances from the infant. This result indicated that 5-month-olds have the motor skills and motivation necessary to exhibit a reaching preference under the conditions of this study. The results are consistent with the hypothesis that depth perception based on cast shadows first appears between 5 and 7 months of age.

In the 15th century, Leonardo da Vinci described a set of techniques that painters could use to portray three-dimensionality on a flat canvas. In his description, he distinguished between two types of shadows: attached shadows and cast shadows. Attached shadows occur when an object's shadow is visible on that same object, such as shading on a ball. A cast shadow occurs when the shadow of one object is seen on a different object or surface, such as a ball's shadow on the ground. Leonardo noted that attached shadows provide information for surface relief, whereas cast shadows indicate a separation in space between objects (Woodworm, 1938).

More recently, computer vision researchers have made detailed descriptions of the information provided by shadows and shading for the perception of object shape and spatial layout (e.g., Horn, 1975; Waltz, 1975), and perception researchers have studied how the visual system uses this information in object and space perception. For example, Koenderink, van Doorn, and Kappers (1992, 1996) have shown that shading (i.e., attached shadows) provides a rich source of information used by the adult visual system for perceiving local surface orientation and three-dimensional object shape. In addition, Kersten, Knill, Mamassian, and Bülthoff (1996) have demonstrated that adults use cast shadow information to perceive objects' distances and movements in three-dimensional space.

Several studies have been done in which the development of space perception based on shading and shadows has been investigated by testing children's and infants' sensitivity to these depth cues. Benson and Yonas (1973) found that 3-year-old children perceive surface relief on the basis of attached shadows. The children in their study discriminated bumps and dents that were specified by shading. In a subsequent study, Yonas, Goldsmith, and Hallstrom (1978) found that 3-year-old children can judge objects' relative distances on the basis of cast shadows. In the first study of infants' sensitivity to depth information provided by shading, Granrud, Yonas, and upland (1985) found that 7-month-old infants perceive surface relief on the basis of shading but found no evidence of this ability in 5-month-old infants. This developmental pattern was similar to that found in studies of infants' sensitivity to other pictorial depth cues, including linear perspective (Arterberry, Bensen, & Yonas, 1991; Yonas, Cleaves, & Pettersen, 1978; Yonas, Granrud, Arterberry, & Hanson, 1986), interposition (Granrud & Yonas, 1984), line junctions (Yonas & Arterberry, 1994), familiar size (Granrud, Haake, & Yonas, 1985; Yonas, Pettersen, & Granrud, 1982), relative size (Yonas, Granrud, & Pettersen, 1985), and surface contours (Sen, Yonas, & Knill, 2001).

It has remained unknown whether infants perceive depth on the basis of cast shadows or whether sensitivity to cast shadows as depth information emerges at the same point in development as sensitivity to depth based on shading or other pictorial depth cues. …

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