A novel quantitative measure of Tracé-alternant EEG activity and its association with sleep states of preterm infants.

This study describes the application of a novel quantitative method for classifying patterns of EEG activity that are associated with the predominant sleep-states of newborn infants. Periods in which there are bursts of high-voltage slow wave activity in the EEG that alternate with periods of low-voltage activity are termed Tracé-alternant. During active or REM sleep. Tracé-alternant is absent and EEG activity is characterized by a variable mixture of frequencies including intermittent high frequency (10-20 Hz) activity superimposed on slower frequencies. Results show that an analytic method previously developed in fetal baboons for identifying EEG segments with and without Tracé-alternant successfully distinguishes homologous patterns of EEG activity in preterm infants. This method provides an excellent objective approach for monitoring changes in EEG patterns that are coincident with behaviorally defined sleep states.

Contrast-dependent responses in the human visual system: childhood through adulthood.

Visual evoked potentials (VEPs) to temporal modulation of spatial patterns, recorded from humans ranging in age from 4-42 years, demonstrated that contrast-dependent responses exist in early childhood and change dramatically throughout childhood. Bright or dark isolated-check stimuli were used to emphasize contributions from ON or OFF pathways to the VEP. (ON and OFF pathways constitute one major pair of parallel subsystems, which process brightness [positive-contrast] and darkness [negative-contrast] information, respectively.) The developmental effects observed for each pathway were similar in magnitude and time course, suggesting maturation of a common physiological mechanism dependent on spatial contrast. Children's responses were more variable and larger than those of adults, and exhibited a relative phase lag. In addition, we recorded transient VEPs to a conventional contrast-reversing checkerboard pattern. The latency of the major positive wave (P100) was found to decrease, while the latency of the initial positive wave (P60) was found to increase, with increasing age. We propose a vector-summation model, which posits a relative decrease in cortical excitation with increasing age, to explain our major findings.

Temporal tuning and the development of lateral interactions in the human visual system.

PURPOSE: The authors examined the development of lateral interactions between neurons in the human visual system through the use of visual evoked potentials (VEPs) elicited by windmill-dartboard stimuli. Previously, these VEPs have revealed two distinct types of lateral interactions (short-range and long-range) in adults. This study aims to track the development of these interactions in the first 6 months of life. METHOD: Windmill-dartboard stimuli were generated by a computer-controlled visual stimulator and presented on an oscilloscope display. VEPs to these stimuli were obtained from a group of human infants between 14 days and 6 months of age and from a group of adults who served as a basis for comparison. Fourier analysis was used to retrieve amplitude and phase measures of the relevant frequency components of the response. RESULTS: Amplitude measures of the VEP components elicited by the windmill-dartboard stimulus showed that the attenuation of the second harmonic frequency component (reflecting long-range lateral interactions) was essentially adultlike at all temporal frequencies for the majority of infants. In contrast, the amplitude of the fundamental frequency component (thought to reflect short-range lateral interactions) exhibited a low-pass temporal tuning function in infants that differed dramatically from adults. Additional immaturities were observable in the phase of the fundamental component of the infant VEPs. CONCLUSIONS: Evidence for the presence of some degree of lateral interaction was seen in even the youngest infant. Long-range lateral interactions appear to mature rapidly in infancy, whereas short-range lateral interactions show a much longer developmental time-course, and their properties are dependent on temporal frequency.

Development of accommodation and convergence in infancy.

Paraxial photorefraction was used to assess the development of accommodation and convergence in a large sample of infants under 1 year of age. The infants viewed small dolls placed at various distances (200-25 cm). The majority of infants at all ages demonstrated appropriate convergence for target distance, regardless of manifest refractive error. However, accommodation lagged behind convergence in development. Infants under 2 months tended to demonstrate either flat accommodation responses with a fixed plane of focus at around 30 cm, or accommodated appropriately for near targets, but failed to relax their accommodation sufficiently for the more distant targets. Thus, the focussing error increased with increasing target distance. Since the manifest refractive error was estimated by extrapolating the accommodation function to 0 diopters demand, these infants demonstrated spuriously myopic behavior. After 2 months, the majority of infants showing emmetropic behavior had accommodation responses that changed appropriately with target distance. However, infants with myopic or hyperopic manifest refractive errors displayed a variety of accommodative styles.