Psycholinguistics, or the psychology of language, encompasses a great many aspects of the study of language. Psycholinguistics is typically understood to include the study of the acquisition of language by children; the processing of language at all levels; and the storage and access of words in the lexicon. Child language acquisition provides us with a great deal of evidence for our innate capacity for language. Neuropsychologist Eric Lenneberg compares the acquisition of language to innate abilities in other species, including eyesight in cats and flight in birds. Children exposed to language for the first time after this developmental period are less likely to acquire language fluently or natively. Research and psycholinguistic investigation on first language acquisition explores this question from a variety of different perspectives, revealing many fascinating aspects of this biological behavior. The study of sign languages, by linguists, including psycholinguists, has enriched our understanding of many aspects of language.