In adults, spatially segregated 1D-motion components have, in fact, been shown to produce a coherent motion percept. This occurs when the apertures through which the moving contours are viewed are very small (Mingolla et al.,
1992; Alais, van der Smagt, van den Berg, & van de Grind,
1998) or when the apertures are relatively large but viewed in the periphery, where motion mechanisms are thought to summate over a relatively large area (Adelson & Movshon,
1983, “Split Herring Bone Illusion”; and see Lorenceau & Shiffrar,
1992). These adult psychophysical results are supported by neural data from area MT, which show that a proportion of pattern-selective neurons maintain their selectivity when stimuli consist of spatially segregated 1D-motion components (Majaj, Carandini, Smith, & Movshon,
1999). In infants, the use of spatially segregated 1D-motion components may be a particularly suitable approach because there is reason to believe that infants summate over larger areas than do adults, even in central vision. For example, in experiments investigating the ability to detect variously sized luminance discs, it has been shown that 3-month-old infants exhibit spatial summation over an area four times larger than that of adults (Hamer & Schneck,
1984; and see Schneck, Hamer, Packer, & Teller
1984; Hansen, Hamer, & Fulton,
1992). In addition, within the infant motion literature it has been suggested that the breakdown in infants’ ability to detect “relative motion” (i.e., patches of
oppositely directed motion) under some conditions may result from summation (and thus cancellation) across visual space (Skoczenski & Aslin,
1992; Wattam-Bell,
1994; Roessler & Dannemiller,
1997; and see Banton, Bertenthal, & Seaks,
1999, for similar conclusions based on infants’ sensitivity to statistical distributions of direction in moving dot stimuli). These psychophysical results in humans are consistent with the finding that receptive field sizes of neurons in young cats (Rusoff & Dubin,
1977; Norton,
1981) and monkeys (e.g., Blakemore & Vital-Durand,
1979) are significantly larger than those of adult animals.