The purpose of this paper is twofold: first, to draw attention to the research on speech perception and, second, to use the results for a reassessment of the contribution of innate capacities versus external stimulation in conjunction with age in first and second language acquisition. The theoretical framework is the universal theory of language acquisition. The focus is on the functional potential of the biological substrate rather than its anatomy.
Neonates are innately capable of two major modes of auditory perception, namely, categorical and continuous perception. The interaction of these two modes allows infants to develop the perceptual categories of their ambient language(s). The continuous mode functions as a monitoring device in shaping the categories of the target(s). Various kinds of evidence are reviewed that suggest that these original sensory abilities remain unchanged throughout an individual's lifespan, but they become difficult to access during later stages of life, such as in adult L2 acquisition, because of the way perceptual-phonological information is stored in memory and/or activated in language processing. There are no biologically based restrictions as to the number of languages that can be learned, or the age ranges during which this can happen.