known as Vernier acuity, is finer than the spacing of the cone lattice that forms the limiting factor for grating acuity in the adult (Westheimer and McKee, 1977).

Positional sensitivity can be inferred from the VEP by showing that the response is dependent on the relative position or configuration of the stimulus. A demonstration of position sensitivity in infants is illustrated in Figure 13.9, which plots data from Skoczenski and Norcia (1999). Responses were measured using stimuli composed of a series of oscillating grating panels that were sandwiched between static grating panels. The motion of the oscillating panels was constant and equal to one-fourth of a period of the grating making up the oscillating pattern. The author compared the responses to the oscillating pattern when the static pattern was positioned symmetrically with respect to the two extremes of the motion (symmetric misalignment/misalignment), and when the static pattern was shifted by 90° of spatial phase, which yielded a stimulus that alternated between a simple grating and a grating with offsets (alignment/misalignment). If all that is being detected is the motion of the moving panels, the two stimulus configurations should produce the same response because the amount of motion is the same. However, the response to the alignment/misalignment condition has a large first harmonic component that is not present in the symmetric misalignment/misalignment condition. Zemon and Ratliff (1984) performed the original experiments in adults, upon which the author's experiment is based. Their study revealed that the first harmonic was actually a nonlinear term owing to an interaction between the moving elements (frequency of 3 Hz, in this case) and the static elements (frequency of 0 Hz), yielding a sum/difference frequency component of 3 Hz. Grose-Fifer et al. (1994) showed that this nonlinear interaction was present in infants, but that it had a very different temporal tuning than in adults.

The response at the first harmonic in the offset grating task is a marker of position sensitivity that persists down to very small offsets equal to the psychophysical threshold for discriminated left/right shifts of the moving elements relative to the static elements (Norcia et al., 1999). In their experiment, vernier offset size was swept over a range of values spanning the psychophysical discrimination threshold, and VEP thresholds were estimated by extrapolating the VEP amplitude versus displacement function to zero amplitude.

Vernier acuity, measured with the swept-parameter VEP, shows a long developmental sequence in humans, with adult values first being reached between 10 and 14 years of age (Skoczenski and Norcia, 2002; see also Fig. 13.10). Vernier thresholds were 20 to 40 times lower than adult levels in the youngest infant tested. By the second semester, thresholds were between 1 and 2 arc min, or a factor of 4 to 8 below those of the adults (0.25 minutes or 15 arc sec). It is only after the age of 6 years that VEP Vernier acuity exceeds grating acuity.

Development of direction selectivity and sensitivity

The detection of motion involves the determination of speed and direction. Directionally appropriate eye movements can be seen at term or even before term (Dubowitz et al., 1980; Hainline, 1993). However, it is unclear whether these early ocular following responses are controlled by cortical or subcortical pathways. Direction selectivity has been demonstrated behaviorally using forced-choice preferential looking (FPL) and looking-time habituation methods by 6 to 8 weeks of age (Wattam-Bell, 1996a; Wattam-Bell, 1996b). In these studies, 6- to 8-week-old infants demonstrated a

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