Dynamics of chromatic visual system processing differ in complexity between children and adults School of Optometry and Vision Science, UNSW, Sydney, NSW, Australia Mei Ying Boon School of Optometry and Vision Science, UNSW, Sydney, NSW, Australia Catherine M. Suttle School of Mathematics and Statistics, UNSW, Sydney, NSW, Australia Bruce I. Henry School of Optometry and Vision Science, UNSW, Sydney, NSW, Australia Stephen J. Dain Measures of chromatic contrast sensitivity in children are lower than those of adults. This may be related to immaturities in signal processing at or near threshold. We have found that children’s VEPs in response to low contrast supra-threshold chromatic stimuli are more intra-individually variable than those recorded from adults. Here, we report on linear and nonlinear analyses of chromatic VEPs recorded from children and adults. Two measures of signal-to-noise ratio are similar between the adults and children, suggesting that relatively high noise is unlikely to account for the poor clarity of negative and positive peak components in the children’s VEPs. Nonlinear analysis indicates higher complexity of adults’ than children’s chromatic VEPs, at levels of chromatic contrast around and well above threshold. Keywords: color vision, development, children, visual evoked potential, nonlinear, linear, correlation dimension Citation: Boon, M. Y., Suttle, C. M., Henry, B. I., & Dain, S. J. (2009). Dynamics of chromatic visual system processing differ in complexity between children and adults. Journal of Vision, 9(6):22, 1–17, http://journalofvision.org/9/6/22/, doi:10.1167/9.6.22. Introduction Psychophysical (Knoblauch, Vital-Durand, & Barbur, 2001; Ling, 2004; Ling & Dain, 2008) and VEP (Boon, Suttle, & Dain, 2007) measures of chromatic contrast sensitivity are lower in children than in adults. In children but not in adults, VEP estimates of chromatic contrast sensitivity (assessed on the basis of the smallest chromatic contrast, which elicits a repeatable VEP) are higher than psychophysical estimates (Boon et al., 2007). VEPs recorded from children, in response to stimuli of moderate to high chromatic contrast, are repeatable and clearly defined but differ from adults’ responses in terms of morphology (Boon et al., 2007; Madrid & Crognale, 2000; Pompe, Kranjc, & Brecelj, 2006). However, children’s VEPs in response to stimuli set at their individual psychophysically determined chromatic contrast threshold (T%) or at twice this level (2T%), unlike adults’, do not contain recognizable VEP components, which are distin- guishable from baseline activity in terms of morphology or are repeatable in latency (defined as having a difference in latency of no longer than 10% of the longest latency on successive averaged VEPs; Boon et al., 2007). Represen- tative VEPs illustrating this trend are presented in Figure 1. In this case, the VEPs are in response to a 2-Hz chromatic onset–offset VEP, which should result in two distinctive VEP responses in the 1000-ms epoch shown here. The adult’s VEPs show the characteristic N–P complex for stimuli set at psychophysical threshold (T%), 2T% and 42% for the two averaged sweeps (indicated in light gray) and over the 1000-ms epoch. However, the child’s VEPs were not repeatable either between averaged sweeps or over the 1000-ms epoch. Although there is a level of examiner subjectivity in identifying peaks and troughs in VEP morphology, VEP findings indicate immaturity of the chromatic visual system in children, which is characterized by decreased VEP component repeatability at low to moderate chromatic contrasts. The child’s VEPs at T% and 2T% also show a strong rhythmic alpha frequency component at baseline, T% and 2T% that also creates ambiguity in the clarity of any rhythmic VEP components, such as negative and positive peaks related to chromatic contrast. Recently, we have found that transient chromatic VEPs recorded from adults may be modeled as arising from a nonlinear deterministic (future states arising from initial states) dynamical (evolving over time) system (Boon, Henry, Suttle, & Dain, 2008) according to coupled differential equations, which contain nonlinear terms Journal of Vision (2009) 9(6):22, 1–17 http://journalofvision.org/9/6/22/ 1 doi: 10.1167/9.6.22 Received November 8, 2008; published June 30, 2009 ISSN 1534-7362 * ARVO