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Jin Lee, Dee Birtles, John Wattam-Bell, Janette Atkinson, Oliver Braddick; Development of visual motion processing: Latency of direction-specific VEP in infants compared to adults. Journal of Vision 2011;11(11):418. doi: 10.1167/11.11.418.
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Pattern-reversal (PR) VEPs are present in newborns, while direction-reversal (DR) VEPs emerge around age 9–11 weeks age (Wattam-Bell, Vis Res, 1991). Comparing the latency of these responses can reveal the emerging properties of direction-selective mechanisms in infants' visual cortex.
DR and PR-VEPs were tested in 46 adults at 1–4 reversal/sec (r/s). DR mean latency was 90 ms, close to the P100 latency for PR responses. However, 35% of participants showed a dominant early peak of 60ms (Braddick et al, VSS 2009). For 38 infants aged 4–54 weeks, 76% showed dominant early peaks (average latency 84 ms) in addition to the later peak of 145 ms. The early peak suggests a fast motion pathway, possibly bypassing V1, which may precede the slower pathway in development.
Latency measures were also calculated from the slopes of phase vs temporal frequency plots (adults: 1–16 r/s, infants: 1–8 r/s). Adults' calculated latency for PR was similar to the transient peak latency. However, the calculated DR latency was approximately double that for the transient peak, presumably reflecting extended cortical processing beyond the initial directional response.
For both stimuli, infants' calculated latency shows a delayed developmental course compared to the transient peak latency. Infants reach adult PR latency values by 15 weeks, but for DR only by 50 weeks. However, the calculated latencies for both PR and DR asymptote to adult values around 35 weeks, suggesting different developmental processes determining the initial peak and the time required for later cortical processing.
In summary, peak latency for motion reflects two pathways, possibly one through V1 and another direct to MT. Even though initial latencies were similar for contrast and motion, the subsequent cortical processing results in a longer calculated latency for motion. This difference also results in separate developmental trajectories.
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