the response at the first and second harmonics (Jampolsky et al., 1994; Norcia et al., 1995). At moderate temporal frequencies (6 Hz) and low spatial frequencies (1c/deg), a symmetric response is achieved by 5 to 6 months of age (Birch et al., 2000; Jampolsky et al., 1994; Mason et al., 2001; Norcia et al., 1995; see Fig. 13.12, left). The age at which a symmetric motion response is achieved depends on both spatial and temporal frequency (Norcia et al., 1990a). Figure 13.13 shows data obtained at four different spatiotemporal frequencies (the combinations of 1 and 3c/deg and 6- and 10-Hz oscillation frequency). At each spatiotemporal combination, the first harmonic shows a decline with age and the second harmonic shows an increase with age. If one considers the age at which the two curves cross (the point where first and second harmonics are of equal amplitude), one observes that this criterion value is reached first for the 6-Hz and 1 c/deg condition (14 weeks), next at 6 Hz and 3 c/deg (27 weeks), then at 10 Hz and 1 c/deg (57 weeks), and finally, at 78 weeks for the 10-Hz and 3 c/deg condition. Brown et al. (1998) found that infant monkeys achieved adult levels of symmetry at 6 Hz and 0.25 c/deg by 6 weeks of age, which is similar to the case in humans, given the 4:1 correction for equivalent developmental ages that has been applied in other contexts (Boothe et al., 1985).

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