Our aim has been to test the hypothesis that the age of immersion in a second language has differential effects on the neural subsystems involved in language processing. This hypothesis arises from consideration of studies of the development and organization of visual, auditory, and somatosensory systems. Within these systems, the nature of sensory input significantly affects the development of specific neurophysiological and behavioral processes (Freeman & Thibos, 1973; Kaas, 1991; Knudsen, 1988; Patkowski, 1980; Wiesel & Hubel, 1963, 1965). Morever, different functions within a system display distinct vulnerabilities to altered timing of input during development. For example, within the visual system, the timing of abnormal visual experience differentially affects the development of stereopsis, monocular spatial resolution, and spectral sensitivity (Harwerth, Smith, Duncan, Crawford & von Noorden, 1986). Although plasticity has been shown to characterize sensory and motor maps even in adult mammalian brains (Kaas, 1991; Kaas, Merzenich, & Killackey, 1983), many such experience-dependent changes occur only during specific critical or sensitive periods. A

general principle that emerges from a variety of studies is that the impact of altered sensory experience for many functions diminishes with maturation.