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Janette Atkinson, Deirdre Birtles, John Wattam-Bell, Oliver Braddick; Global form and global motion: Which develops first in infancy? VERP evidence. Journal of Vision 2005;5(8):490. doi: 10.1167/5.8.490.
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Sensitivity to global form and to global motion are dissociable measures of processing in the ventral and dorsal cortical streams, in adults and in developmental disorders. How do these two forms of processing develop in infancy? Changes in contour orientation elicit visual event-related potentials (VERP) earlier in development than do reversals of motion direction (Atkinson, ARVO 2002); however, both these responses may arise from purely local mechanisms. Here we tested infants aged 8–16 weeks, and adults, for VERPs dependent on global processing. In both form and motion stimuli, a display containing a region of concentrically organized elements alternated at 1–2 Hz with a display of similar, randomly oriented, elements. For the global form test, the elements were short arc segments; for global motion they were dots moving along short trajectories forming similar arcs. In each case, a signal at the stimulus frequency (F1) indicates differential responses to globally coherent and incoherent displays, while a second-harmonic signal (F2) could arise from responses to local changes in orientation or direction. A large proportion of the infants show a statistically reliable F1 response to coherent motion, while few do for coherent form. Infants show a larger F1 for motion than form; this ratio declines with age and averages about 1 in adults. The ratio shows the opposite behaviour for F2, so the motion advantage is specific to global processing. This suggests a developmental advantage for global motion processing over form processing in infancy, even though local processing develops earlier for form. This result will be discussed in relation to the apparently opposite effect in middle childhood whereby dorsal stream function tends to lag behind ventral and show more vulnerability to neurodevelopmental disorders (Atkinson et al, NeuroReport, 1997; Gunn et al, NeuroReport, 2002; Braddick et al, Neuropsychologia, 2003).
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